ABOS Part I Orthopedic Oncology Review: Chondrosarcoma, Osteosarcoma, NOF | Part 22301

ABOS Part I Comprehensive Review - Batch 98

This module contains 30 advanced orthopedic multiple-choice questions developed to mirror the American Board of Orthopaedic Surgery (ABOS) Part I and AAOS OITE examinations. Questions are derived directly from high-yield clinical teaching cases.

Generated MCQ Transcript

Question 1:

A 50-year-old male presents with a 6-month history of dull, aching pain in his left proximal humerus, not relieved by rest. Radiographs reveal a lytic lesion with punctate and ring-and-arc calcifications within the medullary cavity, associated with cortical thickening and periosteal reaction. Which of the following is the most appropriate initial diagnostic step?

• A: Observation with serial radiographs
• B: CT-guided core needle biopsy
• C: Immediate wide en bloc resection
• D: Intralesional curettage
• E: Empiric systemic chemotherapy

Explanation:

Correct Answer: B

The patient's age, insidious pain, and radiographic findings (lytic lesion with chondroid matrix calcifications, cortical thickening, and periosteal reaction) are highly suspicious for a chondrosarcoma, particularly a low-grade lesion. A CT-guided core needle biopsy is the most appropriate initial diagnostic step to confirm the diagnosis, grade the tumor, and guide definitive surgical planning. This is crucial before committing to a definitive surgical procedure.

• Observation with serial radiographs is inappropriate given the suspicious features of malignancy.
• Immediate wide en bloc resection is the definitive treatment but should only be performed after a confirmed diagnosis and precise staging.
• Intralesional curettage is inadequate for chondrosarcoma due to high recurrence rates and is generally contraindicated for malignant lesions.
• Empiric systemic chemotherapy is not indicated as chondrosarcomas are largely resistant to chemotherapy, and a definitive diagnosis is required before any systemic treatment.

Question 2:

A 45-year-old female is diagnosed with a Grade 2 conventional chondrosarcoma of the distal femur. Imaging shows no evidence of metastatic disease. What is the most crucial principle in the surgical management of this tumor?

• A: Intralesional curettage to preserve bone stock
• B: Marginal excision to minimize morbidity
• C: Wide en bloc resection with clear surgical margins
• D: Adjuvant chemotherapy followed by delayed surgery
• E: Preoperative radiation therapy to shrink the tumor

Explanation:

Correct Answer: C

Wide en bloc resection with clear surgical margins is the gold standard and most crucial principle in the surgical management of conventional chondrosarcoma, especially for Grade 2 lesions. These tumors are highly resistant to chemotherapy and radiation, making surgical extirpation the primary curative modality. Achieving clear margins is paramount for local control and to prevent recurrence.

• Intralesional curettage or marginal excision is associated with unacceptably high local recurrence rates for Grade 2 chondrosarcoma.
• Adjuvant chemotherapy and preoperative radiation therapy are generally ineffective for conventional chondrosarcoma and are not considered primary treatment modalities, though they might be considered in very specific, high-risk, or unresectable cases, or for certain variants like mesenchymal chondrosarcoma.

Question 3:

When attempting to differentiate a benign enchondroma from a low-grade central chondrosarcoma in an appendicular skeleton lesion, which of the following radiographic features on plain film or CT is most suggestive of chondrosarcoma?

• A: Lobulated morphology with internal calcifications
• B: Presence of an intact sclerotic rim
• C: Cortical scalloping greater than 2/3 of the cortical thickness
• D: Well-defined lucent lesion in the diaphysis
• E: High signal intensity on T2-weighted MRI

Explanation:

Correct Answer: C

Cortical scalloping greater than 2/3 of the cortical thickness, or actual cortical breakthrough/destruction, is a strong indicator of a low-grade chondrosarcoma rather than an enchondroma. This signifies an infiltrative and slowly aggressive growth pattern. While lobulated morphology and internal calcifications (e.g., punctate or ring-and-arc) can be seen in both benign and low-grade malignant cartilaginous lesions, significant cortical erosion points towards malignancy.

• An intact sclerotic rim usually favors a benign lesion like an enchondroma.
• A well-defined lucent lesion in the diaphysis is a non-specific finding.
• High signal intensity on T2-weighted MRI is characteristic of cartilaginous lesions due to their high water content but does not reliably differentiate benign from low-grade malignant.

Question 4:

A 68-year-old male with Hereditary Multiple Exostoses (HME) presents with increasing pain and a palpable mass in a previously asymptomatic lesion on his distal femur. Radiographs show a thickened cartilaginous cap. What specific finding on imaging is most concerning for malignant transformation into a secondary peripheral chondrosarcoma?

• A: The presence of a cartilage cap
• B: A lobulated contour of the lesion
• C: A cartilaginous cap thickness exceeding 2 cm in an adult
• D: Location in the appendicular skeleton
• E: Pain with activity

Explanation:

Correct Answer: C

In a patient with Hereditary Multiple Exostoses, an increase in pain and a cartilaginous cap thickness exceeding 1-2 cm (with 2 cm often used as a more definitive cutoff in adults) on an osteochondroma are highly suspicious for malignant transformation into a secondary peripheral chondrosarcoma. This is the most reliable radiographic and pathological distinguishing feature.

• The presence of a cartilage cap is normal for an osteochondroma.
• A lobulated contour can be seen in both benign and malignant cartilaginous lesions.
• Location in the appendicular skeleton is common for osteochondromas and does not, by itself, indicate malignancy.
• Pain with activity can be a symptom of malignant transformation but is also seen in benign osteochondromas due to bursitis, fracture, or nerve impingement; therefore, it is less specific than cap thickness.

Question 5:

Which of the following is considered the single most important prognostic indicator in conventional chondrosarcoma?

• A: Patient age at diagnosis
• B: Tumor size
• C: Histological grade (Grade 1, 2, or 3)
• D: Location of the tumor (axial vs. appendicular)
• E: Duration of symptoms before diagnosis

Explanation:

Correct Answer: C

The histological grade (Grade 1, 2, or 3) is the single most important factor in determining the prognosis of conventional chondrosarcoma. It directly correlates with the tumor's metastatic potential, local recurrence risk, and overall patient survival. Higher grades (Grade 2 and especially Grade 3) are associated with a significantly worse prognosis.

• While tumor size and location (axial lesions generally having a worse prognosis due to difficulty in achieving wide margins) do impact management and can influence prognosis, they are secondary to the histological grade.
• Patient age and duration of symptoms are less direct prognostic indicators compared to the tumor's intrinsic biological aggressiveness as reflected by its grade.

Question 6:

A 60-year-old patient with a history of a resected Grade 1 chondrosarcoma of the rib presents with a new, rapidly growing mass at the previous surgical site. Biopsy reveals a high-grade pleomorphic sarcoma with no cartilaginous matrix. What is the most likely diagnosis?

• A: Recurrent Grade 1 conventional chondrosarcoma
• B: Post-radiation sarcoma
• C: Dedifferentiated chondrosarcoma
• D: Metastatic carcinoma
• E: A new primary osteosarcoma

Explanation:

Correct Answer: C

The scenario describes a recurrence of a previously resected low-grade chondrosarcoma, but this time with a high-grade, non-cartilaginous sarcomatous component (pleomorphic sarcoma). This abrupt change in histology from a well-differentiated conventional chondrosarcoma to a high-grade, non-cartilaginous sarcoma is the hallmark of dedifferentiated chondrosarcoma. This variant carries a very poor prognosis.

• Recurrent Grade 1 conventional chondrosarcoma would retain its low-grade cartilaginous features, which is not consistent with a high-grade pleomorphic sarcoma.
• Post-radiation sarcoma would require a history of radiation to the area, which is not mentioned in the vignette.
• Metastatic carcinoma is less likely given the history of a primary bone sarcoma at the same site.
• A new primary osteosarcoma is possible but less likely than dedifferentiation given the direct history of chondrosarcoma at the same site.

Question 7:

A 50-year-old patient undergoes wide en bloc resection for a Grade 2 conventional chondrosarcoma of the proximal femur. Postoperative MRI at 6 months shows no evidence of local recurrence. What is the most appropriate long-term follow-up strategy for this patient?

• A: No further imaging is needed if the patient is asymptomatic.
• B: Annual chest X-ray for 5 years, then biannually.
• C: CT scan of the chest, abdomen, and pelvis every 6 months for 2 years, then annually.
• D: MRI of the local site and chest X-ray every 6-12 months for 5-10 years.
• E: Bone scan every year indefinitely.

Explanation:

Correct Answer: C

Follow-up for resected chondrosarcoma, especially Grade 2 or higher, typically involves surveillance for both local recurrence and distant metastases. The lungs are the most common site of metastasis. A comprehensive strategy involves regular imaging. For a Grade 2 chondrosarcoma, aggressive surveillance is warranted, usually involving CT of the chest, abdomen, and pelvis every 6 months for 2 years, then annually, to detect both lung and other potential metastases, as well as local recurrence (often with MRI of the local site).

• No further imaging is negligent for a malignant tumor.
• Annual chest X-ray is insufficient as CT is more sensitive for detecting lung metastases.
• MRI of the local site and chest X-ray is better but still insufficient due to the lower sensitivity of X-ray compared to CT for lung metastases.
• Bone scan every year indefinitely is not the primary imaging modality for detecting chondrosarcoma metastases, which are typically hematogenous to the lungs.

Question 8:

Which of the following genetic mutations is most commonly associated with the development of central enchondromas and conventional central chondrosarcomas, including those seen in Ollier's disease and Maffucci syndrome?

• A: TP53
• B: IDH1/IDH2
• C: MYC amplification
• D: TERT promoter mutations
• E: H3F3A

Explanation:

Correct Answer: B

Somatic mutations in Isocitrate Dehydrogenase 1 and 2 (IDH1/IDH2) genes are found in a significant proportion (approximately 50-70%) of central enchondromas and conventional central chondrosarcomas (Grades 1 and 2). They are considered early events in chondrosarcoma development and are also highly prevalent in enchondromas and chondrosarcomas associated with Ollier's disease and Maffucci syndrome. These mutations are key oncogenic drivers in chondrogenesis.

• TP53 is a tumor suppressor gene associated with many cancers, including high-grade sarcomas, but not specifically central chondrosarcoma.
• MYC amplification and TERT promoter mutations are found in some aggressive tumors but are not as common or specific as IDH mutations in central chondrosarcoma.
• H3F3A mutations are characteristic of chondroblastoma, not central chondrosarcoma.

Question 9:

A 62-year-old patient undergoes an unplanned intralesional excision for what was thought to be an enchondroma of the proximal femur. Final pathology reveals a Grade 2 chondrosarcoma with positive surgical margins. What is the most appropriate next step in management?

• A: Observation with serial imaging
• B: Adjuvant radiation therapy alone
• C: Systemic chemotherapy
• D: Re-excision with wide margins
• E: Palliative care

Explanation:

Correct Answer: D

An unplanned intralesional excision of a Grade 2 chondrosarcoma with positive margins necessitates a planned re-excision with wide margins. This is crucial for achieving local control and preventing recurrence and potential dedifferentiation. The goal of chondrosarcoma surgery is complete removal with clear margins, which was not achieved in the initial unplanned procedure.

• Observation with serial imaging is inadequate for a Grade 2 malignant tumor with known positive margins.
• Adjuvant radiation therapy alone is generally ineffective for conventional chondrosarcoma due to its radioresistance.
• Systemic chemotherapy is not a primary treatment for conventional chondrosarcoma due to its chemorefractory nature.
• Palliative care is not appropriate given the potential for cure with adequate surgical intervention.

Question 10:

What is the primary reason for the inherent resistance of conventional chondrosarcoma to conventional chemotherapy and external beam radiation therapy?

• A: Rapid proliferation rate of chondrocytes
• B: Lack of specific growth factor receptors
• C: Poor vascularity and hypoxic environment of cartilaginous tissue
• D: High expression of multi-drug resistance proteins
• E: Inability of drugs to penetrate the cartilaginous matrix

Explanation:

Correct Answer: C

The primary reason for conventional chondrosarcoma's resistance to chemotherapy and radiation therapy is attributed to the inherent poor vascularity and hypoxic environment of cartilaginous tissue. This physiological characteristic limits drug delivery to the tumor cells and reduces the effectiveness of radiation, which relies on oxygen-dependent free radical formation to damage DNA. Chondrosarcomas generally have a slow growth rate, and while multi-drug resistance proteins can play a role, the fundamental nature of cartilage is the key factor.

• Rapid proliferation rate is characteristic of many chemosensitive tumors, not chondrosarcoma.
• While lack of specific growth factor receptors and high expression of multi-drug resistance proteins can contribute, the poor vascularity and hypoxia are considered more fundamental.
• Inability of drugs to penetrate the cartilaginous matrix is a consequence of the poor vascularity and density, rather than a separate primary reason.

Question 11:

A 15-year-old male presents with a 4-month history of progressive right distal femoral pain, worse at night. Radiographs show a mixed lytic and blastic lesion with a sunburst periosteal reaction. MRI confirms a large intramedullary mass extending to the physis but not crossing it, with a significant soft tissue component. Staging CT chest is negative for metastases. Biopsy confirms high-grade osteosarcoma. Which of the following Enneking surgical stages best describes this patient's tumor?

• A: Stage IA
• B: Stage IB
• C: Stage IIA
• D: Stage IIB
• E: Stage III

Explanation:

Correct Answer: D

Explanation:

The Enneking surgical staging system classifies musculoskeletal sarcomas based on three parameters: histologic grade (G), local extent of the primary tumor (T), and presence of regional or distant metastases (M).

• Grade (G): The biopsy confirms 'high-grade osteosarcoma', which corresponds to G2.
• Local Extent (T): The MRI shows a 'large intramedullary mass with a significant soft tissue component'. This indicates that the tumor has extended beyond the bone cortex and into the surrounding soft tissues, placing it in an extra-compartmental location. Therefore, it is T2.
• Metastases (M): The 'Staging CT chest is negative for metastases', meaning M0.

Combining these, a G2, T2, M0 tumor is classified as Stage IIB. Stage IA and IB are for low-grade tumors. Stage IIA is for high-grade, intra-compartmental tumors. Stage III is for any tumor with regional or distant metastases (M1).

Question 12:

A 12-year-old female is diagnosed with a high-grade osteosarcoma of the proximal tibia. Neoadjuvant chemotherapy is initiated. During treatment, she develops severe nausea, vomiting, and acute kidney injury with elevated creatinine. Which chemotherapy agent is the most likely cause of these specific toxicities?

• A: Doxorubicin
• B: Ifosfamide
• C: Methotrexate
• D: Cisplatin
• E: Etoposide

Explanation:

Correct Answer: D

Explanation:

Cisplatin is a platinum-based alkylating-like agent commonly used in osteosarcoma regimens. Its well-known dose-limiting toxicities include severe nausea and vomiting (highly emetogenic), nephrotoxicity (acute kidney injury), and ototoxicity (hearing loss, tinnitus). Therefore, the patient's symptoms are highly suggestive of cisplatin toxicity.

• Doxorubicin (Adriamycin) is an anthracycline associated with cumulative dose-dependent cardiotoxicity (dilated cardiomyopathy) and myelosuppression.
• Ifosfamide is an alkylating agent known for causing hemorrhagic cystitis (prevented by Mesna) and neurotoxicity.
• Methotrexate, especially high-dose, is associated with myelosuppression, mucositis, and nephrotoxicity, but severe nausea/vomiting are less prominent compared to cisplatin. Leucovorin is used for rescue.
• Etoposide is a topoisomerase II inhibitor primarily causing myelosuppression and mucositis.

Question 13:

A 17-year-old male undergoes limb salvage surgery for a distal femoral osteosarcoma. The reconstruction involves a massive allograft. During follow-up, he develops persistent pain, swelling, and erythema around the surgical site, along with elevated inflammatory markers. Aspiration of the joint reveals purulent fluid. What is the most appropriate initial management step?

• A: Initiate a new course of systemic chemotherapy.
• B: Perform a CT-guided biopsy to rule out local recurrence.
• C: Administer broad-spectrum intravenous antibiotics and obtain cultures.
• D: Perform an immediate surgical debridement and irrigation with hardware retention if possible.
• E: Observe with serial inflammatory markers and consider oral antibiotics.

Explanation:

Correct Answer: D

Explanation:

The patient's presentation (persistent pain, swelling, erythema, elevated inflammatory markers, purulent fluid on aspiration) is highly indicative of a deep surgical site infection, a devastating complication following massive allograft reconstruction, especially in immunocompromised oncology patients. For a deep infection involving an allograft or prosthetic implant, immediate surgical debridement and irrigation are crucial. Cultures should be obtained during surgery to guide targeted antibiotic therapy. While antibiotics are necessary, they are rarely sufficient alone for deep infections involving implants or allografts. Delaying surgical intervention significantly increases the risk of chronic infection, allograft failure, and potentially amputation.

• A. Initiate a new course of systemic chemotherapy: This is inappropriate as the primary issue is infection, not recurrence. Chemotherapy would further compromise the immune system.
• B. Perform a CT-guided biopsy to rule out local recurrence: While recurrence is always a concern, the signs of acute inflammation and purulent fluid point strongly to infection. A biopsy would delay definitive treatment of the infection.
• C. Administer broad-spectrum intravenous antibiotics and obtain cultures: While antibiotics are essential, they are typically adjunctive to surgical debridement for deep implant-related infections.
• E. Observe with serial inflammatory markers and consider oral antibiotics: Observation and oral antibiotics are insufficient for a suspected deep infection with purulent fluid, which requires urgent surgical intervention.

Question 14:

A 68-year-old female with a long-standing history of Paget's disease affecting her left humerus presents with a rapidly enlarging, painful mass in the same bone. Radiographs show an aggressive, mixed lytic and blastic lesion. Biopsy confirms osteosarcoma. What is the most significant prognostic factor for this patient compared to an adolescent with conventional osteosarcoma?

• A: The specific location in the humerus.
• B: The patient's gender.
• C: The presence of Paget's disease as a predisposing factor.
• D: The likelihood of a good response to neoadjuvant chemotherapy.
• E: The absence of metastatic disease at presentation.

Explanation:

Correct Answer: C

Explanation:

Osteosarcoma arising in the setting of Paget's disease of bone (secondary osteosarcoma) carries a significantly worse prognosis compared to conventional osteosarcoma in adolescents. This is a well-established clinical fact. The reasons for this include:

• Older age: Patients are typically older, often with more comorbidities, which can limit aggressive treatment.
• Higher tumor grade: These tumors are often high-grade.
• Advanced stage at presentation: They are frequently diagnosed at a larger size and/or with metastases.
• Poorer response to chemotherapy: Secondary osteosarcomas, including those from Paget's, tend to respond less favorably to standard chemotherapy regimens compared to primary osteosarcomas in younger patients.

Therefore, the presence of Paget's disease as a predisposing factor is a significant adverse prognostic indicator. The location in the humerus is less impactful than the underlying condition. Gender is not a consistent prognostic factor. A good response to chemotherapy would be a favorable prognostic factor, but it is less likely in this scenario. The absence of metastatic disease at presentation is always favorable, but the question asks for the most significant prognostic factor compared to an adolescent, which points to the underlying Paget's disease.

Question 15:

During pre-operative planning for a distal femoral osteosarcoma, the MRI reveals a separate, distinct intramedullary lesion in the proximal femur, approximately 5 cm away from the primary tumor, with normal intervening marrow. This finding is most accurately described as:

• A: A reactive bone marrow edema pattern.
• B: A synchronous multicentric osteosarcoma.
• C: A skip lesion.
• D: A benign fibrous cortical defect.
• E: An artifact of the MRI sequence.

Explanation:

Correct Answer: C

Explanation:

A 'skip lesion' in osteosarcoma refers to a separate, distinct focus of tumor within the same bone or a contiguous bone, discontinuous from the primary lesion but originating from it. The description of a 'separate, distinct intramedullary lesion in the proximal femur, approximately 5 cm away from the primary tumor, with normal intervening marrow' perfectly fits the definition of a skip lesion. These are true intraosseous metastases and necessitate a wider surgical margin to ensure complete tumor removal, as they carry a high risk of local recurrence if not adequately resected.

• A. A reactive bone marrow edema pattern: While edema can occur, it would typically be contiguous with the primary tumor or related to stress, not a distinct tumor focus.
• B. A synchronous multicentric osteosarcoma: This refers to two or more primary osteosarcomas arising independently in different bones, not a separate focus within the same bone.
• D. A benign fibrous cortical defect: These are common benign lesions, typically small, cortical, and have characteristic radiographic features distinct from an intramedullary tumor.
• E. An artifact of the MRI sequence: While artifacts can occur, a distinct lesion with normal intervening marrow is a real finding that requires careful interpretation.

Question 16:

A 10-year-old boy with a distal femoral osteosarcoma is undergoing limb salvage with a planned expandable endoprosthesis. What is the primary advantage of using an expandable endoprosthesis over a conventional fixed-length endoprosthesis in this patient?

• A: Lower risk of deep infection.
• B: Superior bone integration and reduced aseptic loosening.
• C: Ability to achieve limb lengthening to compensate for growth.
• D: Elimination of the need for future revision surgeries.
• E: Reduced cost and shorter operative time.

Explanation:

Correct Answer: C

Explanation:

The primary advantage of an expandable endoprosthesis (also known as a growing prosthesis) in a skeletally immature patient is its ability to be lengthened over time. This feature allows the reconstructed limb to keep pace with the growth of the contralateral limb, thereby preventing or minimizing significant limb length discrepancy as the child grows. This is crucial for maintaining function and avoiding multiple complex lengthening procedures in the future.

• A. Lower risk of deep infection: Expandable prostheses are complex and do not inherently have a lower infection risk; in fact, their complexity might sometimes increase it.
• B. Superior bone integration and reduced aseptic loosening: While implant design aims for good integration, expandability itself doesn't guarantee superior integration or reduced loosening compared to fixed prostheses, which are also prone to these issues.
• D. Elimination of the need for future revision surgeries: Expandable prostheses have a finite lifespan and are still subject to mechanical failure, infection, and loosening, often requiring revision surgeries in the long term.
• E. Reduced cost and shorter operative time: Expandable prostheses are typically more complex and expensive than fixed-length prostheses, and their implantation may not necessarily be shorter.

Question 17:

A 14-year-old female with a high-grade osteosarcoma of the proximal humerus has completed neoadjuvant chemotherapy. Post-chemotherapy MRI shows a good response with significant tumor shrinkage. The surgical plan is for limb salvage. Which of the following nerves is at highest risk of injury during the surgical approach to the proximal humerus, particularly when dissecting around the surgical neck and deltoid?

• A: Radial nerve
• B: Ulnar nerve
• C: Median nerve
• D: Axillary nerve
• E: Musculocutaneous nerve

Explanation:

Correct Answer: D

Explanation:

The axillary nerve is the nerve at highest risk of injury during surgical approaches to the proximal humerus, especially when the dissection involves the surgical neck and the deltoid muscle. The axillary nerve originates from the posterior cord of the brachial plexus (C5, C6), passes posteriorly around the surgical neck of the humerus, and innervates the deltoid and teres minor muscles. Damage to this nerve results in paralysis of the deltoid, leading to significant impairment of shoulder abduction and external rotation, and sensory loss over the lateral shoulder.

• A. Radial nerve: The radial nerve also originates from the posterior cord and spirals around the posterior aspect of the humerus in the radial groove, making it vulnerable in mid-shaft humeral fractures or approaches to the posterior humerus, but less so in the immediate proximal humeral surgical neck region compared to the axillary nerve.
• B. Ulnar nerve: The ulnar nerve runs medially in the arm and is typically not at high risk during proximal humeral approaches unless dissection extends significantly medially or distally.
• C. Median nerve: The median nerve runs with the brachial artery in the anterior compartment of the arm and is generally not at high risk during standard proximal humeral approaches.
• E. Musculocutaneous nerve: This nerve innervates the anterior compartment muscles of the arm (biceps, brachialis) and is typically more anterior and distal to the immediate surgical neck area.

Question 18:

A 16-year-old male with a distal femoral osteosarcoma undergoes neoadjuvant chemotherapy. After resection, the pathology report indicates 85% tumor necrosis. What is the most accurate prognostic implication of this finding?

• A: This indicates an excellent response to chemotherapy and a favorable prognosis.
• B: This suggests a poor response to chemotherapy and is an adverse prognostic factor.
• C: This level of necrosis is considered average and has no significant prognostic value.
• D: This implies the tumor was originally low-grade, despite initial diagnosis.
• E: This means that adjuvant chemotherapy is no longer necessary.

Explanation:

Correct Answer: B

Explanation:

The percentage of tumor necrosis after neoadjuvant chemotherapy is one of the most significant prognostic factors in osteosarcoma. A good response to chemotherapy is typically defined as greater than 90% (or sometimes 95%) tumor necrosis in the resected specimen. This correlates with improved event-free survival and overall survival. Conversely, a poor response, such as 85% necrosis, which is less than the 90-95% threshold, is considered an adverse prognostic factor. It indicates that the tumor was relatively resistant to the chemotherapy regimen, and these patients have a higher risk of local recurrence and distant metastasis.

• A. This indicates an excellent response to chemotherapy and a favorable prognosis: Incorrect. 85% is below the threshold for an 'excellent' response.
• C. This level of necrosis is considered average and has no significant prognostic value: Incorrect. It is below the favorable threshold and is prognostically significant.
• D. This implies the tumor was originally low-grade, despite initial diagnosis: Incorrect. Tumor necrosis reflects response to treatment, not the original grade.
• E. This means that adjuvant chemotherapy is no longer necessary: Incorrect. Adjuvant chemotherapy is almost always necessary for high-grade osteosarcoma, and a poor response might even prompt consideration of modifying the adjuvant regimen.

Question 19:

A 13-year-old female with a high-grade osteosarcoma of the proximal tibia is undergoing limb salvage. The surgeon plans to perform a Van Nes rotationplasty. What is the primary functional advantage of this procedure compared to a conventional above-knee amputation with a prosthesis?

• A: Improved cosmetic appearance of the limb.
• B: Elimination of the need for a prosthetic device.
• C: Retention of the patient's own ankle joint for active prosthetic knee control.
• D: Lower risk of infection and mechanical complications.
• E: Faster rehabilitation and return to full activity.

Explanation:

Correct Answer: C

Explanation:

Van Nes rotationplasty is a specialized limb salvage procedure primarily used for distal femoral or proximal tibial tumors in children. The key functional advantage is the retention of the patient's own ankle joint, which, after being rotated 180 degrees and reattached, functions as a knee joint for a custom-fitted prosthesis. This allows the patient to use their native ankle dorsiflexion and plantarflexion muscles to actively control the prosthetic knee, providing superior proprioception, balance, and endurance compared to a conventional above-knee amputation with a passive or externally powered prosthetic knee. This leads to excellent functional outcomes, especially for active children.

• A. Improved cosmetic appearance of the limb: Incorrect. Rotationplasty has a unique cosmetic appearance that can be challenging for some patients, though functional outcomes are often prioritized.
• B. Elimination of the need for a prosthetic device: Incorrect. A prosthesis is still required for the lower leg and foot, but it is a more functional prosthesis.
• D. Lower risk of infection and mechanical complications: Rotationplasty has its own set of potential complications, including non-union, nerve issues, and skin problems, and does not inherently have a lower risk of infection or mechanical issues compared to other complex reconstructions.
• E. Faster rehabilitation and return to full activity: Rehabilitation is extensive and prolonged, similar to other complex limb salvage procedures.

Question 20:

Which of the following conditions is most strongly associated with the development of secondary osteosarcoma in the craniofacial bones, particularly the jaw, in older adults?

• A: Fibrous dysplasia
• B: Hereditary retinoblastoma
• C: Multiple hereditary exostoses
• D: Paget's disease of bone
• E: Ollier's disease

Explanation:

Correct Answer: D

Explanation:

Paget's disease of bone is a well-recognized predisposing factor for the development of secondary osteosarcoma, especially in older adults. The craniofacial bones, including the maxilla and mandible (jaw), are common sites for Paget's disease. Therefore, osteosarcoma arising in the jaw in an older adult should raise suspicion for underlying Paget's disease. These secondary osteosarcomas generally carry a worse prognosis than conventional osteosarcomas.

• A. Fibrous dysplasia: While fibrous dysplasia can rarely undergo malignant transformation into osteosarcoma, it is not the most common predisposing factor for jaw osteosarcoma in older adults.
• B. Hereditary retinoblastoma: This condition is associated with germline RB1 mutations and a significantly increased risk of osteosarcoma, but typically in younger individuals and not specifically localized to the jaw.
• C. Multiple hereditary exostoses (HME): HME is a genetic disorder predisposing to the development of multiple osteochondromas, which can transform into chondrosarcoma, not osteosarcoma.
• E. Ollier's disease (multiple enchondromatosis): Similar to HME, Ollier's disease is associated with multiple enchondromas and carries a risk of malignant transformation into chondrosarcoma, not osteosarcoma.

Question 21:

A 10-year-old male presents with an incidental finding of a lucent lesion in the distal femur. Radiographs show an eccentric, lobulated, sclerotic-rimmed lesion in the metaphysis. Which of the following statements regarding the natural history of this likely diagnosis is most accurate?

• A: It typically progresses to an aggressive osteosarcoma in adulthood.
• B: It often spontaneously resolves by skeletal maturity, with osseous remodeling.
• C: Surgical excision is always indicated due to high risk of malignant transformation.
• D: It is a precursor lesion to Paget's disease of bone.
• E: It commonly recurs after surgical curettage.

Explanation:

Correct Answer: B

Non-ossifying fibroma (NOF) is a benign, self-limiting fibrous lesion that characteristically regresses spontaneously, often filling in with normal bone by skeletal maturity. Malignant transformation is exceedingly rare to non-existent. Surgical excision is not always indicated and is reserved for specific situations like impending or actual pathological fracture. It is not a precursor to osteosarcoma or Paget's disease, and recurrence after complete curettage is uncommon.

Question 22:

A biopsy from a well-circumscribed, eccentrically located metaphyseal lesion in a child reveals spindle cells arranged in a storiform pattern, admixed with multinucleated giant cells and hemosiderin deposition. These histological features are most characteristic of which of the following?

• A: Chondroblastoma
• B: Giant Cell Tumor
• C: Non-ossifying Fibroma
• D: Osteoid Osteoma
• E: Fibrous Dysplasia

Explanation:

Correct Answer: C

The classic histological description of a non-ossifying fibroma (NOF) includes a proliferation of benign spindle cells, often arranged in a storiform (pinwheel or cartwheel) pattern, admixed with scattered multinucleated giant cells, foam cells (lipid-laden macrophages), and areas of hemosiderin deposition. Chondroblastoma has chondroblast-like cells. Giant cell tumor lacks the storiform pattern and has uniform giant cells. Osteoid osteoma has osteoid seams. Fibrous dysplasia has woven bone in a fibrous stroma.

Question 23:

Which of the following best describes the typical radiographic appearance of a Non-Ossifying Fibroma on a plain X-ray?

• A: Central calcification with a periosteal reaction.
• B: Permeative osteolytic lesion with soft tissue mass.
• C: Well-defined, eccentric, lytic lesion with a sclerotic margin and often multiloculated appearance.
• D: Sunburst periosteal reaction with Codman's triangle.
• E: Ground-glass matrix with endosteal scalloping.

Explanation:

Correct Answer: C

NOFs typically present as well-defined, eccentric, purely lytic lesions with a characteristic sclerotic margin. They often have a lobulated or 'bubbly' appearance, indicating fibrous septa. Cortical thinning and mild expansion are also common. The other options describe features of other bone tumors (osteosarcoma for B and D, fibrous dysplasia for E, and calcification for enchondroma).

Question 24:

A 12-year-old boy has an asymptomatic 4 cm non-ossifying fibroma of the proximal tibia, incidentally discovered on radiographs for knee pain attributed to Osgood-Schlatter disease. The cortical involvement is approximately 30%. What is the most appropriate initial management?

• A: Immediate curettage and bone grafting
• B: Prophylactic internal fixation
• C: Serial radiographic observation
• D: Biopsy to rule out malignancy
• E: Radiation therapy

Explanation:

Correct Answer: C

For asymptomatic non-ossifying fibromas (NOFs) that are less than 50% of the cortical diameter and not associated with an impending or actual pathological fracture, the most appropriate initial management is serial radiographic observation. NOFs are benign and often regress spontaneously. Surgical intervention (curettage, grafting) is reserved for larger lesions (typically >50% cortical involvement), symptomatic lesions, or those with pathological fracture risk. Biopsy is generally not needed if characteristic radiographic features are present in the appropriate age group. Radiation therapy is contraindicated for benign bone lesions.

Question 25:

A 14-year-old competitive soccer player sustains a pathological fracture through a 6 cm non-ossifying fibroma located in the distal femoral metaphysis. The lesion involves approximately 60% of the cortical circumference. After initial immobilization, what is the most appropriate next step in management?

• A: Continued conservative management with cast immobilization until union
• B: Open biopsy followed by aggressive chemotherapy
• C: Intralesional steroid injection
• D: Curettage and bone grafting, with or without internal fixation
• E: Amputation

Explanation:

Correct Answer: D

A pathological fracture through a large non-ossifying fibroma (NOF), especially one involving more than 50% of the cortex, is a clear indication for surgical intervention. While conservative management might be considered for small, non-displaced fractures through very small lesions, a 6 cm lesion with 60% cortical involvement and a fracture warrants curettage and bone grafting. Internal fixation may be added to provide stability and protect the construct, especially in a young, active individual. Chemotherapy and amputation are inappropriate for a benign lesion. Intralesional steroids are used for other benign lesions like unicameral bone cysts, not NOF.

Question 26:

Which of the following is crucial to include in the differential diagnosis of a large, expansile, lytic metaphyseal lesion in a child, alongside NOF, particularly if fluid-fluid levels are noted on MRI?

• A: Osteomyelitis
• B: Ewing's Sarcoma
• C: Aneurysmal Bone Cyst (ABC)
• D: Enchondroma
• E: Chordoma

Explanation:

Correct Answer: C

Aneurysmal Bone Cyst (ABC) is a critical differential diagnosis for a large, expansile, lytic metaphyseal lesion, especially in children and if fluid-fluid levels are present on MRI, as these are highly characteristic of ABCs. While other options like Ewing's sarcoma are important for malignant lesions, and enchondromas for cartilaginous lesions, ABC mimics NOF's lucent, expansile nature and shares the pediatric age group. Osteomyelitis can be lytic but usually has other signs of infection. Chordoma typically affects the axial skeleton.

Question 27:

What is the most common anatomical location for a Non-Ossifying Fibroma?

• A: Vertebral body
• B: Diaphysis of long bones
• C: Epiphysis of long bones
• D: Metaphysis of long bones
• E: Small bones of the hand and foot

Explanation:

Correct Answer: D

NOFs almost exclusively occur in the metaphysis of long bones, with the distal femur, proximal tibia, and distal tibia being the most frequently affected sites. They originate in the cortex and grow into the medullary cavity.

Question 28:

What is the primary differentiating feature between a Fibrous Cortical Defect (FCD) and a Non-Ossifying Fibroma (NOF)?

• A: FCDs are always symptomatic, while NOFs are asymptomatic.
• B: NOFs exhibit malignant transformation, FCDs do not.
• C: FCDs are typically smaller and purely cortical, while NOFs are larger and extend into the medullary cavity.
• D: FCDs occur in adults, NOFs in children.
• E: They have completely different histological appearances.

Explanation:

Correct Answer: C

FCDs and NOFs are considered to be part of the same spectrum of fibrous lesions. The primary distinction is size and medullary extension: FCDs are smaller (<2-3 cm) and primarily cortical, whereas NOFs are larger and typically extend into the medullary cavity, often representing a later stage of FCD. Histologically, they are identical. Both are benign and typically asymptomatic, occurring in children.

Question 29:

Which of the following statements regarding the malignant transformation of Non-Ossifying Fibroma is true?

• A: It frequently transforms into low-grade osteosarcoma.
• B: It has a high risk of transforming into fibrosarcoma.
• C: Malignant transformation has been well-documented in multiple cases.
• D: Malignant transformation is extremely rare, with only anecdotal reports, if any, being generally unconvincing.
• E: It transforms into Ewing's sarcoma in 5-10% of cases.

Explanation:

Correct Answer: D

Malignant transformation of a Non-Ossifying Fibroma (NOF) is exceedingly rare, to the point where most authorities consider it practically non-existent or, at best, based on highly controversial or anecdotal reports lacking definitive proof. There is no established risk of malignant transformation into osteosarcoma, fibrosarcoma, or Ewing's sarcoma. This lack of malignant potential is a key feature in its benign nature.

Question 30:

Which statement regarding the cortical involvement of Non-Ossifying Fibroma is most accurate in predicting pathological fracture risk?

• A: Any cortical involvement, regardless of percentage, mandates prophylactic surgery.
• B: Fracture risk is directly proportional to the lesion's length along the bone, not its cortical involvement.
• C: Lesions involving more than 50% of the cortical diameter are considered high risk for pathological fracture.
• D: Cortical thickening is a sign of impending fracture.
• E: Fracture risk is only present if the lesion is located in the diaphysis.

Explanation:

Correct Answer: C

The most widely accepted criterion for increased pathological fracture risk in NOF is involvement of more than 50% of the cortical diameter. This significantly weakens the bone. Cortical thinning is more common than thickening. Length along the bone is less critical than cross-sectional cortical involvement. NOFs are metaphyseal, not diaphyseal, and fracture risk is directly related to structural weakening.

✖

⚠️ Unanswered Questions

You have unanswered questions in this module. Are you sure you want to submit?

Return Submit Exam

ABOS

ABOS Part I Comprehensive Review - Batch 98

Item 0 of 30

Study Mode Exam Mode

Score: 0%

Complete Module & Score

Module Results

0%

Overall Score

0

Correct

0

Incorrect

Review Incorrect Retake Module