Dedifferentiation of Metastatic Recurrent Chondrosarcoma of the Right Distal Femur Into High-Grade Sarcoma Favouring Leiomyosarcoma: A Report of a Rare Case With Review of Literature

Figures

• 

  Figure 1. Axial 18F-FDG PET-CT fusion image at the level of the distal femur (September 2023) Axial 18F-FDG PET-CT fusion image at the level of the distal femur (September 2023). The right thigh (left panel) demonstrates a heterogeneously FDG-avid intramedullary mass with multiple calcific foci and a markedly hypermetabolic peripheral rim (SUVmax: 12.05) surrounding a relatively photopenic central zone (SUVmax: 5.42), consistent with the characteristic metabolic heterogeneity of dedifferentiated chondrosarcoma: high FDG uptake in the dedifferentiated sarcomatous component and low uptake in the chondroid core. Loss of fat planes with adjacent musculature is noted. No distant metastatic disease was identified on this staging scan. The left thigh (right panel) is included for comparison. 18F-FDG PET-CT: 18F-fluorodeoxyglucose positron emission tomography-computed tomography, SUVmax: maximum standardised uptake value

• 

  Figure 2. Post-operative X-ray of the right lower limb (March 2025) Anteroposterior plain radiograph of the right lower limb (March 2025) showing the post-operative reconstruction following limb-salvage surgery performed in 2023. A modular distal femoral endoprosthetic replacement is in situ, comprising an intramedullary stem extending into the femoral diaphysis and a hinged total knee replacement component articulating with the proximal tibia. Tumour recurrence involved the entire right femur, including the region of the prosthesis, and extended into the proximal tibia. A pathological comminuted fracture of the proximal femoral shaft with posterosuperior displacement of the distal fragment and anterior angulation of the proximal prosthesis tip is evident. These findings, in conjunction with MRI-confirmed nodal metastases and the extent of soft tissue involvement, rendered further limb-preserving surgery technically unfeasible and necessitated palliative hip disarticulation. MRI: magnetic resonance imaging

• 

  Figure 3. Clinical timeline Clinical timeline illustrating the chronological sequence of events in the management of dedifferentiated chondrosarcoma with leiomyosarcomatous dedifferentiation. Events are displayed bilaterally along a central time axis. Key surgical milestones (primary limb-salvage surgery, 2023; palliative hip disarticulation, April 2025) are denoted by enlarged nodes. The disease-free interval of approximately two years between primary surgery and metastatic recurrence is marked on the central spine. DDCS: dedifferentiated chondrosarcoma, HPE: histopathological examination, IHC: immunohistochemistry, LMS: leiomyosarcoma, MRI: magnetic resonance imaging, PET-CT: positron emission tomography-computed tomography, SMA: smooth muscle actin, SUVmax: maximum standardised uptake value

• 

  Figure 4. H&E stain at 10× H&E stain at 10× magnification demonstrating a high-grade spindle cell neoplasm with scattered atypical cartilaginous islands (pale blue lacunar structures) embedded within a cellular sarcomatous background. The abrupt juxtaposition of chondroid and non-chondroid elements is characteristic of the biphasic morphology of dedifferentiated chondrosarcoma. H&E: haematoxylin and eosin

• 

  Figure 5. H&E at 40× H&E at 40× magnification displaying the high-grade spindle cell component at higher power. Tumour cells exhibit markedly enlarged, hyperchromatic, and pleomorphic nuclei with irregular nuclear contours. The surrounding matrix shows residual atypical cartilaginous differentiation, confirming the dedifferentiated nature of the neoplasm. H&E: haematoxylin and eosin

• 

  Figure 6. H&E at 40× H&E at 40× magnification illustrating the characteristic architectural pattern of the sarcomatous component, with tumour cells arranged in long, sweeping intersecting fascicles, a growth pattern resembling smooth muscle neoplasia and forming the morphological basis for the suspicion of leiomyosarcomatous differentiation. H&E: haematoxylin and eosin

• 

  Figure 7. IHC SMA at 40× IHC for SMA at 40× magnification showing strong and diffuse cytoplasmic positivity throughout the spindle cell population. This widespread SMA expression supports a myogenic phenotype and is a key marker supporting leiomyosarcomatous differentiation in this case. IHC: immunohistochemistry, SMA: smooth muscle actin

• 

  Figure 8. IHC SMA at 4× IHC for SMA at 4× magnification demonstrating the broad architectural distribution of SMA positivity across the entire tumour section, confirming the diffuse rather than focal nature of the myogenic marker expression observed at higher magnification. IHC: immunohistochemistry, SMA: smooth muscle actin

• 

  Figure 9. IHC caldesmon at 40× IHC for caldesmon at 40× magnification revealing focal and weak cytoplasmic staining within a subset of the spindle cell population. H-caldesmon is a highly specific marker for smooth muscle differentiation; its focal and weak reactivity in this case, alongside strong SMA positivity, is consistent with partially dedifferentiated leiomyosarcomatous histology. IHC: immunohistochemistry, SMA: smooth muscle actin

• 

  Figure 10. Ki-67: 60% Immunohistochemistry for Ki-67 demonstrating nuclear labelling in approximately 60% of tumour cells, confirming a high proliferative index commensurate with high-grade sarcoma. Elevated Ki-67 indices have been associated with shortened survival in dedifferentiated chondrosarcoma.