ECP 2023 Abstracts

S357 Virchows Archiv (2023) 483 (Suppl 1):S1–S391 13 Methods: A 66-year-old man had complained bilateral knee pain which was diagnosed as osteoarthritis and treated conservatively for ten years. One month prior to surgery, he had buckling and bilateral leg numbness. He had a rupture of Achilles tendon and urolithiasis. Results: Physical exam revealed a black pigmentation in the conjunc- tiva, posterior auricle and lip. The urine was coloured black and urine homogentisic acid was increased. Radiology revealed osteoporosis with vertebral disc narrowing. Based on the imaging finding, his symptom was supposed to be due to the instability at T10/11 level and surgery was performed for posterior decompression and posterior fixation of kyphosis. Vertebral bone was sampled for histological evaluation. Histologically, brown pigment was deposited in the bone fragments, which was coloured by methylene blue stain. Molecular study revealed homogentisic acid oxidase mutation. Conclusion: Ochronosis is caused by the accumulation of a homogen- tisic acid in connective tissue, and the endogenous type is called alkap- tonuaria, which is an autosomal recessive disease. Histological exami- nation of vertebral ochronosis is rare and our case will reinforce the understanding of ochronosis. E-PS-22-019 Clear cell sarcoma of the knee - an unusual presentation masquer- ading benign bursitis J. Ichikawa*, T. Kawasaki, H. Imada, S. Kanno, T. Torigoe, Y. Yazawa *Department of Orthopedic Surgery, University of Yamanashi, Japan Background & objectives: Clear cell sarcoma is a rare soft tissue sarcoma with melanocytic differentiation. It usually forms a well- demarcated, expansile mass, however, pericystic growth is uncom- mon. Herein, we report an unusual presentation of clear cell sarcoma mimicking bursitis. Methods: A 63-year-old woman had presented with left knee pain that worsened after turning over. Conservative therapy failed to improve her symptoms. An magnetic resonance imaging revealed a clinically intra- articular lesion and the tumour surrounded the joint of the left knee. It was low on MRI T1-weight imaging. Focal invasion of the posterior side of the femur was also observed. Results: Because the tumour was found to be malignant upon biopsy, it was surgically resected. Gross examination of the resected specimen showed peri-capsular/peri-articular extension of the whitish tumour with focal intramedullary invasion of the femur. Histologically, the tumour cells had round nuclei with prominent nucleoli. While most of the synovial cells were intact, focal intra-articular extension was seen. Immunohistochemical analysis revealed positivity for MITF, HMB45, SOX10, and S100 protein. Fluorescence in situ hybridization demon- strated breakapart signals of EWSR1, consistent with clear cell sarcoma. Conclusion: Our case highlights the unique extension of clear cell sar- coma, which can be clinically challenging to diagnose and may masquer- ade as benign synovitis or tenosynovial giant cell tumour, diffuse type. The current case also expands the variation of clear cell sarcoma exten- sion, emphasizing the importance of careful consideration of differential diagnoses in patients with atypical presentation of this rare malignancy. E-PS-22-020 Spinal metastasis of well-differentiated liposarcoma component in retroperitoneal dedifferentiated liposarcoma treated by minimally invasive surgery J. Ichikawa*, T. Kawasaki, H. Imada, S. Kanno, T. Ohba, H. Haro *Department of Orthopedic Surgery, University of Yamanashi, Japan Background & objectives: Generally, well-differentiated liposarcoma (WDL) has the potential to become a dedifferentiated liposarcoma (DDL) but lacks metastatic potential. We present a rare case of spinal metastasis of a WDL component in retroperitoneal DDL treated by Minimally Invasive Surgery (MIS). Methods: A 65-year-old woman presented with low back pain and left sciatica. Magnetic Resonance Imaging showed a mass with high signal intensity on both T1 and T2-weighted images in L1. She had undergone resection of retroperitoneal DDL previously, with two recurrences, but had no metastasis except to L1. Therefore, we diagnosed L1 metastasis of the WDL component in DDL. Results: Therefore, we diagnosed the vertebral tumour as metastasis of the WDL component of DDL, and planned surgery to achieve symp- tomatic improvement. Tumour curettage and laminectomy followed by percutaneous pedicle screw fixation with CT navigation were per- formed. Histological examination in the primary retroperitoneal lesion showed mixed well-differentiated and dedifferentiated liposarcoma. Lipoblasts containing hyperchromatic nuclei were apparent in the well- differentiated area. On the other hand, there was a well-differentiated component in the spinal metastasis lesion. Positive immunohisto- chemical staining for MDM2 and CDK4 confirmed dedifferentiated liposarcoma. She could walk with neither pain nor palsy. However, the retroperitoneal mass enlarged and she died 1.5 years after surgery. Conclusion: We have described an extremely rare case of spinal metas- tasis of a well-differentiated liposarcoma component of retroperitoneal dedifferentiated liposarcoma. The clinical behaviour of WDL differed markedly between the extremities and retroperitoneum, as reflected by dedifferentiation and the high recurrence rate. Although surgical indications for metastasis are of course restricted, considering the slow growth of DDL and relatively long survival, MIS including percutane- ous pedicle screw fixation is a promising tool. E-PS-22-021 Atypical spindle cell/pleomorphic lipomatous tumour masquerad- ing as a myxoid tumour J. Ichikawa*, T. Kawasaki, H. Imada, S. Kanno, N. Taniguchi, T. Ashi- zawa, H. Haro *Department of Orthopedic Surgery, University of Yamanashi, Japan Background & objectives: Atypical spindle cell/pleomorphic lipo- matous tumours (ASPLT) can now be categorized and immunohis- tochemistry (IHC) and fluorescence in situ hybridization (FISH) are essential for accurate diagnosis. Herein, we report a case with ASPLT masquerading as a myxoid tumour. Methods: A 64-year-old man presented with a mass on his right but- tock. Magnetic resonance imaging (MRI) revealed a low signal on T1-weighted images with a few high signals suggesting the presence of intratumoral fat and a high signal on T2 short tau inversion recovery images suggesting the presence of myxoid components. Taken together, the imaging findings suggested myxoid liposarcoma. Results: Needle biopsy findings suggested myxoma, because there were few atypical spindle cells. Collectively, the preoperative find- ings indicated intramuscular myxoma. Marginal resection was thus performed. Macroscopically, the tumour consisted of mainly myxoid components with little fat tissue. Histopathologically, the tumour had an ill-defined border and most of the tumour cells were bland and short-spindled with a myxoid matrix, with some showing cytological atypia and pleomorphism. IHC indicated the tumour to be positive for CD34 and negative for retinoblastoma. FISH showed no amplifica- tion of MDM2 and no rearrangement of FUS or EWSR1. The final diagnosis was ASPLT. One year after surgery, the patient remains free of recurrence. Conclusion: We have described an atypical case of ASPLT and dif- ficulties with differential diagnosis of this tumour. Pathological findings and molecular confirmation including IHC and FISH are more useful than imaging for ASPLT because MRI findings are heterogeneous. Although wide resection is recommended, some cases, such as our case, have an inconclusive preoperative diagnosis. Since ASPLT is a benign tumour that does not metastasize, careful, long-term follow-up may benefit patients more than additional surgery.