Retroperitoneal extraskeletal osteosarcoma with cystic change arising from heterotopic ossification: a rare entity

Kiran Agarwal 1 , Savita Agarwal* 1 , Lokesh Kumar 2 1 Department of Pathology, Lady Hardinge Medical College and Smt S.K. Hospital, New Delhi, India 2 Department of Radiodiagnosis, FH Medial College, Tundla, Firozabad, UP, India Extraskeletal osteosarcoma arising from myositis ossificans or heterotopic benign ossification is an extremely rare entity. Heterotopic ossification under rare circumstances may undergo malignant transformation and give rise to extraskeletal osteosarcoma however diagnosis of such transformation requires the presence of a remnant of the benign precursor lesion. Most of the cases reported as secondary extraskeletal osteosarcoma did not contain residual tissue of the previous benign condition and its presence was suggested solely on the basis of clinical course without any histopathological or radiological evidences. We describe a case of secondary extraskeletal osteosarcoma of retroperitoneum showing morphologic evidence of heterotopic benign ossification with extensive cystic change in an eightyfive year old female. C-43 Retroperitoneal extraskeletal osteosarcoma Annals of Pathology and Laboratory Medicine, Vol. 02, No. 01, Jan-Mar 2015 Introduction Extraskeletal osteosarcoma is a malignant mesenchymal tumor of soft tissues without attachment to the bone or periosteum, composed of neoplastic cells that recapitulate the phenotype of osteoblasts and synthesize bone. Extraskeletal osteosarcoma arising from myositis ossificans or heterotopic benign ossification is an extremely rare entity. Heterotopic ossification is a benign process that can occur after an operation, a neurological injury, poliomyelitis, thermal or electrical burn [1] and rarely it may develop spontaneously without prior history of trauma [2, 3] . It frequently occurs in tendons, fasciae, skeletal muscle, subcutaneous fat, and organs like kidney or breast. Under rare circumstances it may undergo malignant transformation and give rise to extraskeletal osteosarcoma and diagnosis of such transformation requires the presence of a remnant of the benign precursor lesion. Most of the cases reported as secondary extraskeletal osteosarcoma did not contain residual tissue of the previous benign condition and its presence was suggested solely on the basis of clinical course without any histopathological or radiological evidences [4,5,6,7] . We describe a case of secondary extraskeletal osteosarcoma of retroperitoneum showing morphologic evidence of heterotopic benign ossification with extensive cystic change in an eighty five year old female. Case Report An Asian female patient in her ninth decade presented with gradually increasing mass in abdomen of two year duration. There were no complains pertaining to bladder or bowel habits. There was no history of previous trauma, surgery or radiotherapy. Per abdominal examination revealed a non tender firm mass in upper abdomen measuring 14x10cms. There was no ascites or any other palpable mass. Patient’s biochemical tests were within normal limits except for mildly elevated S. Alkaline Phosphatase levels. CECT revealed a predominantly cystic mass with extensive curvilinear calcification in the anterior wall. Mass was displacing adjacent organs and was separate from adjacent bony structures. Right kidney was seen separate from the mass and appeared morphologically normal [Fig.1]. Mass was abutting left kidney but not arising from it [Fig. 2]. Based on these findings a radiologic diagnosis of isolated hydatid cyst was suggested and thus FNAC was not performed. On laprotomy mass was lying in retroperitoneum with splaying of pancreas over anterior surface. It was adhered to spleen, pancreas and gall bladder but not infiltrating into these organs. Mass with adhered spleen, body of pancreas and gall bladder was resected. Grossly it was a cystic mass measuring 17x17x7cms, filled with hemorrhagic necrotic material adhered to its inner wall. Cyst wall thickness varied from 0.5 to 2.5cms with few gritty areas in its anterior wall [Fig. 3]. Fig 1: Axial section of delayed phase CT showing hypodense round lesion with rim calcification on left side of abdomen. Right kidney is also vizualised in this section in right renal fossa with excreted contrast material in the calyceal system. Fig 2: Reformatted sagittal CT image showing hypodense lesion abutting left kidney. The lesion is causing mass effect on left kidney but clearly not arising from it. Microscopic sections from cyst wall revealed thick fibrocollagenous tissue with adhered hemorrhagic necrotic material on its inner side. Rest of the cyst wall showed varied morphology with proliferation of