Two cases of parosteal osteosarcoma with unusual histological and molecular features.

Osteosarcomas arising from the cortical surface of bone can be subdivided into parosteal, periosteal, and high grade surface osteosarcomas, with parosteal osteosarcomas being the most well differentiated.1Hang J.F. Chen P.C.H.C. Parosteal steosarcoma.Arch Pathol Lab Med. 2014; 138: 694-699Crossref PubMed Scopus (36) Google Scholar Clinically, parosteal osteosarcomas show female predominance and generally occur in the third decade of life, with the distal femur, proximal tibia, and proximal humerus being the common sites of origin.2Okada K. Frassica F.J. Sim F.H. et al.Parosteal osteosarcoma: a clinicopathological study.J Bone Joint Surg Am. 1994; 76: 366-378Crossref PubMed Scopus (176) Google Scholar In general, they are low grade tumours with good prognosis.3Laitnen M. Parry M. Albergo J.I. et al.The prognostic and therapeutic factors which influence the oncological outcome of parosteal osteosarcoma.Bone Joint J. 2015; 97-B: 1698-1703Crossref PubMed Scopus (18) Google Scholar Radiologically, they appear as a lobulated mass with a broad base of attachment to the bone, and irregular mineralisation1Hang J.F. Chen P.C.H.C. Parosteal steosarcoma.Arch Pathol Lab Med. 2014; 138: 694-699Crossref PubMed Scopus (36) Google Scholar; a cleavage plane between the tumour and the bone is sometimes noted.4Jelinik J.S. Murphey M.D. Kransdorf M.J. et al.Parosteal osteosarcoma: value of MR imaging and CT in the prediction of histologic grade.Radiology. 1996; 201: 837-842Crossref PubMed Scopus (66) Google Scholar On gross examination, they appear as ossified masses, which may have a cartilaginous cap; soft areas representing dedifferentiation may be present.1Hang J.F. Chen P.C.H.C. Parosteal steosarcoma.Arch Pathol Lab Med. 2014; 138: 694-699Crossref PubMed Scopus (36) Google Scholar Classically, they are described as having a biphasic histological appearance with parallel trabeculae of woven bone, with intervening spindle cells showing moderate atypia and collagenous stroma; this is the so called ‘streamer pattern’. Occasionally, other histological patterns are encountered, which may resemble desmoplastic fibroma or fibrous dysplasia.1Hang J.F. Chen P.C.H.C. Parosteal steosarcoma.Arch Pathol Lab Med. 2014; 138: 694-699Crossref PubMed Scopus (36) Google Scholar Dedifferentiation (presence of high grade areas arising from a classic parosteal osteosarcoma) may occur, and is associated with a poorer prognosis.5Ruengwanichayakun P. Gambarotti M. Frisoni T. et al.Parosteal osteosarcoma: a monocentric retrospective analysis of 195 patients.Hum Pathol. 2019; 91: 11-18Crossref PubMed Scopus (9) Google Scholar Molecular studies show parosteal osteosarcomas to be associated with amplification of Cyclin dependent kinase 4 (CDK4) and Mouse double minute 2 (MDM2), with 87–100% of cases positive for either (or both) of these markers by immunohistochemical staining in two studies,6Yoshida A. Ushiku T. Motoi T. et al.Immunohistochemical analysis of MDM2 and CDK4 distinguishes low-grade osteosarcoma from benign mimics.Mod Pathol. 2010; 23: 1279-1288Crossref PubMed Scopus (111) Google Scholar,7Dujardin F. Binh M.B. Bouvier C. et al.MDM2 and CDK4 immunohistochemistry is a valuable tool in the differential diagnosis of low-grade osteosarcomas and other primary fibro-osseous lesions of the bone.Mod Pathol. 2011; 24: 624-637Crossref PubMed Scopus (129) Google Scholar and amplification of MDM2 by fluorescent in situ hybridization (FISH) seen in 83% of cases examined in a 2012 work.8Duhamel L.A.E. Ye H. Halai D. et al.Frequency of mouse double minute 2 (MDM2) and mouse double minute 4 (MDM4) amplification in parosteal and conventional osteosarcoma subtypes.Histopathology. 2012; 60: 357-359Crossref PubMed Scopus (46) Google Scholar As such, cases which present without the classical histological pattern or without typical molecular changes may mimic benign lesions and present a diagnostic challenge. We present two cases of parosteal osteosarcoma with atypical histological features diagnosed recently in our institution, and expand the morphological spectrum of parosteal osteosarcoma. The first case is that of a 21-year-old Chinese female, with no relevant past medical history, who presented with a 2 year history of posterior left knee pain, swelling, and limited range of motion. She had tried acupuncture to no effect, and underwent magnetic resonance imaging (MRI) in a neighbouring country, revealing a 3.1 cm lobulated mass apparently arising from the left posterior knee joint. She presented to our institution seeking a second opinion where plain radiographs were performed, demonstrating a bony lesion with circumferential sclerosis arising from the posterior aspect of the left lower femur, with ill-defined cortical margins and no association with any soft tissue mass. Radiological differential diagnoses considered were chondromyxoid tumour and parosteal osteosarcoma (Fig. 1A,B). Computed tomography scan of the leg revealed a 2.8 cm exophytic lesion at the posterior aspect of left lower femur with a rim of ossification. At this point the differential diagnoses considered by the clinical team included osteochondroma, parosteal osteosarcoma, chondromyxoid tumour, or chondrosarcoma. She underwent en bloc resection of the mass, and the post-operative working diagnosis was osteochondroma. Examination of the resected specimen revealed a 3.6 cm bony mass, with firm, rough, bony tissue intermixed with softer, white, fibrous areas (Fig. 1C,D). Microscopic examination showed the centre of the lesion to resemble myositis ossificans and consist primarily of plump spindled to stellate cells with vesicular chromatin and small nucleoli, mild to moderate atypia, and occasional mitotic figures, set in abundant collagenous stroma (Fig. 1E–G). Irregular woven bone was noted primarily at the periphery of the lesion, and a rim of reactive bone was noted at the edge of the specimen (Fig. 1E–G). Nodules of hyaline cartilage were seen within the lesion. Based on the histomorphology, a diagnosis of myositis ossificans was first considered. Further immunohistochemical studies showed focal positivity for desmin, smooth muscle actin, and special AT-rich sequence binding protein2 (SATB2) in the lesional cells (Fig. 1H), with Ki-67 proliferative index of 5% (Fig. 1I). Given the location and clinical concern for parosteal osteosarcoma, the specimen was sent for FISH studies, which showed amplification of MDM2 (Fig. 1J). Given the discrepancy between the morphological and molecular findings, the case was sent to an overseas expert for second opinion, and a final diagnosis of parosteal osteosarcoma, grade I, was made. The patient has had no evidence of recurrent disease 11 months after resection. The second case is a 45-year-old Chinese female, with a past medical history of hypertension, hyperlipidaemia, and benign uterine leiomyomata who presented to a general practitioner with a 3 month history of distal left humerus swelling. Clinical examination revealed a 3.0 cm bony swelling at the left distal humerus that was not tethered to the skin. Plain radiographs were obtained, which demonstrated a 2.5 cm osseous lesion at the posterolateral margin of the supracondylar region of the distal humerus (Fig. 2A,B). She was referred to our centre and MRI revealed a parosteal mass in the same location, with surrounding cortical and soft tissue oedema. The radiological differential diagnoses included florid reactive periostitis, bizarre periosteal osteochondromatous proliferation, myositis ossificans traumatic, periosteal osteosarcoma, and parosteal osteosarcoma. En bloc resection of the lesion with wide margins was performed. Gross examination of the specimen revealed a white, partially encapsulated bony nodule which measured 2.8 cm in maximum dimension (Fig. 2C,D). Microscopic examination revealed the lesion to have a nodular architecture with a surface composed of cartilage merging with underlying short, irregular trabeculae of blue to purple appearing woven bone, surrounded by plump spindle cells with oval nuclei, fine chromatin, and small nucleoli, showing mild atypia and occasional mitoses (Fig. 2E–I). These spindle cells infiltrated the cortex and percolated through the Haversian systems (Fig. 2E–G). Based on this histological appearance, the lesion was provisionally diagnosed as a bizarre parosteal osteochondromatous proliferation, but was sent for MDM2 amplification analysis by FISH, and for an external consultation. The FISH study showed no amplification of MDM2 (Fig. 2J), and immunohistochemical studies in the external consultant's laboratory showed focal positivity for CDK4 and negativity for MDM2. Despite the lack of MDM2 amplification, and non-specific immunohistochemical profile, a final diagnosis of grade I parosteal osteosarcoma was made based on the cellular atypia, and the infiltrative growth pattern; the external consultant agreed with this interpretation. The patient has been well with no recurrence 29 months after resection. Parosteal osteosarcoma is one of the more frequently misdiagnosed osteosarcomas,9Grimer R.J. Carter S.R. Spooner D. Sneath R.S. Diagnosing musculoskeletal tumours.Sarcoma. 2001; 5: 89-94Crossref PubMed Scopus (16) Google Scholar especially when the classic ‘streamer’ histological pattern is absent. It is unlikely to be misdiagnosed as a periosteal, or high grade surface osteosarcoma, as these higher grade malignancies show characteristic radiological patterns, and histologically would show much more marked atypia.1Hang J.F. Chen P.C.H.C. Parosteal steosarcoma.Arch Pathol Lab Med. 2014; 138: 694-699Crossref PubMed Scopus (36) Google Scholar However, if there is a dilemma between diagnosing a parosteal osteosarcoma with dedifferentiation and a conventional, high grade osteosarcoma, use of MDM2 FISH can be useful, as MDM2 amplification tends to be retained in dedifferentiated parosteal osteosarcoma, and is not commonly seen in other high grade osteosarcomas.8Duhamel L.A.E. Ye H. Halai D. et al.Frequency of mouse double minute 2 (MDM2) and mouse double minute 4 (MDM4) amplification in parosteal and conventional osteosarcoma subtypes.Histopathology. 2012; 60: 357-359Crossref PubMed Scopus (46) Google Scholar,10Yoshida A. Ushiku T. Motoi T. et al.MDM2 and CDK4 immunohistochemical coexpression in high-grade osteosarcoma: correlation with a dedifferentiated subtype.Am J Surg Pathol. 2012; 36: 423-431Crossref PubMed Scopus (58) Google Scholar,11Wunder J.S. Eppert K. Burrow S.R. et al.Co-amplification and overexpression of CDK4, SAS and MDM2 occurs frequently in human parosteal osteosarcomas.Oncogene. 1999; 18: 783-788Crossref PubMed Scopus (127) Google Scholar Confusion between parosteal osteosarcoma and other benign or reactive lesions is more likely, as they all represent lesions with only mild atypia. Both osteochondroma and parosteal osteosarcoma may show cartilaginous differentiation, but the cartilage seen in osteochondroma shows perpendicular columns of chondrocytes with intact polarity, with superficial clusters of chondrocytes in the superficial aspect, more mature chondrocytes at the base, and endochondral ossification. Furthermore, no interosseous spindle cell proliferation should be seen. Parosteal osteosarcoma may also be mistaken for myositis ossificans, as both lesions show myofibroblast-like cells intermixed with bony trabaculae; however, myositis ossificans should show a ‘zonation’ pattern with bony trabeculae appearing more mature at the periphery of the lesion.1Hang J.F. Chen P.C.H.C. Parosteal steosarcoma.Arch Pathol Lab Med. 2014; 138: 694-699Crossref PubMed Scopus (36) Google Scholar This was not observed in our first case, raising the suspicion that the lesion was in fact a parosteal osteosarcoma. Fibrous dysplasia is another mimic of parosteal osteosarcoma, as it may present as an exophytic mass and have similar histomorphological features, but fibrous dysplasia should show an intramedullary component.12Dorfman H.D. Ishida T. Tsuneyoshi M. Exophytic variant of fibrous dysplasia (fibrous dysplasia protuberans).Hum Pathol. 1994; 25: 1234-1237Crossref PubMed Scopus (31) Google Scholar Finally, bizarre parosteal osteochondromatous proliferation (BPOP, Nora's lesion) may present as an exophytic bony lesion, which shows proliferating spindle cells, atypical-appearing chondrocytes, and endchondral ossification with ‘blue’ or ‘purple’ bony trabeculae, however, it is usually seen in the bones of the hands and the feet, and the spindle cells usually do not show nuclear hyperchromasia or cellular atypia.1Hang J.F. Chen P.C.H.C. Parosteal steosarcoma.Arch Pathol Lab Med. 2014; 138: 694-699Crossref PubMed Scopus (36) Google Scholar The presence of atypia, and infiltration of malignant cells into the cortex seen in the second case, raised the possibility that the lesion was actually a parosteal osteosarcoma. Examining CDK4 or MDM2 expression by immunohistochemistry, or alternatively, performing FISH or polymerase chain reaction (PCR) studies for MDM2 amplification would be helpful to differentiate these benign mimics from parosteal osteosarcoma, as these markers appear to be quite specific for low-grade osteosarcoma in this context. In one study immunohistochemical stains for CDK4 and MDM2 were performed on 23 low grade osteosarcomas and 40 benign mimics; all of the osteosarcomas showed expression of either or both of these markers, whereas only one case of a benign mimic (a bizarre parosteal osteochondromatous proliferation) showed weak staining for CDK4 and MDM2.6Yoshida A. Ushiku T. Motoi T. et al.Immunohistochemical analysis of MDM2 and CDK4 distinguishes low-grade osteosarcoma from benign mimics.Mod Pathol. 2010; 23: 1279-1288Crossref PubMed Scopus (111) Google Scholar In another study no expression of CDK4 and MDM2 was seen by immunohistochemistry in 107 benign fibro-osseous lesions but expression of either or both of these markers was seen in 89% of the low grade osteosarcomas studied.7Dujardin F. Binh M.B. Bouvier C. et al.MDM2 and CDK4 immunohistochemistry is a valuable tool in the differential diagnosis of low-grade osteosarcomas and other primary fibro-osseous lesions of the bone.Mod Pathol. 2011; 24: 624-637Crossref PubMed Scopus (129) Google Scholar Furthermore, no amplification of MDM2 by PCR or FISH was seen in the benign fibro-osseous lesions that the authors examined.7Dujardin F. Binh M.B. Bouvier C. et al.MDM2 and CDK4 immunohistochemistry is a valuable tool in the differential diagnosis of low-grade osteosarcomas and other primary fibro-osseous lesions of the bone.Mod Pathol. 2011; 24: 624-637Crossref PubMed Scopus (129) Google Scholar In our work, the positivity for MDM2 by FISH clinched the diagnosis in the first case, but the lack of positivity in the second required us to fall back to the morphology on routine haematoxylin and eosin stained sections for the final diagnosis. Even though parosteal osteosarcoma is considered to be a low grade lesion with good clinical outcomes, there is a need to differentiate it from benign lesions. If wide margins are not taken, there are relatively high rates of local recurrence; with multiple recurrences, dedifferentiation can occur, which drastically negatively affects overall survival.3Laitnen M. Parry M. Albergo J.I. et al.The prognostic and therapeutic factors which influence the oncological outcome of parosteal osteosarcoma.Bone Joint J. 2015; 97-B: 1698-1703Crossref PubMed Scopus (18) Google Scholar Fortunately, with careful histological examination, and proper use of ancillary techniques, the proper diagnosis should be reached in most instances, sparing patients these negative outcomes. The authors would like to thank Dr Andrew Rosenberg, University of Miami, Miller School of Medicine, for his review of the two cases presented herein. The authors state that there are no conflicts of interest to disclose.