Final Diagnosis --Low-grade Central Osteosarcoma

FINAL DIAGNOSIS

Low-grade central osteosarcoma .

DISCUSSION

Low-grade central osteosarcomas comprise < 2% of all osteosarcomas, most commonly arise from the long bones, rarely from flat bones, and can invade adjacent soft tissue (1). Microscopically, they are formed of spindle cells that permeate the marrow fat and surrounding bony trabeculae while producing an osteogenic matrix along with well-formed trabeculae. They show minimal cytologic atypia and few mitotic figures. These lesions are characterized by amplification of chromosomal region 12q13-15, to include the MDM2 and CDK4 genes.

The differential diagnosis for a bland spindle cell lesion in this location includes fibrous dysplasia, meningioma, hemangiopericytoma/solitary fibrous tumor (HPC/SFT), and schwannoma. In contrast to low-grade central osteosarcoma, fibrous dysplasia typically lacks permeative growth and activating GNAS mutations are implicated in their development. Meningiomas have characteristic histologic features including whorls and psammoma bodies; SSTR2a is a highly sensitive marker for this entity. Bone invasion is occasionally encountered even in low grade meningioma. HPC/SFT is composed of randomly oriented spindle cells with distinct 'staghorn' blood vessels; they are characterized by the NAB2-STAT6 gene fusion and thus exhibit nuclear immunopositivity for STAT6.

While low-grade central osteosarcoma is a rare entity, particularly in this location, it is in the differential diagnosis of spindle cell dural-based tumors. Our patient had previously been misdiagnosed twice with meningioma. The second recurrence prompted further investigation, including review of previous pathology, additional stains not originally available (particularly SSTR2a and STAT6), and molecular studies including FISH, using material from the first resection specimen, which provided characterization of untreated tumor.

Evaluation of the MDM2 (12q15) locus was coupled with a chromosome 12 centromeric probe (CEP12) as the copy number control. The ETV6 (12p13.2) locus was also assessed. FISH studies were conducted on FFPE tissue sections as previously described.(2) Co-amplification (10-30 copies) of the MDM2 and CEP12 loci was observed in 95% of the 400 interphase nuclei analyzed (MDM2/CEP12 ratio of 1.1); a subpopulation of cells (~5%) exhibited slightly higher copies of MDM2 vs CEP12, however the MDM2/CEP12 ratio did not exceed 2.0. ETV6 was negative for amplification. The salient cytogenetic feature of parosteal and low-grade central osteosarcomas is supernumerary ring or giant rod-shaped chromosomes, which are mainly composed of amplified 12q13~q15 region to include most commonly MDM2 at 12q15 (3). Notably, well-differentiated and dedifferentiated liposarcomas (WDL/DDL) harbor similar genetic alterations. Co-amplification of MDM2 and centromere 12, as in our case, is an unusual FISH pattern for low-grade osteosarcoma, although it has been reported in a few cases of parosteal osteosarcoma (4). Moreover, while the presence of multiple alphoid 12 (alpha 12 satellite sequences) signals rather than centromere 12 signals was an interpretative consideration (multiple alphoid 12 signals have been reported in a small subset of WDL/DDL and osteosarcoma) (5), the signal intensity was supportive of the latter. Amplification of MDM2 can be seen in both low-grade and high-grade osteosarcomas, however, concurrent overrepresentation of 12p sequences is more common in high-grade (grade III-IV) osteosarcomas (5/6 cases) rather than grade I-II parosteal osteosarcomas (1/5 cases). Assessment of the current case for overrepresentation of ETV6 (12p13) by FISH was negative, however, NGS studies revealed amplification of other 12p loci to include KRAS (12p12.1), CCND2 (12p13.32), FGF23 (12p13.32), and FGF6 (12p13.32). Similar to our case, a previously reported grade II parosteal osteosarcoma with amplification of 12p sequences accompanying MDM2 amplification, demonstrated amplification of CCND2 and KRAS loci but not ETV6 [3].

Grading of central osteosarcoma remains unsettled; this case is probably best assigned to the WHO grade 2 category on the basis of mitotic activity and molecular findings. The corrected diagnosis provided justification for the clinical team to add radiation therapy to the treatment regimen; however, a subsequent third tumor recurrence was also treated surgically. The patient remains well 8 years after the initial tumor resection.

REFERENCES

1. Fletcher DM, Bridge JA, Hogendoorn PCW, Mertens F (Eds.) (2013) WHO classification of tumours of soft tissue and bone. 4th Ed: 281-282, IARC (Lyon).
2. Lyle PL, Bridge JA, Simpson JF, Cates JM, Sanders ME (2016) Liposarcomatous differentiation in malignant phyllodes tumors is unassociated with MDM2 or CDK4 amplification. Histopathology. 68(7):1040-5.
3. Sandberg AA & Bridge JA. Updates on the cytogenetics and molecular genetics of bone and soft tissue tumors: osteosarcoma and related tumors. Cancer Genet Cytogenet. 2003; 145(1):1-30.
4. Gisselsson D, Pälsson E, Hogland M, Domanski H, Mertens F, Pandis N, Sciot R, Dal Cin P, Bridge JA, Mandahl N (2002) Differentially amplified chromosome 12 sequences in low- and high-grade osteosarcomas. Genes Chromosomes Cancer, 33:133-140.
5. Kashima T, Halai D, Ye H, Hing S, Delaney D, Pollack R, O'Donnell P, Tirabosco R, Flanagan AM (2012) Sensitivity of MDM2 amplification and unexpected multiple faint alphoid 12 (alpha 12 satellite sequences) signals in atypical lipomatous tumor. Mod Pathol. 25(10):1384-96.

Contributed by Ashley R. Perkins, DO, MS; Robert J.B. Macaulay, MD; Marilyn M. Bui, MD, PhD; Julia A. Bridge, MD; Arnold B. Etame, MD, PhD