Calcified Chondroid Mesenchymal Neoplasm of the Temporomandibular Joint Region: Three Cases Illustrating Recent Developments and Diagnostic Pitfalls.

Calcified chondroid mesenchymal neoplasm (CCMN) is a recently described and evolving entity that predominantly arises in the distal extremities andtemporomandibular joint (TMJ) region. CCMN is characterized by FN1 and PDGFRA gene fusions. In the TMJ region, most cases previously described as tophaceous pseudogout (i.e., mass-forming calcium pyrophosphate deposition disease) are now being increasingly recognized as CCMN. There is considerable morphologic overlap between CCMN and tophaceous pseudogout (TPG), and the detection of FN1 and PDGFRA gene rearrangements is critical in distinguishing CCMN from TPG. We aim to identify, from our archival records, cases of CCMN that were previously diagnosed as TPG. We retrospectively reviewed clinical history and histopathology slides of three cases of TMJ region masses initially diagnosed as TPG, and submitted all three cases for targeted RNA sequencing. Case 1 harbored a canonical FN1::FGFR2 fusion. Case 2 failed targeted RNA sequencing. Case 3 harbored a canonical PDGFRA::USP8 fusion. None of the three cases had clinical history of pseudogout. All three cases were subsequently reclassified as CCMN on review. CCMN should be considered as a differential diagnosis in first presentations of 'tophaceous pseudogout' associated with chondroid metaplasia, with targeted molecular testing done as part of the diagnostic process.