Phosphaturic mesenchymal tumour mixed connective tissue variant: report of three cases with unusual histological findings.

Phosphaturic mesenchymal tumour mixed connective tissue variant (PMTMCT) is a rare tumour occurring in bone and soft tissue that usually behaves in a benign manner. Elaboration of biologically active substances by this tumour gives rise to a paraneoplastic syndrome known as oncogenic osteomalacia, manifesting clinically as bone pain, generalized weakness and pathological fractures. Recognition of PMTMCT and its associated syndrome is important, as resection of the tumour in most instances results in prompt resolution of symptoms. Previously reported cases of this tumour have emphasized the consistent presence of certain histological features that are considered prerequisite for making the diagnosis of PMTMCT. We describe three cases of PMTMCT, of which two first presented with progressive symptoms of osteomalacia and one remained clinically silent aside from the symptom of a palpable lump. Our cases highlight the wide-ranging histological patterns displayed by these tumours, and draw attention to certain microscopic findings that until now have been given little if any mention. Tentacular growth pattern and satellite nodules appear to be common findings in PMTMCTs, and can make complete surgical excision of these tumours challenging. The ability of this otherwise histologically benign tumour to permeate vascular spaces has to our knowledge never been described previously. One tumour lacked the characteristic calcifying matrix of PMTMCT, suggesting that in some tumours this defining feature may be focal if not entirely absent. PMTMCT shares features with and can resemble a variety of bone and soft tissue neoplasms, requiring the surgical pathologist to be familiar with this entity.