Aims The histological distinction between gastrointestinal smooth muscle neoplasms (SMNs) and their differential diagnoses, especially gastrointestinal stromal tumours (GISTs), has important clinical management implications. This study aimed to investigate preliminary data suggesting that loss of the α5 chain of collagen IV (α5(IV)), placental acid phosphatase (PLAP) expression and smoothelin expression can be used diagnostically as markers of gastrointestinal SMNs. To aid this investigation, these potential markers were directly compared against caldesmon.

Methods 31 SMNs and 111 potential differential diagnoses (16 different neoplasm types) were immunostained for caldesmon, PLAP and smoothelin.

Results A pilot study indicated that loss of α5(IV) positivity was neither a specific nor sensitive marker of SMN. Caldesmon, PLAP and smoothelin were expressed by all 31 SMNs though leiomyosarcomas showed some loss of staining proportion and/or intensity. Caldesmon positivity was commonly shown by GISTs, glomus tumours and angiomyolipomas. Cytoplasmic smoothelin positivity was commonly shown by glomus tumours and angiomyolipomas, whereas PLAP positivity was shown by one desmoplastic small round cell tumour studied and only infrequently shown by angiomyolipomas. Nuclear smoothelin positivity was seen among four leiomyosarcomas but also a wide range of non-SMNs.

Conclusions α5(IV) immunohistochemistry with the A7 antibody is not a diagnostically useful marker of gastrointestinal SMNs. Both PLAP and smoothelin (cytoplasmic expression only) are as sensitive as but are more specific than caldesmon as such a marker. Further, PLAP and smoothelin immunostainings are technically reliable and can be reproducibly assessed.