Incidental finding of an (11)C-choline PET-positive solitary plasmacytoma lesion.

A 72-year-old man with monoclonal gammopathy of unknown significance presented with pain at the left hip level 4 years after radical prostatectomy for a malignant tumour. At the time of diagnosis of the primary tumour, the PSA level had increased to 7.4 ng/ml. Biopsy revealed a Gleason score of 4+3 and T2aN0M0 disease. Following radical prostatectomy (pathological stage pT3c, 1/15 positive lymph nodes, perineural infiltration), androgen deprivation was started promptly. At the onset of pain at the left hip level, PSA, MRI, bone scintigraphy and 11Ccholine PET/CT were performed owing to suspicion of recurrence of prostate cancer [1–4]. The anatomical imaging features (osteolytic lesion at the left ilium on MRI, negative total bone scintigraphy) and the low PSA level (<0.4 ng/ml) [5] were not suggestive for metastatic spread of the prostate tumour. 11C-choline PET/CT showed pathological uptake at the left ilium (a,b), confirming the neoplastic nature of the osteolytic lesion documented by MRI. Histological evaluation revealed the presence of moderately differentiated plasmacytoma cells, while bone marrow biopsy showed no signs of abnormal plasma cell infiltration. The presence of a solitary bone lesion associated with monoclonal plasma cell infiltration on tissue biopsy led to the diagnosis of solitary plasmacytoma of the bone. Local radiation therapy was initiated promptly (total radiation dose 56 Gy), and androgen ablation was continued. Although 11C-choline PET is increasingly used when recurrence of prostate cancer is suspected, positive findings should be interpreted with caution since choline-avid bone lesions of different origin may be present, for which treatment options and clinical management are very different