[A Case of Pelvic Lymph Node Recurrence Detected by PSMA PET/CT after Primary Proton Beam Radiotherapy for Prostate Cancer].

The patient is a 72-year-old man who was referred to our hospital with an elevated prostate specific antigen (PSA) level. He was diagnosed with prostate cancer (cT2aN0M0) at the age of 62 years. He had undergone radical proton beam radiotherapy. The PSA level decreased to a nadir of 0.217 ng/ml after 5 years, gradually increasing thereafter to 1.595 ng/ml during the next 5 years. Although magnetic resonance imaging of the prostate showed an abnormal signal area in the prostate, repeated biopsies of the prostate revealed no malignant findings. Contrast-enhanced abdominal computed tomography (CT), bone scintigraphy and fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET)/CT did not detect any abnormalities in the prostate or metastatic lesions. ¹⁸F-prostate specific membrane antigen (PSMA)-PET/CT showed no accumulation in the prostate, but some accumulation in a left obturator lymph node. Open pelvic lymph node dissection was performed, and pathological examination confirmed lymph node metastasis from the prostate cancer. The PSA level decreased from 2.482 ng/ml preoperatively to 0.391 ng/ml at 3 months postoperatively. PSMA-PET/CT might be useful for early localization of recurrent lesions in biochemical recurrence after radical treatment for prostate cancer.