A 70-year-old man with a history of localized prostate cancer treated with whole-pelvis radiation therapy with a boost to the prostate, in conjunction with androgen deprivation therapy 7 years prior, presented with lower back pain. Evaluation by his primary care physician led to a bone scan, which revealed an area of activity in the sacrum. The patient’s prostate-specific antigen (PSA) level has been stable for the last 3 years. He has no other areas of pain and is otherwise healthy.
The nuclear medicine bone scan image is shown (left).
Based on the radiographic appearance and clinical scenario, what is the most likely diagnosis?