Pitfalls in bone density monitoring in prostate cancer during anti-resorptive treatment

A 74-year-old man presented to the Andrology Clinic for management of potential complications of androgen deprivation therapy for prostate cancer. He had a rising prostate-specific antigen with a concurrent rise in alkaline phosphatase and bone remodeling markers. This was despite treatment with a radical prostatectomy, androgen deprivation, and anti-resorptive therapy. A follow-up dual-energy X-ray absorptiometry scan revealed a marked increase in his bone mineral density at both the lumbar spine and femoral neck. This increase, especially in the context of rising bone remodeling markers, was unlikely due to the effect of anti-resorptive therapy alone. Subsequent whole-body bone scintigraphy demonstrated a “superscan” phenomenon which is characterized by uniform and avid tracer retention throughout the skeleton, in this case due to widespread skeletal metastasis, so that the usual physiological uptake in the kidneys is no longer observed and can be misinterpreted as a “normal” scan if the absence of the kidneys is not recognized. This case highlights the importance of considering diffuse metastatic disease when there is a rapid increase in bone mineral density, even in individuals treated with anti-resorptive therapy.