How would you manage prostate cancer in a patient who received prior chemoradiation followed by an APR for rectal cancer?

Patients who have been treated with chemoradiation followed by an APR for rectal cancer and now have prostate cancer and are not surgical candidates do have definitive treatment options despite the previous treatment. Although HDR brachytherapy is usually a great option for patients who have already had radiation to the pelvis, it is not an option in this situation. For patients who cannot be treated with brachytherapy, a technique that my colleagues at Fox Chase have extensive experience developing and using is pulsed low dose rate IMRT. This technique of pulsing each IMRT fraction into short bursts uniquely allows for repair of normal tissue (Paly et al., PMID 32769406). In our experience, we can maximally give up to 60 Gy in 2 Gy fractions. Depending on the stage of disease (PSA, Gleason score, and T stage), our threshold for using ADT, especially short-term, is lower since the dose is lower.

I agree with Dr. @Dr. First Last that salvage CyberKnife is a good option in this setting, and with Dr. @Dr. First Last that salvage pulsed low dose rate IMRT is intriguing, while HDR brachytherapy is considerably more difficult following APR. However, I have treated 2 patients with HDR brachytherapy in this setting, and a few more have been similarly managed by my partners, admittedly in a very busy brachytherapy practice. You could certainly consider taking your patient to the OR, placing him under spinal anesthesia, inserting brachytherapy catheters initially under fluoroscopic guidance, fully recognizing they will need subsequent adjustment, then him to CT where needle positioning can be optimized via CT. This is a bit cumbersome, but can be very effective.