When treating inguinal lymph nodes in the setting of pelvic RT, what is your preferred setup in order to minimize dose to the penis?

I have tried a variety of different setups but have found that building a scrotal shelf works the best. The patient is set up supine in the frog leg position using your immobilization method of choice. I then use a custom moldable headrest and place it against the perineum to serve as a shelf for the genitalia which helps to separate it from the perineum and out of high dose areas. I ensure that the penis is kept midline, pointing inferiorly, and out of the inguinal fields. I have found this setup to be easy to reproduce during treatment, and it helps prevent inadvertent radiation dose to the male genitalia.

We use a pair of disposable hospital undergarments and pull the scrotum and penis midline up onto the mons pubis. The patient then does this each day when he gets on the linear accelerator and the position is checked by the technologists. There can be some variation day to day with this approach, but in general, it helps to pull the genitalia away from the perianal area and into an area of lower dose. The patient gets a clean new pair each day for treatment provided by our department. 

We've been using proton therapy (IMPT) more recently for anal canal cancers, which has some benefits over IMRT. One is better marrow sparing which can help reduce cytopenias during concurrent chemo and this may help reduce the risk of hospitalizations (grade 3+ toxicity) according to the UPENN study of patients on concurrent chemo.

Another is better genital sparing when treating the inguinal nodes, as there is less dose into the labia, clitoris, testes, and penis with IMPT. When using a vaginal dilator during treatment delivery, it may be possible to spare the anterior vaginal wall better as well - something we've done with IMRT too. CBCT is needed to confirm the positioning of the dilator daily.

Another benefit is to pencil beam protons is sparing the small bowel and bladder when treating pelvic nodes, as there is no bridging or scattered dose across the midline, which really helps avoid the sigmoid too. Counterintuitively, using a nearly Empty bladder sometimes provides better OAR sparing than a Full bladder when treating this way with protons, because it avoids pushing the sigmoid up against the pre-sacral nodes and helps keep the lateral bladder edges away from the pelvic sidewall nodes.