Proposed genitalia contouring guidelines in anal cancer intensity-modulated radiotherapy

Objective: Intensity-modulated radiotherapy (IMRT) for anal canal carcinoma (ACC) is associated with favourable toxicity outcomes. Side effects include sexual dysfunction, skin desquamation, pain and fibrosis to perineum and genitalia region. The genitalia are situated anterior to the primary ACC between two inguinal regions providing a challenging structure to avoid. Techniques improving outcomes require robust, consistent genitalia contouring to ensure standardization and production of fully optimized IMRT plans. Official recommendations for genitalia contouring are lacking. We describe a potential genitalia contouring atlas for ACC radiotherapy. Methods: Following a review of genitalia CT anatomy, a contouring atlas was generated for male and female patients positioned prone and supine. Particular attention was paid to the reproducibility of the genitalia contour in all planes. Results: Male and female genitalia positioned prone and supine are described and represented visually through a contouring atlas. Contoured areas in males include penis and scrotum, and in females include clitoris, labia majora and minora. The muscles, bone, prostate, vagina, cervix and uterus should be excluded. The genitalia contour extends laterally to inguinal creases and includes areas of fat and skin anterior to the symphysis pubis for both genders. Conclusion: This atlas provides descriptive and visual guidance enabling more consistent genitalia delineation for both genders when prone and supine. The atlas can be used for other sites requiring radiotherapy planning. Advances in knowledge: This atlas presents visual contouring guidance for genitalia in ACC radiotherapy for the first time. Contouring methods provide reproducible genitalia contours that allow the provision of accurate dose toxicity data in future studies.