Adaptive Radiation in Anal Cancer

The standard treatment for Human Papilloma Virus (HPV)-positive locally advanced anal cancer (described as a tumor that is greater than 4 cm in size, or with positive lymph nodes) is 54 Gy of radiation treatment to the anal canal and primary tumor planning total volume (PTV), 50.4-54 Gy to positive nodal PTV and 45 Gy to elective lymph node PTV with 5-fluorouracil (5-FU) and mitomycin-C chemotherapy administered at the same time as radiation in 30 fraction (treatments) delivery. During the six week course of radiotherapy, there is often a notable decrease in volume of the tumor (both primary and regional nodes), as early as one week into treatment, detected on weekly on-board Cone Beam Computed Tomography (CBCT), which is a scan done on the treatment machine while patients receive radiation to ensure that the tumor is being treated and normal tissue is not. However, CT simulation (a CT scan used to plan radiation treatment) and re-planning of the treatment to account for the tumor shrinkage are not routinely performed due to time, patient inconvenience and staffing resources. As such, daily adaptive radiation, which can generate a new CT-based plan using the anatomy of the day, may be a time efficient method to both plan and treat the patient.