1. Reducing excess imaging dose to cancer patients receiving radiotherapy Adam Schwertner, Justin Guan, Xiaofei Ying, Darrin Pelland, Ann Morris, Ryan Flynn Department of Radiation Oncology, University of Iowa Hospitals and Clinics a. b. Introduction Radiation therapy is used to treat patients with cancer by focusing the radiation energy on the tumor to damage its DNA and leave surrounding normal tissue relatively unharmed. The radiation is supplied over many days and before each treatment session many patients have a CT taken to ensure he/she is positioned correctly below the radiation source as seen in figure 1. Imaging radiation can add 5-10% to the radiation dose from the therapeutic beams. We propose to treat the tumor with the imaging beams, reducing excess dose due to imaging in normal tissue. This study explores the effects of a wide range of pre-treatment imaging doses and assesses the benefits of inclusion in the plan. Materials and Methods This treatment planning study was accomplished by acquiring 10 previously delivered, physician approved treatment plans for head and neck cancer patients. In each of the 10 treatment plans, a new parameter was created which included the imaging dose as part of the radiation therapy treatment. For each plan, 5 different imaging doses were considered: 0, 10, 20, 30, and 40 cGy. Multiple imaging doses were considered in order to generalize the results, and there are time when a patient is imaged twice prior to treatment. For each imaging dose a computer optimization was run to incorporate the imaging dose into the treatment plan. Comparisons between dose inclusion and non-dose inclusion techniques were made for the total dose to the patient and for the parotid gland, which secretes saliva into the mouth. Results Plans that included imaging dose in the treatment planning optimization process always restricted spinal cord and brainstem doses to safe levels as seen in figure 2 and 3. By including the pre-treatment imaging dose as part of the radiation therapy, the parotid glands were spared 40% of the dose or more 95% of the time and the overall dose to the patient was reduced by 50% or more 95% of the time. This effect can be seen in figures 4 and 5. The plans that did not include imaging dose in the treatment planning optimization process delivered potentially damaging spinal cord and brain stem doses 95% of the time when an imaging dose of 20 cGy per day or more was applied. Typical imaging doses are 10 cGy per day or less. Conclusions Including pre-treatment imaging doses into radiation therapy plans successfully reduces excess radiation dose to patients as seen in figure 3. Dose inclusion also allows for the imaging dose to be increased to create a higher quality image to ensure precise patient position. Figure 3: Radiation dose distribution overlays for each of the 5 imaging doses considered. IDI stands for Imaging Dose Incorporation. This figure demonstrates how including imaging dose into the treatment plan manages dose escalation because as the imaging dose increases the IDI columns remain virtually unchanged while the non-IDI column exhibits dose increase as seen by the patches of red. Figure 2: Dose volume histogram for a patient in which imaging dose was not included in the treatment plan (left) and when imaging dose was included (right). PTV stands for Primary treatment volume. This figure demonstrates that as imaging dose is increased and included in the treatment plan the prescribed dose to the treatment volumes remains unchanged and dose to vital organs is managed. Figure 4: Scatter plot of dose increase to the parotid glands for each of the 10 patients when imaging dose is included in the treatment plan vs. not included. Figure 5: Scatter plot of overall dose increase for each of the 10 patients when imaging dose is included in the treatment plan vs. not included. Figure 1: Linear accelerator located at The University of Iowa Hospitals and Clinic. This is the machine which produces the pre-treatment CT and also provides the therapeutic radiation.