Monte Carlo Dosimetry of Organ Doses from a Sweeping-Beam Total Body Irradiation Technique: Feasibility and First Results

Total body irradiation (TBI) is a radiation treatment often purposed to suppress the immune system prior to a bone marrow transplant. Several toxicities can arise in TBI, and high-quality dose volume data for organs at risk are required if one is considering any change from a well-established technique. We present a novel Monte Carlo (MC) dosimetry technique to acquire this data based on our current TBI technique that accounts for a sweeping-beam Cobalt-60 delivery, a stationary flattening filter, patient-specific lung compensators, and two patient treatment positions. For each patient, a virtual MC phantom is created including the planning CT image in each treatment position (supine and prone). The results from dose simulations on each phantom were summed together geometrically with a deformable registration tool. Dose volume statistics for lungs, liver, thyroid, and kidneys are obtained. The preliminary results of a retrospective study using this technique on patients who have received TBI at our clinic indicate that, for a total body prescription dose of 12 Gy ± 10%, the mean body dose ranged from 11.19 to 12.15 Gy with smaller patients receiving lower mean body doses than larger patients. The mean dose delivered to the thyroid was the highest of the contoured organs receiving up to 12.84 Gy, and the lung doses were the most heterogeneous, with standard deviations up to 0.73 Gy in individual patients. This high-quality dose data shows promise for use in both routine quality assurance of our current technique, and to provide baseline data for development of a new technique. The technique could also be adapted to TBI techniques at other clinics that include compensators, flattening filters, moving beams, and/or multiple treatment positions.