SU-E-T-320: Low Dose Rate Pediatric Total Body Irradiation (TBI) Using TrueBeam

Abstract

Purpose: To develop a Low Dose Rate Pediatric Total Body Irradiation (TBI) methodology using a linear accelerator with dose rates less than 100 MU/min (TrueBeam), specifically, using this feature to deliver an Anterior-to-Posterior beam and a Posterior-to-Anterior beam, i.e. AP-PA treatment, with a delivered dose rate < 10 cGy/min for thin pediatric patients (< 14 cm). Methods: A raised platform was constructed to sit approximately 6 cm off of the floor (approximately 2 meters from the source), the patient positioned supine with head turned for the AP beam and supine with head turned the same direction for the PA beam. A scattering plate was placed on foam pillars approximately 20 cm from the patient's surface. This also used to hold custom organ blocks and/or compensators. Output, percent depth dose, surface dose, block transmission, and off axis measurements were taken using solid water slabs, ion chambers, and film. From these measurements the optimal dose rate and number of monitor units could be selected for a given patient thickness. Results: Depth dose and off axis measurements confirmed a dose distribution within ±5% of the central axis dose at all positions for an open field. Dose measurements taken at 3 mm depth with varied scattering plate distance indicated that a 20 cm distance would allow quick access to the patient as they would be under anesthesia and not compromise surface dose coverage. Conclusions: This work has shown that for certain patients, particularly pediatric patients, linear accelerators that allow the user more low dose rate options can be used to deliver low dose rate TBI (< 10 cGy/min) without the need to construct heavy beam attenuators placed above the patient. This allows greater flexibility and increased patient safety.