AbstractID: 7471 Title: A New AP/PA Lung-Sparing Technique for Photon Total Body Irradiation (TBI) of Children Delivering photon total body irradiation (TBI) with optimal sparing of the lung tissue is particularly challenging in young children. AP-PA techniques that rely on the patient to stand motionless and upright are not applicable to the active or sedated child. Lateral techniques are not conducive to precision lung blocking. We have designed a technique that addresses these issues without requiring patient cooperation or expensive devices. TBI is administered with the patient lying in the right decubitus position with his left arm at his side and his right forearm under his head on a gurney either awake or sedated. Partial-transmission blocks are used to reduce the lung dose. A custom thermoplastic shell (HipFix R, MedTec) covering from the top of the shoulders to the waist or below is created on the simulation table for reproducible torso positioning. Lung blocks and diodes (positions defined by simulation films) are attached with VelcroR to a plastic sheet which is then Velcro’ed to the shell. Lung blocking position was measured in 5 consecutive children, aged 2 to 7 years, treated with this technique in either 6 or 9 fractions over 3 to 5 days. Lung block positioning accuracy and reproducibility were determined by assessing whether the blocks shadowed tissues outside the lungs on the initial verification film taken prior to each fraction. This safe and inexpensive technique permits lung block margins to be less than 1 cm reproducibly without shielding of tissues outside the lungs.