AbstractID: 7042 Title: Data analysis and tissue compensation for total body irradiation
The total body irradiation technique employed at our institution utilizes a Mick TBI frame and an Eleckta SL25 accelerator with a 22 MV photon beam at 5 meters SSD. To form our calculation data set, we measured depth dose at the beam axis, as well as beam profiles at 5, 10, and 20 cm depths. A tissue phantom ratio (TPR) curve was generated from the depth dose data. Noting the TPR curve is approximately exponential; we derived off axis TPR correction factors from ratios of the beam profile measurements at each point off axis. Exponential attenuation factors were derived and the difference between the off axis factors and the central axis factor yielded an off axis correction for
TPR. The first term in the series expansion for the exponential is multiplied by the central axis TPR to yield the off axis TPR. To calculate doses at various points off axis, a digital photograph is made of the patient, and off axis distances are measured from that picture. Diameter and air gap measurements are made directly with the patient, and doses are calculated by multiplying the TPR by the off axis TPR correction and the beam profile function. The inverse square air gap factor is included. The digital photograph is then used in fabricating a compensator out of lead strips.