SU-E-T-559: TBI Lung Dose Comparison Using Bilateral and Anterior-Posterior Delivery Techniques and Tissue Density Corrections

Abstract

Purpose: To compare lung dose distributions for two common techniques of total body photon irradiation (TBI) at extended source‐to‐surface distance, with and without tissue density correction (TDC). Lung dose correction factors as a function of lateral thorax separation are approximated for bilateral opposed TBI (supine), similar to those published for anterior‐posterior (AP‐PA) techniques in AAPM Report 17. Methods: 3D treatment plans were created retrospectively for eight patients treated with bilateral TBI, and for whom CT data had been acquired from the head to mid‐thigh. These plans included bilateral opposed and AP‐PA techniques, each with and without TDC, using source‐to‐axis distance of 377 cm and largest possible field size. Results: On average, bilateral TBI requires 40% more monitor units than AP‐PA TBI due to increased separation. Calculation of midline thorax dose without TDC leads to dose underestimation of 17% on average (st. dev. 3%) for bilateral TBI. Lung dose correction factors (CF) are calculated as the ratio of mid‐lung dose (with TDC) to midline thorax dose (without TDC). Bilateral CF generally increase with patient separation, with high variability. However, bilateral CF are 4% (st. dev. 2%) higher than the same corrections calculated for AP‐PA TBI. On average, mid‐lung dose is 4% (st. dev. 3%) lower in bilateral TBI than in AP‐PA TBI. However, maximum lung dose is higher with bilateral TBI (up to 30% higher than prescribed, per patient) due to the absence of arm tissue blocking the anterior chest. Conclusions: Dose calculations for bilateral TBI without TDC are incorrect by up to 21% in the thorax. Bilateral lung CF may be calculated as 1.04 times the values published in Table 6 of AAPM Report 17, though a larger patient pool is necessary to better quantify this trend. Bolus or other compensation will reduce lung maximum dose in the anterior thorax.