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

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.