TBI lung dose comparisons using bilateral and anteroposterior delivery techniques and tissue density corrections.

Daniel W Bailey,Tara Lakeman,Lee D Hales,Iris Z Wang,Matthew B Podgorsak,Anurag K Singh

doi:10.1120/jacmp.v16i2.5293

Abstract

This study compares lung dose distributions for two common techniques of total body photon irradiation (TBI) at extended source‐to‐surface distance calculated 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 anteroposterior–posteroanterior (AP–PA) techniques in AAPM Report 17 (i.e., Task Group 29). 3D treatment plans were created retrospectively for 24 patients treated with bilateral TBI, and for whom CT data had been acquired from the head to the lower leg. 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. On average, bilateral TBI requires 40% more monitor units than AP–PA TBI due to increased separation (26% more for 23 MV). Calculation of midline thorax dose without TDC leads to dose underestimation of 17% on average (standard deviation, 4%) for bilateral 6 MV TBI, and 11% on average (standard deviation, 3%) for 23 MV. Lung dose correction factors (CF) are calculated as the ratio of midlung dose (with TDC) to midline thorax dose (without TDC). Bilateral CF generally increases with patient separation, though with high variability due to individual uniqueness of anatomy. Bilateral CF are 5% (standard deviation, 4%) higher than the same corrections calculated for AP–PA TBI in the 6 MV case, and 4% higher (standard deviation, 2%) for 23 MV. The maximum lung dose is much higher with bilateral TBI (up to 40% higher than prescribed, depending on patient anatomy) due to the absence of arm tissue blocking the anterior chest. Dose calculations for bilateral TBI without TDC are incorrect by up to 24% in the thorax for 6 MV and up to 16% for 23 MV. Bilateral lung CF may be calculated as 1.05 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 customized shielding will reduce lung maximum dose in the anterior thorax.PACS numbers: 87.55.D, 87.55.Dk, 87.55.Ne, 87.56.Bd, 87.57.Qp

Highlights

IntroductionRadiation pneumonitis is one of the most serious complications associated with total- and halfbody irradiation, resulting in fatality in up to 80% of presenting patients who have received uniform or near-uniform dose to the total lung.[1]
Radiation pneumonitis is one of the most serious complications associated with total- and halfbody irradiation, resulting in fatality in up to 80% of presenting patients who have received uniform or near-uniform dose to the total lung.[1] an accurate analysis of wholelung dose is essential in any large-field thorax irradiation setting
A. 6 MV total body photon irradiation (TBI) In terms of dose to the umbilicus, both AP–PA and bilateral TBI techniques calculated without inhomogeneity corrections were capable of delivering dose to the prescription point with good accuracy

Summary

Introduction

Radiation pneumonitis is one of the most serious complications associated with total- and halfbody irradiation, resulting in fatality in up to 80% of presenting patients who have received uniform or near-uniform dose to the total lung.[1]. Lung dose distributions are compared for two common delivery techniques of total body photon irradiation (TBI). The goal of this study is two-fold: to examine the differences between lung doses calculated with inhomogeneity corrections and without these corrections (as is commonly performed in simple TBI calculations); and to compare lung dose distributions between the bilateral and AP–PA techniques. We aim to approximate lung dose correction factors for bilateral TBI simple calculations (i.e., non-3D, no tissue density correction) based solely on the index of bilateral thorax separation, similar to the correction factors reported previously for the AP–PA technique by Van Dyk et al[2,6]

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