SU-E-I-42: Size-Specific Dose Estimates (SSDE) Modified for Lung Inhomogeneities: A Practical Approach

Abstract

Purpose: his work recommends a method of estimating the patient effective diameter (ED) parameter in the thoracic area in order to compute the Size‐Specific Dose Estimates (SSDE) for both adult and pediatric patients, taking into account the lung inhomogeneities. Methods: A well‐established method of estimating effective distance/depth (sometimes called radiological depth) in radiotherapy treatment planning for lung inhomogeneities, is recommended here. The method requires the use of an average relative electron density for the lung tissue of rho(Lung)=0.3 to be used so a corrected lateral (LAT) patient thickness and ED is determined. Based on the new ED the dose correction factor, as per TG‐204, can be selected. This method was compared with the current recommendation by TG‐204 and another correction method for lung inhomogeneities from the literature. Results: Improved estimation of patient ED that incorporates lung inhomogeneities was compared to that computed by a radiotherapy treatment planning system. The two methods agreed to within 5% for the cases studied. This difference is smaller than the 12–20% between the recommended method by TG‐204 and another published correction method. In addition, the results showed that the effective AP thickness is not necessary to be accounted for by this approach. Conclusions: A fast and easy method of computing patient effective LAT thickness and ED in order to estimate SSDE is based on the use of a relative electron density for the lung tissue. The more accurate LAT thickness and patient ED allow the selection of a dose correction factor out of the TG‐204 tables that would take into account in some way the tissue heterogeneities in the thoracic area. This method may potentially be used in cone‐beam CT image guided radiotherapy where estimation of patient imaging doses is required.