Analytical model for out-of-field dose in photon craniospinal irradiation

Abstract

Introduction The prediction of late effects after radiotherapy in organs outside a treatment field requires accurate estimations of out-of-field dose. However, out-of-field dose is not calculated accurately by commercial treatment planning systems (TPSs). The purpose of this study was to develop and test an analytical model for out-of-field dose during craniospinal irradiation (CSI) from photon beams produced by a linear accelerator. Materials & Methods In two separate evaluations of the model, we measured absorbed dose for a 6-MV CSI using thermoluminescent dosimeters placed throughout an anthropomorphic phantom and fit the measured data to an analytical model of absorbed dose versus distance outside of the composite field edge. These measurements were performed in two separate clinics—The University of Texas MD Anderson Cancer Center (MD Anderson) and the American University of Beirut Medical Center (AUBMC)—using the same phantom but different linear accelerators and TPSs commissioned for patient treatments. The measurement at AUBMC also included in-field locations. Measured dose values were compared to those predicted by TPSs and parameters were fit to the model in each setting. Results In each clinic, 95% of the measured data were contained within a factor of 0.2 and one root mean square deviation of the model-based values. The root mean square deviations of the mathematical model were 0.91 cGy/Gy and 1.67 cGy/Gy in the MD Anderson and AUBMC clinics, respectively. The TPS predictions agreed poorly with measurements in regions of sharp dose gradient, e.g., near the field edge. At distances greater than 1 cm from the field edge, the TPS underestimated the dose by an average of 14% ± 24% and 44% ± 19% in the MD Anderson and AUBMC clinics, respectively. The in-field measured dose values of the measurement at AUBMC matched the dose values calculated by the TPS to within 2%. Conclusions Dose algorithms in TPSs systematically underestimated the actual out-of-field dose. Therefore, it is important to use an improved model based on measurements when estimating out-of-field dose. The model proposed in this study performed well for this purpose in two clinics and may be applicable in other clinics with similar treatment field configurations.

A simple and fast physics-based analytical method to calculate therapeutic and stray doses from external beam, megavoltage x-ray therapy

“…The dose due to phantom-scattered radiation was calculated as $$D_{\mathrm{normalP}\mathrm{normalS},\text{water}}^{\mathrm{normalC}\mathrm{normalP}\mathrm{normalE}}false(x,dfalse)=D_{\mathrm{normalP}\mathrm{normalS}}false(dfalse)false[Cfalse(dfalse)\times {\tilde{G}}_{\mathrm{normalP}\mathrm{normalS},1}false(xfalse)\times T{F}_{\mathrm{normalP}\mathrm{normalS},1}false(xfalse)+false(1-Cfalse(dfalse)false)\times {\tilde{G}}_{\mathrm{normalP}\mathrm{normalS},2}false(xfalse)\times T{F}_{\mathrm{normalP}\mathrm{normalS},2}false(xfalse)false],$$ where D PS ( d ) is the integral absorbed phantom scatter dose, C ( d ) is a partitioning factor to apportion the two component Gaussians, G̃ PS,1 ( x ) and G̃ PS,2 ( x ) are each un-normalized Gaussians centered on the CAX, and TF PS,1 ( x ) and TF PS,2 ( x ) are the corresponding transmission factors in water. The method in equation 2.28 was previously described in Zhang et al (2011) and Taddei et al (2013).…”

A comparative study on the risks of radiogenic second cancers and cardiac mortality in a set of pediatric medulloblastoma patients treated with photon or proton craniospinal irradiation

“…The stray dose from proton therapy was calculated by our Monte Carlo Proton Radiotherapy Treatment Planning system [11], which uses the Monte Carlo N-particle eXtended code (version 2.6; Los Alamos National Laboratory, Los Alamos, NM) [21] as a dose calculation engine. The secondary dose from photon therapy was obtained from the TPS and measurement [22]. Our method was previously described in the literature [23,24] and briefly summarized here: For organs in close proximity to the treatment field, doses are accurately reported by the TPS and as such doses were directly taken from the TPS.…”

Measurement and modeling of out-of-field doses from various advanced post-mastectomy radiotherapy techniques

“…Out-of-field dose data based on measurements or Monte Carlo simulations for static and IMRT photon fields were extensively reported in the literature (Stovall et al , 1995; Mutic and Low, 1998; Mutic and Klein, 1999; Stern, 1999; Meeks et al , 2002; Kry et al , 2005; Kry et al , 2006; Sharma et al , 2006; Wang and Xu, 2008; Bednarz and Xu, 2009; Ruben et al , 2011; Halg et al , 2012; Kaderka et al , 2012; La Tessa et al , 2012; Taddei et al , 2013; Covington et al , 2016), while analytical models of out-of-field dose from photon external therapy had only been proposed in a few studies (McParland and Fair, 1992; Taddei et al , 2013; Jagetic and Newhauser, 2015; Hauri et al , 2016). Most of the previous modeling studies focused on open beams or conventional field-in-field beams, and the modeling of IMRT beams required intensive information of each field (Hauri et al , 2016).…”