Experimental Validation of an Analytical Program and a Monte Carlo Simulation for the Computation of the Far Out-of-Field Dose in External Beam Photon Therapy Applied to Pediatric Patients

Abstract

BackgroundThe out-of-the-field absorbed dose affects the probability of primary second radiation-induced cancers. This is particularly relevant in the case of pediatric treatments. There are currently no methods employed in the clinical routine for the computation of dose distributions from stray radiation in radiotherapy. To overcome this limitation in the framework of conventional teletherapy with photon beams, two computational tools have been developed—one based on an analytical approach and another depending on a fast Monte Carlo algorithm. The purpose of this work is to evaluate the accuracy of these approaches by comparison with experimental data obtained from anthropomorphic phantom irradiations.Materials and MethodsAn anthropomorphic phantom representing a 5-year-old child (ATOM, CIRS) was irradiated considering a brain tumor using a Varian TrueBeam linac. Two treatments for the same planned target volume (PTV) were considered, namely, intensity-modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT). In all cases, the irradiation was conducted with a 6-MV energy beam using the flattening filter for a prescribed dose of 3.6 Gy to the PTV. The phantom had natLiF : Mg, Cu, P (MCP-N) thermoluminescent dosimeters (TLDs) in its 180 holes. The uncertainty of the experimental data was around 20%, which was mostly attributed to the MCP-N energy dependence. To calculate the out-of-field dose, an analytical algorithm was implemented to be run from a Varian Eclipse TPS. This algorithm considers that all anatomical structures are filled with water, with the exception of the lungs which are made of air. The fast Monte Carlo code dose planning method was also used for computing the out-of-field dose. It was executed from the dose verification system PRIMO using a phase-space file containing 3x109 histories, reaching an average standard statistical uncertainty of less than 0.2% (coverage factor k = 1 ) on all voxels scoring more than 50% of the maximum dose. The standard statistical uncertainty of out-of-field voxels in the Monte Carlo simulation did not exceed 5%. For the Monte Carlo simulation the actual chemical composition of the materials used in ATOM, as provided by the manufacturer, was employed.ResultsIn the out-of-the-field region, the absorbed dose was on average four orders of magnitude lower than the dose at the PTV. For the two modalities employed, the discrepancy between the central values of the TLDs located in the out-of-the-field region and the corresponding positions in the analytic model were in general less than 40%. The discrepancy in the lung doses was more pronounced for IMRT. The same comparison between the experimental and the Monte Carlo data yielded differences which are, in general, smaller than 20%. It was observed that the VMAT irradiation produces the smallest out-of-the-field dose when compared to IMRT.ConclusionsThe proposed computational methods for the routine calculation of the out-of-the-field dose produce results that are similar, in most cases, with the experimental data. It has been experimentally found that the VMAT irradiation produces the smallest out-of-the-field dose when compared to IMRT for a given PTV.

Validation of an in vivo transit dosimetry algorithm using Monte Carlo simulations and ionization chamber measurements

“… 40 validated the HD120 MLC for VMAT plans. Other applications of PRIMO included the use of PRIMO as an independent tool for the beam commissioning of a 6 MV Varian Truebeam STx, 41 simulation of out‐of‐field dose, 42 simulation of conical collimators for stereotactic radiosurgery, 43 and the use of PRIMO as an independent dose verification system for radiosurgery HyperArc plans. 44 …”

“…Esposito et al 39 validated the Millennium 120 MLC for the simulation of IMRT plans and Paganini et al 40 validated the HD120 MLC for VMAT plans. Other applications of PRIMO included the use of PRIMO as an independent tool for the beam commissioning of a 6 MV Varian Truebeam STx, 41 simulation of out-of -field dose, 42 simulation of conical collimators for stereotactic radiosurgery, 43 and the use of PRIMO as an independent dose verification system for radiosurgery HyperArc plans. 44 Table 1 summarizes the simulations details following the scheme proposed by the AAPM Task Group 268 report RECORDS.…”

Complete patient exposure during paediatric brain cancer treatment for photon and proton therapy techniques including imaging procedures

“…The plans were iteratively optimised over several steps using the constraint V7Gy=4% for the eye and V40Gy=5% and V25Gy=5%, for the left and right cochlea respectively, and V98%[PTV] > 95% regarding D prescribed . More details can be found in a recently published paper ( 24 ).…”

“…Within HARMONIC, a tool for calculating the dose from imaging procedures during radiotherapy has been further developed ( 23 ). Furthermore, HARMONIC has invested substantial efforts into validating computational and analytical tools needed to estimate out-of-field organ doses in children treated with photon and proton therapy ( 24 , 25 ). Particularly important for proton therapy are the challenges related to the creation of secondary neutrons and the higher relative biological effectiveness (RBE) of neutrons and protons when compared to photons.…”

Analytical models for external photon beam radiotherapy out-of-field dose calculation: a scoping review

“…Different strategies have been developed by radiotherapy physics research teams as an alternative to TPS to precisely estimate out-of-field doses and to provide large amounts of data for epidemiological studies. To this aim, outstanding experimental works have been performed first ( 23 , 24 ), paving the way to software implementing analytical modeling ( 25 , 26 ) or implementation of Monte Carlo simulations ( 27 , 28 ). Yet, to the best of our knowledge, the use of these calculation software remains so far confined to dosimetry for epidemiological research programs.…”