Abstract
The purpose of this comparative study was to test the accuracy of anisotropic analytical algorithm (AAA) and pencil beam convolution (PBC) algorithms of Eclipse treatment planning system (TPS) for dose calculations in the low‐ and high‐dose buildup regions. AAA and PBC algorithms were used to create two intensity‐modulated radiotherapy (IMRT) plans of the same optimal fluence generated from a clinically simulated oropharynx case in an in‐house fabricated head and neck phantom. The TPS computed buildup doses were compared with the corresponding measured doses in the phantom using thermoluminescence dosimeters (TLD 100). Analysis of dose distribution calculated using PBC and AAA shows an increase in gamma value in the dose buildup region indicating large dose deviation. For the surface areas of 1, 50 and 100cm2, PBC overestimates doses as compared to AAA calculated value in the range of 1.34%–3.62% at 0.6 cm depth, 1.74%–2.96% at 0.4 cm depth, and 1.96%–4.06% at 0.2 cm depth, respectively. In high‐dose buildup region, AAA calculated doses were lower by an average of ‐7.56%(SD=4.73%), while PBC was overestimated by 3.75%(SD=5.70%) as compared to TLD measured doses at 0.2 cm depth. However, at 0.4 and 0.6 cm depth, PBC overestimated TLD measured doses by 5.84%(SD=4.38%) and 2.40%(SD=4.63%), respectively, while AAA underestimated the TLD measured doses by ‐0.82%(SD=4.24%) and ‐1.10%(SD=4.14%) at the same respective depth. In low‐dose buildup region, both AAA and PBC overestimated the TLD measured doses at all depths except ‐2.05%(SD=10.21%) by AAA at 0.2 cm depth. The differences between AAA and PBC at all depths were statistically significant (p<0.05) in high‐dose buildup region, whereas it is not statistically significant in low‐dose buildup region. In conclusion, AAA calculated the dose more accurately than PBC in clinically important high‐dose buildup region at 0.4 cm and 0.6 cm depths. The use of an orfit cast increases the dose buildup effect, and this buildup effect decreases with depth.PACS number: 87.53.Bn
Highlights
106 Oinam et al.: Verification of IMRT dose buildup calculation flattening filter, collimator assembly and, to a lesser extent, secondary scatter photons from the accelerator head.[1,2,3,4,5] The problem is further complicated by oblique incidence of the beam and the use of multileaf collimator (MLC) for beam intensity modulation in treatment techniques like intensity-modulated radiotherapy (IMRT).(6) Several authors have reported measurement of skin dose on patient and buildup dose on phantom from different treatment techniques.[6,7,8] While one study reported increase in skin dose of patients undergoing IMRT treatment,(9) others have reported lesser skin dose as compared to conventional techniques.[6,7] But most of the studies do not address the comparison of treatment planning system (TPS) calculated and measured doses
In an attempt to improve the accuracy of dose calculation in tissue interface or inhomogeneous region, Varian Medical System (Palo Alto, CA) released a new photon dose calculation algorithm known as anisotropic analytical algorithm (AAA).(10,11,12,13) This algorithm uses triple-source modeling for accurate dose calculation at a point whereby it superimposes the doses from photons of both primary component and secondary scatter photon, and from electron contamination originating from flattening filter, collimator jaws, and accessories
A commercially available treatment planning system, Eclipse (V 8.6) (Varian Medical System, Palo Alto, CA), was configured for photon pencil beam convolution (PBC) and AAA algorithm using 6 MV X-rays from Clinac DHX linear accelerator (Varian Medical System, Palo Alto, CA) following manufacturer recommended guidelines and protocols.[10]. Beam profiles and depth dose curves were measured in a water phantom of RFA 300 Plus with OmniPro Accept software (Wellhofer Scanditronix, Germany) in slow speed and high precision of 0.5 mm stepping mode at five different depths for a number of square field sizes ranging from 2 × 2 to 40 × 40 cm2
Summary
The phase space (particle fluence, energy) parameters are modeled using a Monte Carlo simulation-derived multiple source model This consists of a point source for radiation from the primary target, a finite source for extra focal radiation, and a third source to model the electron contamination. Parameters used to characterize the multileaf collimation (MLC) are the leaf transmission factor and the dosimetric leaf separation The latter provides the effective dosimetric opening between mechanically closed leaf pairs due to rounded leaf tips.[10,13,14] While very limited studies[7,15,16] have reported comparison of TPS calculated and measured skin dose in clinical treatment conditions, AAA algorithm has not been tested so far to check its reliability and efficiency in the dose calculation in the dose buildup region. The accuracy of AAA and PBC algorithms available in Eclipse TPS was extensively investigated in non-clinical as well as clinical treatment conditions for the IMRT dose calculation in both high-dose buildup and low-dose buildup regions
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