Abstract
The influence of dose variation due to heterogeneities in narrow photon beams used in stereotactic radiosurgery has been investigated. Since the lateral electronic disequilibrium and existence of steep dose gradients in small fields and the presence of heterogeneities can intensify these problems, in this study the effects of heterogeneities on 6 MV small photon beams produced by circular cone collimators with 5, 10, 15, 20 and 30 mm diameters are investigated. The heterogeneities include 3 cm Cork with density of 0.2 g/cm3 instead of lung and 3 cm Polytetrafluoroethylene (P.T.F.E) with density of 2.2 g/cm3 as bone. The measurements were carried out with EBT2 gafchromic and EDR2 radiographic films. Simulation was done by MCNP Monte Carlo Code (MCNP5). The depth dose curves in heterogeneous phantom were compared with homogeneous phantom. A good agreement was obtained within film and Monte Carlo calculations in presence of low density heterogeneity and also in the presence of high density heterogeneity. Monte Carlo results showed good agreement after stopping power correction.
Highlights
In recent years, treatment by linear accelerator-based stereotactic radiosurgery (SRS) has increased significantly
The aim of this study is to evaluate the effects of heterogeneities of SRS fields irradiated by 6 MV photon beams using EBT2 and EDR2 films and verification by Monte Carlo simulation
The results show dose reduction at central axis by depth increasing, this reduction is higher for smaller field sizes
Summary
Treatment by linear accelerator-based stereotactic radiosurgery (SRS) has increased significantly This type of treatment modality employs narrowly-collimated, 6 - 10 radiation photon beams with field sizes smaller than 4 × 4 cm to limited volume of tumor. The aim of this technique is to deliver high dose with accurate position and dose of ±1 mm of ±5%, respectively [1]. Because of the effect of heterogeneities on inaccuracy of the measurements, especially in small fields, selecting detectors with good spatial resolution, high accuracy and precision, dose rate and energy independence is very important. Monte Carlo method provides a bridge between measurement and calculation and it is one of the best methods for situations in which physical measurements are difficult or impossible
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