Quantification the dosimetric parameters of asymmetric physical wedged-6MV photon beam

Abstract

Introduction: Physical wedge as a useful tool has been utilized in radiotherapy to modify photon beam shape and intensity such that it distributes dose uniformly in tumor site and reduces hot points. Since during Linac commissioning dosimetric parameters like output factors and lateral dose profiles are measured only for symmetric open and wedged fields, so calculation the parameters for asymmetric wedged fields become necessary. The study aims to achieve output factors and dose profiles for symmetric and asymmetric wedged fields of 6 MV photon beams. Materials and Methods: The Siemens PRIMUS Linac head for 6 MV photon beam was simulated by BEAMnrc and all dose calculations were performed by DOSXYZnrc code. Percentage depth dose (PDD) and profiles for open and wedged (15° and 45°) fields were compared with corresponding measurements. Wedge factors for 10 x 10 cm2 field size as a function of lateral distance from central axis were obtained as well for half beam (negative or positive Y jaws were closed) wedged fields. All mesurments were carried out using 0.125 cm3 Farmer type ionization chamber with DOSE1 electrometer (FC65G, Scanditronix-Wellhofer, Germany) at the depth of 10 cm with source to surface distance (SSD) of 100 cm in 50 cm3 PTW-Blue water phantom and processed by dosimetry software RFAplus (Version 5.2, Scanditronix-Wellhofer, Germany). Results: The calculated doses were in agreement with measured data that confirmed the simulated MC model. The output factors on the central axis of symmetric wedged beams decreased to 0.693 and 0.307 for 15˚ and 45˚ wedges, respectively. The total photon fluence of 15˚ and 45˚ physical wedged fields reduced to 71.6% and 27.7% of fluence for open field, respectively. It was found that the output factor for asymmetric wedged fields is lower than ones from symmetric fields, particularly at field edges. Conclusion: The presences of physical wedge across the beamline decrease the photon fluence and cases beam hardening both for symmetric and asymmetric wedged fields. Lack of scattering photons near the half beam edges in asymmetric wedged field causes a dose fall off in these regions where have potential to overestimate the calculated dose by treatment planning system and consequent cold spots at target volume. Our calculated correction coefficients are recommended to calculate doses by TPS in case of asymmetric wedged fields.