Monte Carlo modeling of a 6 and 18 MV Varian Clinac medical accelerator for in-field and out-of-field dose calculations: development and validation

Abstract

There is a serious and growing concern about the increased risk of radiation-induced second cancers and late tissue injuries associated with radiation treatment. To better understand and to more accurately quantify non-target organ doses due to scatter and leakage radiation from medical accelerators, a detailed Monte Carlo model of the medical linear accelerator is needed. This paper describes the development and validation of a detailed accelerator model of the Varian Clinac operating at 6 and 18 MV beam energies. Over 100 accelerator components have been defined and integrated using the Monte Carlo code MCNPX. A series of in-field and out-of-field dose validation studies were performed. In-field dose distributions calculated using the accelerator models were tuned to match measurement data that are considered the de facto ‘gold standard’ for the Varian Clinac accelerator provided by the manufacturer. Field sizes of 4 cm × 4 cm, 10 cm × 10 cm, 20 cm × 20 cm and 40 cm × 40 cm were considered. The local difference between calculated and measured dose on the percent depth dose curve was less than 2% for all locations. The local difference between calculated and measured dose on the dose profile curve was less than 2% in the plateau region and less than 2 mm in the penumbra region for all locations. Out-of-field dose profiles were calculated and compared to measurement data for both beam energies for field sizes of 4 cm × 4 cm, 10 cm × 10 cm and 20 cm × 20 cm. For all field sizes considered in this study, the average local difference between calculated and measured dose for the 6 and 18 MV beams was 14 and 16%, respectively. In addition, a method for determining neutron contamination in the 18 MV operating model was validated by comparing calculated in-air neutron fluence with reported calculations and measurements. The average difference between calculated and measured neutron fluence was 20%. As one of the most detailed accelerator models for both in-field and out-of-field dose calculations, the model will be combined with anatomically realistic computational patient phantoms into a computational framework to calculate non-target organ doses to patients from various radiation treatment plans.

Dosimetric Characteristic of Enhanced Dynamic Wedges used in Radiation Therapy

“…For the 15 MV simulations, 7.0 × 10 6 electrons were sampled for the 20 × 20 cm 2 field size. The field required less electrons since more collisions occurred in the phantom due to larger field sizes [25]. Table 1 The 6 MV dose profiles measured and calculated at the depth of d max were also compared.…”

“…Monte Carlo simulation is one of the most accurate methods use in simulating radiation transport and predicting dose [13,14,22,25]. In this study enhanced dynamic wedge have been studied using the EGSnrc Monte Carlo code.…”

Equivalent Dose From Secondary Neutrons and Scatter Photons in Advance Radiation Therapy Techniques With 15 Mv Photon Beams

“…It was found that with the IMRT technique the neutron equivalent dose normalized to the prescribed dose varied from 2 to 6 mSv․Gy -1 . Several other studies have determined the neutron equivalent dose inside the treatment room by using Monte Carlo simulation and measurement for a variety of photon energies [4][5][6][7][8][9]. Nevertheless, no study has estimated the neutron equivalent dose for the VMAT technique.…”

Secondary Cancer Risks in Out-of-field Organs for 3-D Conformal Radiation Therapy

“…In radiation therapy, organs located far from the tumor volume ("out-of-field" organs) are assumed to receive very low doses of radiation and, therefore, frequently ignored in treatment planning 1) , even though it is well known that small radiation doses to these organs nonetheless can induce cancers. Recently, interest in the implications of such secondary caners has significantly grown, mainly because advanced treatment techniques such as 3-D conformal radiation therapy (CRT) and intensity-modulated radiation therapy (IMRT) tend to increase the radiation dose to out-of-field organs [2][3][4][5] .…”