The angular dependence of a two dimensional monolithic detector array for dosimetry in small radiation fields

N Stansook,M Petasecca,M Carolan,M Newall,A B Rosenfeld,P Metcalfe,M Lerch,K Utitsarn

doi:10.1088/1742-6596/777/1/012020

N Stansook, M Petasecca + Show 6 more

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https://doi.org/10.1088/1742-6596/777/1/012020

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Abstract

The purpose of this study is to investigate the directional dependence of a two dimensional monolithic detector array (M512) under 6 MV photon irradiation and to evaluate the effect of field size on angular dependence. Square fields of sizes: 3x3 cm2 and 10x10 cm2 were measured at the iso-centre of a cylindrical phantom. Beam angles with incidences from 00- 1800 in increments of 150 were used to investigate the central pixel angular response of M512, normalized to the pixel response for normal (0°) beam incidence. The angular response of the detector was compared to the response of EBT3 radiochromic film in the identical geometric orientation. The maximum angular dependence was observed at the angle 90°±15° to be -18.62% and -17.70% for the field sizes 3x3 cm2 and 10x10 cm2, respectively. The angular dependence of M512 showed no significant difference between field sizes of 3x3 cm2 and 10x10 cm2 (p>0.05). The maximum dose difference measured by the central pixel of M512 and EBT3 for all angles are -20% for 3x3 cm2 field size and -18.58% for the 10x10 cm2 field. The diode array’s size and packaging effects the angular response of the detector. The angular correction factor is necessary to apply to increase accuracy in dosimetry for arc treatment delivery.

Highlights

Advanced external beam radiotherapy (EBRT) techniques such as stereotactic body radiotherapy (SBRT) and volumetric modulated arc therapy (VMAT) have been increasingly used in the treatment of cancer patients
The purpose of this study is to investigate the directional dependence of a two dimensional monolithic detector array (M512) under 6 MV photon irradiation and to evaluate the effect of field size on angular dependence
The relative response of M512 which is normalized at the incidence angle zero showed less angular dependence for the angles between 0°- 45° but showed a response change of -19% for the 3 x 3 cm2 field size and -17.70% for 10 x 10 cm2 field at incident angle 900-1050

Summary

Introduction

Advanced external beam radiotherapy (EBRT) techniques such as stereotactic body radiotherapy (SBRT) and volumetric modulated arc therapy (VMAT) have been increasingly used in the treatment of cancer patients Due to their delivery complexity, with small beamlets and high dose gradients modulated throughout the rotation of the gantry around the patient, the accuracy of patient verification before treatment becomes very important. The diode is a widely used radiation detector for small field dosimetry due to their suitable characteristics such as high sensitivity compared with the ionization chamber, small active volume size, high spatial resolution and real time measurement. The dosimetric characterisation of M512 in term of percentage depth dose and dose per pulse dependence has been completed and shows that the M512 is suitable for use as a quality assurance detector in small radiation fields [10], the angular response of the M512 detector for dose verification in arc therapy delivery has to be studied. The purpose of this study is to investigate the angular response of the M512 detector to 6 MV photon beam irradiation for different field sizes

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