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

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.