Optimization of a tissue-equivalent CVD-diamond dosimeter for radiotherapy using the Monte Carlo code PENELOPE

The design of a chemical vapour deposited (CVD) diamond detector is studied by means of the Monte Carlo (MC) code PENELOPE. The energy, field-size and directional dependences have been studied, together with its potential for measurements of depth-dose curves and lateral beam profiles in a water phantom. The CVD-diamond sample modelled in this investigation had a thickness of 50 μ m. It was covered on both sides with 0.2-μ m-thick silver electrodes and encapsulated in polystyrene. The detector was exposed to a reference ⁶⁰Co photon beam as well as to clinically relevant 6 and 18 MV X-ray beams, with field sizes of 2 × 2, 5 × 5 and 10 × 10 cm² at the phantom surface. For the considered photon-beam qualities the calculated detector response varied by 3.4%. The change of the absorbed dose ratio diamond to water with field size was around 1.3% for the three qualities. The device showed a directional dependence with maximal variation of 1.7% and 2.0% for the ⁶⁰Co and 6 MV beams, respectively, but for 18 MV X-rays the dependence was below 1%. MC simulations for narrow beams (2 × 2 cm²) proved that the prototype has an excellent spatial resolution, comparable to the thickness of the diamond layer, and is therefore suitable for lateral beam profile measurements in the presence of high dose gradients as often met in e.g. intensity-modulated radiotherapy. A better construction of the CVD-diamond dosimeter was proposed, where geometrical optimization of the encapsulation and the replacement of the silver electrodes with graphite was shown by further MC calculations to reduce the energy and angular dependences to about 1%.