Use of PTW-microDiamond for relative dosimetry of unflattened photon beams

Abstract

Purpose: The increasing interest in SBRT treatments encourages the use of flattening filter free (FFF) beams. Aim of this work was to evaluate the performance of the PTW60019 microDiamond detector under 6MV and 10MVFFF beams delivered with the EDGE accelerator (Varian Medical System, Palo Alto, USA). A flattened 6MV beam was also considered for comparison.

Methods: Short term stability, dose linearity and dose rate dependence were evaluated. Dose per pulse dependence was investigated in the range 0.2-2.2mGy/pulse. MicroDiamond profiles and output factors (OFs) were compared to those obtained with other detectors for field sizes ranging from 40×40cm² to 0.6×0.6cm². In small fields, volume averaging effects were evaluated and the relevant correction factors were applied for each detector.

Results: MicroDiamond short term stability, dose linearity and dependence on monitor unit rate were less than 0.8% for all energies. Response variations with dose per pulse were found within 1.8%. MicroDiamond output factors (OF) values differed from those measured with the reference ion-chamber for less than 1% up to 40×40cm² fields where silicon diodes overestimate the dose of ≈3%. For small fields (<3×3cm²) microDiamond and the unshielded silicon diode were in good agreement.

Conclusions: MicroDiamond showed optimal characteristics for relative dosimetry even under high dose rate beams. The effects due to dose per pulse dependence up to 2.2mGy/pulse are negligible. Compared to other detectors, microDiamond provides accurate OF measurements in the whole range of field sizes. For fields <1cm correction factors accounting for fluence perturbation and volume averaging could be required.

Keywords: Dose per pulse; MicroDiamond detector; Small field dosimetry; Unflattened photon beams.