Room: Exhibit Hall
Purpose: To assess the suitability of using a single crystal diamond detector to perform absolute dosimetry in photon and electron radiotherapy beams.
Methods: Following TG-51 guidelines, a Varian TrueBeam linear accelerator was calibrated using a Farmer-type ionization chamber to deliver 1 cGy/MU in a 40x40x40 cmÂ³ water tank at d(max) (photons: 6, 6-FFF, 10, 10-FFF, 15, 18 MV; electrons: 6, 9, 12, 15, 18 MeV) under reference geometry (100 SSD, photons: 10x10 cmÂ² field size, electrons: 15x15 cmÂ² applicator). A synthetic single crystal diamond detector (PTW microDiamond TN60019) was then used for dosimetric measurements under the same geometry. Photon and electron measurements were taken at 10 cm depth (%dd(10) used to calculate doses at d(max)) and d(max), respectively. Readings were converted to dose using a manufacturer-specified calibration factor of 82.440 cGy/nC (â?¶â?°Co, detector oriented perpendicularly). As the detector was oriented axially, the factor was corrected for a difference in response (~2.5%, manufacturer-specified). The calculated doses were then compared with nominal values to establish cross-calibration coefficients.
Results: Using the manufacturer-supplied calibration factor and angular correction, the calculated doses at d(max) for both photons and electrons were consistently 1.6% higher than nominal values. After cross-calibration, the calibration coefficient of the detector was found to be 83.199Â±0.264 and 83.146Â±0.105 cGy/nC for photons and electrons, respectively. There appeared to be no dependence on radiation type or energy and thus, an average coefficient of 83.171 cGy/nC was established.
Conclusion: Although TG-51 remains the standard for absolute dosimetry, with cross-calibration, a single crystal diamond detector allows accurate, efficient, and energy-independent dosimetric measurements. Some advantages compared to an ionization chamber include simple setup and elimination of the need to determine various correction factors (e.g., k(Q), P(T,P), P(pol), P(ion), k'(R50), PQ(gr)). Disadvantages include the angular dependence of response (a stem effect), although this can be abated with cross-calibration.