Surface Dose Measurements on Varian Truebeam Linac: Comparison of Ptw Microdiamond and Microsilicon Detectors Using Extrapolation Method
Abstract
Purpose
To estimate surface dose on a Varian TrueBeam linear accelerator using PDD extrapolation with microDiamond and microSilicon detectors, and to assess detector-dependent differences as a function of field size.
Methods
Measurements were performed on a Varian TrueBeam linear accelerator operating at 6 MV FFF. Square radiation fields of 1×1, 5×5, 10×10, 20×20, and 30×30 cm² were collimated by jaws and delivered at a source-to-surface distance (SSD) of 100 cm. Percentage depth dose (PDD) curves were acquired in a PTW BEAMSCAN water phantom using a PTW microDiamond and a PTW microSilicon detectors. Surface dose was estimated by applying a quadratic extrapolation to the PDD data within the 1.5–4 mm depth range closest to the water surface, with the extrapolated dose reported at 0.5 mm depth and normalized to the field-size-specific maximum dose (Dmax).
Results
The surface dose measured with microDiamond were 39%, 47%, 51%, 56%, and 61% across the 1×1, 5×5, 10×10, 20×20, and 30×30 cm² field sizes, respectively. Similarly, the microSilicon produced 44%, 45%, 49%, 47%, and 52%, for the same field sizes. Surface dose exhibited the expected dependence on field size, increasing from the smallest to the largest fields. Notably, for the largest field size, the microDiamond detector reported lower surface-dose values compared with the microSilicon detector. These results are consistent with prior surface-dose measurements using a PTW Advanced Markus chamber, which measured surface doses of 42% and 55% for the 1×1 and 30×30 cm² fields, respectively.
Conclusion
Surface dose was estimated using PDD extrapolation with microDiamond and microSilicon detectors, showing good agreement with prior Advanced Markus chamber measurements. Ongoing work includes additional detectors and extended geometries, including source-to-surface distance dependence.