Author: Wesley S. Culberson, Ryan Gardner, Patrick M Hill, Mark Pankuch, Karsten K. Wake ๐จโ๐ฌ
Affiliation: Department of Medical Physics, School of Medicine and Public Health, University of Wisconsin - Madison, Department of Human Oncology, University of Wisconsin School of Medicine and Public Health, University of Wisconsin, Northwestern Medicine Proton Center ๐
Purpose: To assess the dosimetric capabilities of Sun Nuclear (Melbourne, FL) EDGE Diode Detectorsโข for measurements in the plateau region of pencil beam scanning (PBS) fields. Many clinical dosimeters, including diodes, exhibit changes in response near the Bragg Peak of proton fields due to changes in LET, complicating quality assurance (QA) for proton arc therapy. This study evaluates the Sun Nuclear diodes in a region of relatively constant LET to evaluate the potential applicability of the ArcCHECKยฎ, with its cylindrical diode array, as a QA device for proton arc therapy.
Methods: Monoenergetic PBS fields were created in RayStation (RaySearch Laboratories, Stockholm, Sweden) for 14 cm x 10 cm fields of uniformly weighted spots distributed with a spacing of 0.5 ฯ, delivering 1 Gy at the measurement depth of 3.3 cm in water. A custom detector holder enabled simultaneous readings from a calibrated Advanced Markus ion chamber (PTW Freiburg GmbH) and two EDGE diodes, scaling diode signals to physical dose. Energy and dose rate dependence were assessed by measuring the relative response of the two diodes while varying energy (100 to 220 MeV) and beam current (20 to 136 nA). Radiation damage was quantified with relative response measurements in proton (160 MeV) and electron (6 MeV) fields up to 400 Gy.
Results: Dose linearity was confirmed up to 10 Gy while energy and dose rate dependence tests yielded relative response within 1.5% and just over 2%, respectively. Substantial signal degradation of 5โ9% per 100 Gy was observed in proton fields, whereas electron exposure caused less than 0.2% per 100 Gy.
Conclusion: The diodes exhibited dose linearity and acceptable levels of energy and dose rate dependence. However, due to substantial degradation with accumulated proton dose, any QA device incorporating these diodes would require frequent recalibration in clinical proton fields.