Evaluation of multiple detectors and development of a novel detector for stereotactic radiotherapy/radiosurgery
Creators: name: Esen, Nsikan
Publisher: name: RMIT University
Year: 2024
Subjects: classificationCode: 510502
subject: Medical physics

description: Purpose: Advances in the field of radiation oncology have dramatically improved our ability to effectively target cancer cells. Techniques such as three-dimensional conformal radiotherapy, stereotactic radiotherapy, intensity modulated radiotherapy and volumetric modulated arc therapy allow the dose to be effectively shaped to maximize radiation delivered to the target while minimizing the dose to the surrounding tissue. It is vital to ensure a precise dose is delivered to the target volume and most of the above techniques mandate pre-treatment verification of planned fluences. This could be achieved by several dosimeters available in radiotherapy such as ionization chamber, diode, TLD, 2D Array, etc. Specifically, stereotactic ablative radiotherapy (SABR) uses small fields which mandate the use of miniaturized detectors for pre-treatment verification. Alanine and nanoDot are new classes of dosimeters that need to be characterized for pre-treatment verification. The aim of this project was to evaluate multiple detectors for SABR patient verification and develop a spherical array detector for verifying the dose delivered to cancer patients undergoing stereotactic radiotherapy/radiosurgery or conventional radiotherapy involving non-coplanar beams.<br><br>Methods: Prior to SABR pre-treatment verification, all detectors were assessed for dose linearity, energy, and directional and dose rate dependence. The Bruker EleXsys E500 EPR spectrometer of 9.5MHz was used to read the alanine pellet dosimeters signals; Harshaw QS 5500 automatic TLD reader was used for reading the TLDs and Dose 1 electrometer was used to read the charge response of microDiamond (PTW60019). Microstar Reader from Landauer Inc. was utilized to study the signal of the irradiated nanoDot OSL dosimeters while PC Electrometer from SunNuclear was used to collect the signal response of IN5626-diode distributed on the surface of the designed phantom. All irradiations were performed using a Clinac 21iX at 6 MV x-ray beam. A 3-D novel spherical phantom was designed using Via card software. In total, 120 holes were created on the surface of the phantom that allows the insertion of diodes (IN5626). The phantom was printed using CreatBot 3-D printing machine. Also to check the suitability of the evaluated detectors, 3-D phantoms were designed for each detector and were placed separately inside an indigenous Rod phantom made of Perspex to perform SABR pre-treatment patient verification. The statistical analysis, linear and curve fitting was done using OriginPro® 2018 (Origin Lab® Corporation).<br><br>Results: The relationship between dose measured using Alanine, TLD100H, nanoDot and microDiamond dosimeters, as well as a diode (IN5626), followed a linear fit of R2 = 0.9993, R2 = 0.9999, R2 = 1, R2 = 0.9999) and no significant difference between dose rate and energy was observed for the detectors. For all three sites (sternum, spine and scapula), the average measured and planned dose was recorded for alanine (18.29 ± 0.91 % and 18.57 ± 1.12 %)Gy , microDiamond (17.76 ± 0.65 % and 17.68 ± 0.63)Gy, nanoDot(18.29 ± 0.85 % and 18.64 ± 1.31 %)Gy and TLD100H (18.28 ± 0.70 % and 18.29 ± 0.85 %)Gy respectively. The differences between the measured and the Treatment Planning System ( TPS) computed dose was within 2% for Alanine, nanoDot, and microDiamond, and 3% with TLD100H respectively. In terms of ArcCheck and film dosimetry, the gamma criteria at 3%, 3 mm were found to correlate well with our non-SABR spine routine patient-specific QA results. Significant reduction in QA time on using ArcCheck for SABR QA in pre-treatment verifications. The results obtained from the novel spherical phantom designed shows excellent dose distribution on the surface of the sphere using python code compared well with the treatment planning system (TPS) dose distribution on the surface.<br><br>Conclusion: This study indicates that the evaluated detectors are consistent and agree well between the measured and the planned doses. Particularly, alanine and nanoDot indicates excellent dosimeter quality and therefore are confirmed as valuable dosimeters for SABR pre-treatment verification. ArcCheck results indicate that it could replace film dosimetry for all sites except SABR spine. A prototype spherical array detector has been designed and the diode is characterized by linearity, field size, dose rate, energy and angular dependence. A GUI software has been designed to read the Dicom RT plan and dose from the treatment planning system and the extracted dose correlated to the dose measured by the diodes.<br>