Supervisors info:
Ελευθέριος Παππάς, Επίκουρος Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Παντελής Καραΐσκος, Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Παναγιώτης Παπαγιάννης, Καθηγητής, Ιατρική Σχολή, ΕΚΠΑ
Summary:
Background: Optically Stimulated Luminescence Dosimeters (OSLDs) are passive detectors, that present many favorable characteristics (high sensitivity, multiple readings of the same irradiation session, short waiting time between irradiation and readout etc.) to be incorporated in dosimetry protocols for advanced Radiotherapy applications (Stereotactic Radiosurgery / Radiotherapy and Stereotactic Body Radiotherapy – SRS/SRT and SBRT).
Materials/Methods: The myOSLchip (RadPro, Germany) dosimetry system was employed throughout this study, comprising BeO-based dosimeters with sensitive volume of 4.65 x 4.65 x 0.5 mm3, a reader and a bleacher. The dose-response characteristics and relevant dependencies were assessed according to the AAPM TG-191 guidelines. More specifically, OSLDs individual sensitivity factors, depletion rates and linearity were determined for two batches in the range 52-2600 MU (~0.5-25 Gy). Beam quality dependency (6 and 10MV) and orientation (en face and edge on, where the BeO disk is perpendicular and parallel to the beam’s direction, respectively) was quantified. Moreover, the short and mid-term fading rates were studied for the first 24 hours and up to 25 days post irradiation, respectively, at three dose levels (0.5, 3, 6 Gy) and logarithmic regression curves were fitted. Potential dose rate dependency was investigated in the range of 60-1400 MU/min. Sensitivity changes with increasing lifetime accumulated dose was studied for 7 irradiation-readout-bleaching cycles of 14 Gy each. Furthermore, 17 dosimeters were implemented in 4 SRS/SRT and 2 SBRT plan verification procedures, employing anthropomorphic phantoms with bone-mimicking inhomogeneities (Prime head phantom and SBRT upper abdomen phantom, RTSafe, Greece). The treatment plans were extracted from the Treatment Planning Systems in DICOM format for further processing and comparison with the measured dose distributions. Agreement between calculations and measurements was quantified using the γ index test, considering global and local criteria of 5%/1.5mm, 5%/1mm, 3%/2mm, 3%/1.5mm and 3%/1mm, based on the relevant experimental uncertainties involved (2.8%, 0.5 mm).
Results: Individual sensitivity correction factors in the range 0.559-1.851 with 0.222 standard deviation and 0.891-1.268 with 0.078 standard deviation were determined for the 2 batches, respectively. An individual depletion factor was calculated for each OSLD in the range 1.008-1.031 with 0.006 standard deviation and 1.007-1.013 with 0.001 standard deviation for the 2 batches, respectively. Dose-response was found linear for doses up to 2 Gy and sublinear in the range of 2-25 Gy, with corresponding correction factors of 1.01 and 1.04 in the ranges 2-6 Gy and 6-25 Gy, respectively. There was no significant energy dependence within uncertainties, but the dosimeters exhibit directional
dependence of 2.8% (±2.1%) in the edge on orientation. Fading rates of <16% in the first 24 hours and <19% for the first 25 days post irradiation (normalized to the signal obtained at 3 hours and 1 day, respectively) were calculated while being independent (within uncertainties) on dose, at least in the dose range investigated. Dosimeters’ response is not significantly affected (within uncertainties) by the dose rate. Dose-response sensitivity remains stable up to ~98 Gy of accumulated dose. Plan verification procedures exhibit 100% pass rates for all criteria with 2 mm distance to agreement, whereas 1 of the 4 SRS/SRT plans fails (<90%) the γ index test if a 1-mm distance to agreement criterium is applied. Correspondingly, the γ index test is passed in all applied criteria for the SBRT plans, if a 5% dose cut-off threshold is considered.
Conclusions: A comprehensive characterization of BeO OSLDs was successfully performed, in accordance with the AAPM TG-191 guidelines, which included the basic dose-response dependencies and the necessary correction factors. The dosimetric system investigated was found suitable for implementation in quality assurance protocols of advanced radiotherapy applications (SRS/SRT and SBRT).
Keywords:
Radiotherapy, OSLDs, SRS/SRT, SBRT, γ index
