S1649
Physics - Detectors, dose measurement and phantoms
ESTRO 2026
Digital Poster 2355 Sterilized EBTXD radiochromic film dosimetry. A first approach. Íñigo Recio Gil 1 , Cristina Torcal Arguis 2 , Carla Cases Copestake 1 1 Medical Physics, Hospital Clínic Barcelona, Barcelona, Spain. 2 Surgical Area, Hospital Clínic Barcelona, Barcelona, Spain Purpose/Objective: Radiochromic films are suitable for intraoperative in- vivo dosimetry, but they undergo a sterilization process that causes visible darkening, affecting dosimetry. No information on this effect is available in the literature. This work aims to develop a dosimetry procedure that accounts for sterilization-induced changes. Material/Methods: EBTXD films (Gafchromic) were used with a flatbed scanner (EPSON 12000XL) in transmission mode and software for pixel value (PV) reading (Radiochromic.com). Sterilization was performed with plasma H ₂ O ₂ (V-PRO maX2) using two cycles differing in charge and duration: standard (EBTXDSTD, Lumen1) and express (EBTXDEXP, Fast Non-Lumen1).Irradiation was done with a LIAC (Sordina, Italy) accelerator and a RW3 (PTW) water-equivalent phantom at 10 MeV and 1.6 cm depth (SSD 71.3 cm). PV was measured as the mean within a 2.5×6.0 cm ROI at the film center. Dosimetry was performed using the three-channel method described by Micke et al. (2011)2.Films were sterilized and scanned in separate batches to assess darkening and process repeatability, allowing times between sterilization and irradiation (trad) of 2–150 hours. Films were then irradiated with doses of 2–19 Gy and scanned 24 hours later alongside a non- sterilized, non-irradiated reference film (EBTXDREF). Time dependence was assessed and corrected to obtain virtual calibration curves (VCC) at case-specific trad. Three irradiated EBTXDEXP films with different combinations of trad and dose (47.9 h, 14.3 Gy; 54.6 h, 8.7 Gy; 25.4 h, 3.5 Gy) were used to compare results with a VCC (trad) and a non-sterilized calibration curve (CC). Results: Sterilized films showed lower PV than EBTXDREF. Batch PV standard deviation did not exceed 0.9% for EBTXDEST and 1.6% for EBTXDEXP. PV showed a linear dependence on trad (R² > 0.6), although some blue channel doses deviated slightly (Figure 1). Regression lines varied with dose, channel and cycle and allowed determination of a case-specific VCC (trad) (Figure 2).For the three EBTXDEXP films, dosimetry relative errors (ε) were 9.8%, 17.2% and 20.0% using the non- sterilized CC and 5.6%, 3.4% and 20.0% using the VCC
Bland-Altman plots exhibited negative average deviations between paired GPRs, indicating lower scores for Octavius-4D measurements, especially on IMRT plans. For Test 1&2, the mean biases were -3.4 and -2.9 percentage points, respectively (for VMAT only, -1.2 and -0.7). Adopting 95% as tolerance level [2], the fractions of successful plans were: 24/40 for Octavius-4D, 36/40 for EPIDoct, and 33/40 for EPIDpat; for VMAT only, 24/30, 27/30 and 26/30, respectively. Conclusion: The implemented models achieved satisfactory accuracy for both RC-CCC kernel and RC EPID-air module. For VMAT plans, pre-treatment verification showed similar performances to Octavius system. Further work is ongoing to establish center-specific tolerance and action limits for EPID-dosimetry in clinical pre-treatment QA, based on the collected results. References: [1] Esposito M, Baldoni R, Bossuyt E, et al. A commissioning protocol for portal imaging-based radiotherapy in vivo dosimetry systems. Phys Imaging Radiat Oncol. 2024;32:100666. [2] Miften M, Olch A, Mihailidis D, et al. Tolerance limits and methodologies for IMRT measurement-based verification QA: Recommendations of AAPM Task Group No. 218. Med Phys. 2018;45(4):e53-e83. Keywords: EPID dosimetry, Secondary-check, VMAT plans
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