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Reference radiochromic film dosimetry in kilovoltage photon beams during CBCT image acquisition
Author(s) -
Tomic Nada,
Devic Slobodan,
DeBlois François,
Seuntjens Jan
Publication year - 2010
Publication title -
medical physics
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.473
H-Index - 180
eISSN - 2473-4209
pISSN - 0094-2405
DOI - 10.1118/1.3302140
Subject(s) - imaging phantom , dosimetry , cone beam computed tomography , kerma , nuclear medicine , dose profile , ionization chamber , thermoluminescent dosimeter , image quality , image guided radiation therapy , thermoluminescent dosimetry , medical imaging , materials science , medicine , dosimeter , physics , radiology , computed tomography , computer science , ion , quantum mechanics , artificial intelligence , image (mathematics) , ionization
Purpose: A common approach for dose assessment during cone beam computed tomography (CBCT) acquisition is to use thermoluminescent detectors for skin dose measurements (on patients or phantoms) or ionization chamber (in phantoms) for body dose measurements. However, the benefits of a daily CBCT image acquisition such as margin reduction in planning target volume and the image quality must be weighted against the extra dose received during CBCT acquisitions. Methods: The authors describe a two‐dimensional reference dosimetry technique for measuring dose from CBCT scans using the on‐board imaging system on a Varian Clinac‐iX linear accelerator that employs the XR‐QA radiochromic film model, specifically designed for dose measurements at low energy photons. The CBCT dose measurements were performed for three different body regions (head and neck, pelvis, and thorax) using humanoid Rando phantom. Results: The authors report on both surface dose and dose profiles measurements during clinical CBCT procedures carried out on a humanoid Rando phantom. Our measurements show that the surface doses per CBCT scan can range anywhere between 0.1 and 4.7 cGy, with the lowest surface dose observed in the head and neck region, while the highest surface dose was observed for the Pelvis spot light CBCT protocol in the pelvic region, on the posterior side of the Rando phantom. The authors also present results of the uncertainty analysis of our XR‐QA radiochromic film dosimetry system. Conclusions: Radiochromic film dosimetry protocol described in this work was used to perform dose measurements during CBCT acquisitions with the one‐sigma dose measurement uncertainty of up to 3% for doses above 1 cGy. Our protocol is based on film exposure calibration in terms of “air kerma in air,” which simplifies both the calibration procedure and reference dosimetry measurements. The results from a full Monte Carlo investigation of the dose conversion of measured XR‐QA film dose at the surface into dose to water (or water kerma) at the surface of the phantom indicate that, for typical beam qualities used in CBCT, this conversion can be approximated by simple mass‐energy absorption coefficient ratios water‐to‐air.