SU‐DD‐A1‐04: In Vivo Prostate IMRT Dosimetry With MOSFET Detectors Using Brass Build‐Up Caps

Abstract

Purpose: To develop a general formalism with various correction factors to predict dmax entrance dose with the new hemispherical brass buildup caps to be used with MOSFET detectors in anterior prostate IMRT fields and thereby integrate in vivo IMRT dose measurement as part of routine QA process in IMRT radiotherapy Method and Materials: We have used the new wide energy hemispherical build‐up caps for this study. Due to its high density and high atomic number it provides the minimal amount of metal needed to achieve full build‐up at Dmax for a range of photon energies. We have developed a general formalism to predict Dmax entrance dose by applying necessary correction factors after studying the response of MOSFET with brass build up caps for energy, dose rate, dose reproducibility, SSD and patient specific IMRT correction factor. Results: In vivo Prostate IMRT dose measurements with MOSFET detectors using brass buildup caps was performed and compared against dose predicted by two different treatment planning systems. We used both 6 MV and 10 MV for this study and compared the in vivo MOSFET detector reading with dose predicted by Philips Pinnacle (6 MV) and CMS XiO (10 MV) treatment planning systems respectively. We achieved a overall accuracy of better than ± 5% on measured patient doses. Conclusion: Routine IMRT QA in most institutions today only involves verifying the optimized fluence map delivered to the patient in a test phantom at a certain preset depth. Based on our work here, we believe adding in vivo IMRT dosimetry with MOSFET detectors using the new brass build up caps along with routine fluence map verification in phantoms and MLC quality assurance offers greater accuracy and confidence in actual dose delivered to the patient.

A metallic buildup cap for glass rod dosimeters in radiotherapeutic in vivo dosimetry

“…Since the phantom is generally considered an oversimplification of the patient's heterogeneous anatomy, and the phantom dedicated for routine pre-treatment QA is hard to be applied to some unique treatments such as total body irradiation, several detectors for in vivo measurements have been studied. Thermoluminescence dosimeters (TLDs) have been widely used over the past few decades [1][2][3], along with several other dosimeters that have improved the usability of TLD have been introduced, such as optically-stimulated luminescent dosimeters [4], metal-oxide-semiconductor field effect transistors (MOSFETs) [5], and glass rod dosimeters (GRDs) [6,7]. Owing to its high reproducibility and linearity and low dependencies with respect to major conditions of radiation treatments such as beam energy, dose rate, field size, and source-to-surface distance [8], GRD has been used as a primary in vivo dosimeter in our clinic.…”

“…In 2006, the characteristics of a metallic buildup cap dedicated to MOSFET dosimeters were studied by Varadhan et al [5] Using the buildup cap, they estimated an overall accuracy of better than 5% when compared to doses calculated using RTP for both 6 and 10 MV and reported a total uncertainty of 4.6%. As the buildup cap was hemispherical, made of highdensity metal, both size and beam angle dependence could be minimized.…”