In vivo prostate IMRT dosimetry with MOSFET detectors using brass buildup caps

Raj Varadhan,John Miller,Brenden Garrity,Michael Weber

doi:10.1120/jacmp.v7i4.2278

Abstract

The feasibility of using dual bias metal oxide semiconductor field effect transistor (MOSFET) detectors with the new hemispherical brass buildup cap for in vivo dose measurements in prostate intensity‐modulated radiotherapy (IMRT) treatments was investigated and achieved. In this work, MOSFET detectors with brass buildup caps placed on the patient's skin surface on the central axis of the individual IMRT beams are used to determine the maximum entrance dose (Dmax) from the prostate IMRT fields. A general formalism with various correction factors taken into account to predict Dmax entrance dose for the IMRT fields with MOSFETs was developed and compared against predicted dose from the treatment‐planning system (TPS). We achieved an overall accuracy of better than ±5% on all measured fields for both 6‐MV and 10‐MV beams when compared to predicted doses from the Philips Pinnacle 3 and CMS XiO TPSs, respectively. We also estimate the total uncertainty in estimation of MOSFET dose in the high‐sensitivity mode for IMRT therapy to be 4.6%.PACS numbers: 87.53Xd, 87.56Fc

Highlights

Routine intensity-modulated radiotherapy (IMRT) quality assurance in most institutions only involves verifying the optimized fluence map delivered to the patient in a test phantom at a certain preset depth
We have developed a general formalism with all the correction factors taken into account to predict Dmax entrance dose for anterior IMRT fields
The results indicate that the minimum dose that can be measured in relation to the sensitivity of Metal oxide semiconductor field effect transistor (MOSFET) is around 25 cGy, where the percent standard deviation falls below 1%, which is consistent with the results reported elsewhere.[4]. At a low dose of 5 cGy, our percent standard deviation was 5.3% compared to a standard deviation of 20% for the same dose, as reported in the literature.[4]. This is because our measurements were made in the high-sensitivity mode, while the data in Ref. 4 were collected in the low-sensitivity mode

Summary

Introduction

Routine intensity-modulated radiotherapy (IMRT) quality assurance in most institutions only involves verifying the optimized fluence map delivered to the patient in a test phantom at a certain preset depth. Reconstruction of actual patient dose delivered to the patient through electronic portal imaging systems is still in its infancy. In this scenario, verification of actual dose delivered to the patient through in vivo dosimetry can play a vital part in the entire chain of the quality assurance process. Metal oxide semiconductor field effect transistor (MOSFET) dosimeters with plastic water buildup caps have been in use in our institution for some time to verify routine patient entrance Dmax doses, and we have had excellent agreement with predicted doses as reported by other groups.[1,2,3] More recently, Thomson Nielsen, the manufacturer of MOSFETs, has introduced single, wide-energy hemispherical buildup caps for patient dosimetry that have full buildup at Dmax for all photon energies.

Methods

Results

Conclusion