Abstract
Portal dosimetry (PD) was performed for 181 fields from 14 IMRT plans of various clinical sites at gantry zero and source‐to‐detector distance (SDD) of 100 cm. PD was realized using aSi1000 electronic portal imaging device (EPID) and portal dose prediction (PDP) algorithm implemented in Eclipse treatment planning system (TPS). Agreement of PDP predicted and EPID measured photon fluence/dose distribution were evaluated using gamma (γ) index set at 3% at 3 mm distance to point agreement (DTA). Three gamma scaling parameters, maximum γ(γ)max, average γ(γ)avg and percentage of points with γ≤1(γ%≤1) were estimated for each field. An independent measurement was carried out using MatriXX 2D ion chamber array with detector plane at 100 cm and γmax,γavg and γ%≤1 were estimated using OmniPro IMRT analyzing software. Effect of extended SDD and gantry rotation on portal dosimetry outcome was also investigated for another 45 IMRT fields. PDP predicted and EPID measured photon fluence agrees well with overall mean values of γmax,γavg and γ%≤1 1 at 2.02, 0.24 and 99.43%, respectively. γ(γ)max value was lower in 15 MV compared to 6 MV IMRT plan. Independent verification using MatriXX showed comparable overall mean values of γ(γ)avg and γ%≤1 at 0.25 and 99.80%. However, in all plans, MatriXX showed significantly lower γmax(p<0.05) with an overall mean value of 1.35. In portal dosimetry, compared to gamma values at 100 cm SDD, γmax,γavg and γ%≤1 values improve from a mean of 0.16, 0.03 and 0.26 at 110 cm SDD to 0.35, 0.05 and 0.29 at 140 cm SDD. PD outcome was independent of gantry rotation. In conclusion, both MatriXX 2D ion chamber array and portal dosimetry showed comparable results and can be use as an alternative to each other for relative photon fluence verification.PACS number (s): 87.55.D‐, 87.55.de, 87.55.kd,87.55.km,87.55.Qr,87.56Fc:
Highlights
239 Sharma et al.: Comparison of portal dosimetry and 2D detector array for intensity-modulated radiotherapy (IMRT) quality assurance (QA).its excellent dose and dose rate linearity, reproducibility, and insignificant energy and field size dependence for megavoltage photon beams.[6,8,10]Van Herk[11] proposed using electronic portal imaging device (EPID), available for patient set-up verification for dose measurement
The commissioning of portal dosimetry involves the configuration of portal dose prediction (PDP) software and calibration of EPID response at different locations of the imager to the dose measured by a reference dosimeter following manufacturer recommended calibration protocol.[22]. Calibration was performed in special, integrated portal image acquisition mode for 6 and 15 MV X-ray at 100 and 140 cm source-to-detector distance (SDD) and 400 monitor units (MU)/min, which is the dose rate used for IMRT delivery
In plans using portal dosimetry (PPD), γmax was found to be lesser for IMRT plans with 15 MV X-rays, as compared to 6 MV
Summary
239 Sharma et al.: Comparison of portal dosimetry and 2D detector array for IMRT QA.its excellent dose and dose rate linearity, reproducibility, and insignificant energy and field size dependence for megavoltage photon beams.[6,8,10]Van Herk[11] proposed using electronic portal imaging device (EPID), available for patient set-up verification for dose measurement. The dosimetric characteristics and calibration procedures of various types of EPIDs, strategies to use EPIDs for dose verification, clinical approaches to EPID dosimetry and current clinical experiences were extensively reviewed and reported in a recent article by van Elmpt et al[12] The majority of the studies reporting EPID dosimetry have focused on dose response characteristic of detectors, and various methods and algorithms in use.[11,12,13,14,15,16,17] a few have demonstrated the usefulness of portal dosimetry for pretreatment verification of IMRT, mostly using their in-house developed portal image to dose conversion software.[18,19,20,21] While limited data on the clinical application of these two emerging quality assurance (QA) methods are available separately, to our knowledge, no data are available comparing MatriXX results with that from portal dosimetry for the same IMRT fluence. Effect of gantry rotation and extended sourceto-detector distance (SDD) were investigated on portal dosimetry outcome
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