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SU‐GG‐T‐213: Portal Dosimetry for Pre‐Treatment Verification of IMRT Plan: A Comparison with 2D Ion Chamber Array
Author(s) -
Sharma D,
Mhatre V,
Heigrujam M,
Talapatra K,
Mallik S
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 - 10.1118/1.3468602
Subject(s) - dosimetry , nuclear medicine , ionization chamber , fluence , radiation treatment planning , image guided radiation therapy , detector , medical imaging , physics , optics , radiation therapy , medicine , ion , radiology , laser , quantum mechanics , ionization
Purpose : To compare portal dosimetry (PD) and 2D pixel ion chamber (MatriXX) for pre‐treatment verification of photon fluence. Method and Materials : PD was performed for 181 fields from 14 IMRT plans of various clinical sites 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 were evaluated using gamma (γ) index set at 3% at 3 mm. Three γ 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 with detector plane at 100 cm and γ max , γ avg and γ%≤1 were estimated using Omnipro IMRT analyzing software. Effect of extended source to detector distance (SDD) and gantry rotation on PD outcome was also investigated for another 45 IMRT fields. Results : PDP predicted and EPID measured photon fluence agrees well with overall mean values of γ max , γ avg and γ%≤1 at 2.02, 0.24 and 99.43% respectively. 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 avg and γ%≤1 at 0.25 and 99.80%. However, in all plans, MatriXX showed significantly lower γ max (p<0.05) with an overall mean value of 1.35. In PD, compare to γ values at 100 cm SDD, γ max , γ avg and γ%≤1 values improve from a mean of 0.16, 0.03 and 0.25 at 110 cm SDD to 0.34, 0.05 and 0.29 at 140 cm SDD. PD outcome was independent of gantry rotation. Conclusion : Both PD and MatriXX showed comparable result and can be use as an alternative to each other for relative photon fluence verification. While portal dosimetry reduces measurement time, absolute dose measurement needs further investigation.