The aim of this study was to demonstrate how dosimetry with an amorphous silicon electronic portal imaging device (EPID) replaced film and ionization chamber measurements for routine pre-treatment dosimetry in our clinic. Furthermore, we described how EPIDdosimetry was used to solve a clinical problem. IMRT prostate plans were delivered to a homogeneous slab phantom. EPID transit images were acquired for each segment. A previously developed in-house back-projection algorithm was used to reconstruct the dose distribution in the phantom mid-plane (intersecting the isocenter). Segment doseimages were summed to obtain an EPID mid-plane doseimage for each field. Fields were compared using profiles and in two dimensions with the evaluation (criteria: ). To quantify results, the average , maximum , and the percentage of points with were calculated within the 20% isodose line of each field. For 10 patient plans, all fields were measured with EPID and film at gantry set to . The film was located in the phantom coronal mid-plane ( depth), and compared with the back-projected EPID mid-plane absolute dose.EPID and film measurements agreed well for all 50 fields, with , , and . Based on these results, film measurements were discontinued for verification of prostate IMRT plans. For 20 patient plans, the dose distribution was re-calculated with the phantom CT scan and delivered to the phantom with the original gantry angles. The planned isocenter dose was verified with the EPID and an ionization chamber. The average ratio, , was 1.00 . Both measurements were systematically lower than planned, with and . EPID mid-plane doseimages for each field were also compared with the corresponding plane derived from the three dimensional (3D) dose grid calculated with the phantom CT scan. Comparisons of 100 fields yielded , , and . Seven plans revealed under-dosage in individual fields ranging from 5% to 16%, occurring at small regions of overlapping segments or along the junction of abutting segments (tongue-and-groove side). Test fields were designed to simulate errors and gave similar results. The agreement was improved after adjusting an incorrectly set tongue-and-groove width parameter in the treatment planning system (TPS), reducing from 2.19 to 0.80 for the test field. Mid-plane dose distributions determined with the EPID were consistent with film measurements in a slab phantom for all IMRT fields. Isocenter doses of the total plan measured with an EPID and an ionization chamber also agreed. The EPID can therefore replace these dosimetry devices for field-by-field and isocenter IMRT pre-treatment verification. Systematic errors were detected using EPIDdosimetry, resulting in the adjustment of a TPS parameter and alteration of two clinical patient plans. One set of EPID measurements (i.e., one open and transit image acquired for each segment of the plan) is sufficient to check each IMRT plan field-by-field and at the isocenter, making it a useful, efficient, and accurate dosimetric tool.
This work was financially supported by the Dutch Cancer Society (Grant no. NKI 2000-2255). The authors are indebted to Rene Tielenburg, Karel van Ingen, and Edwin Roosjen for assistance with measurement and calculation of patient plans on phantoms.
II. METHOD AND MATERIALS
II.A. Patient plans
II.C. Dose comparison methods
II.D. Verification of EPIDdoseimages with film
II.E. Verification of the planned dose: isocentre
II.F. Verification of the planned dose: field-by-field
II.G. Test fields
III.A. Verification of EPIDdoseimages with film
III.B. Verification of the planned dose: isocenter
III.C. Verification of the planned dose: field-by-field
III.D. Test fields
IV.A. Clinical application
IV.B. Replacing EDR2 film with EPIDdose verification
IV.C. Finding the source of the problem
IV.D. The TPS tongue-and-groove width parameter
IV.E. Future directions
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