Clinical experience with EPID dosimetry for prostate IMRT pre-treatment dose verification
- Department of Radiation Oncology, Netherlands Cancer Institute-Antoni van Leeuwenhoek Hospital, Amsterdam (Netherlands)
The aim of this study was to demonstrate how dosimetry with an amorphous silicon electronic portal imaging device (a-Si EPID) replaced film and ionization chamber measurements for routine pre-treatment dosimetry in our clinic. Furthermore, we described how EPID dosimetry 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 dose images were summed to obtain an EPID mid-plane dose image for each field. Fields were compared using profiles and in two dimensions with the {gamma} evaluation (criteria: 3%/3 mm). To quantify results, the average {gamma} ({gamma}{sub avg}), maximum {gamma} ({gamma}{sub max}), and the percentage of points with {gamma}<1(P{sub {gamma}}{sub lt1}) 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 0 deg. . The film was located in the phantom coronal mid-plane (10 cm depth), and compared with the back-projected EPID mid-plane absolute dose. EPID and film measurements agreed well for all 50 fields, with <{gamma}{sub avg}>=0.16, <{gamma}{sub max}>=1.00, and <P{sub {gamma}}{sub lt1}>=100%. 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 (plan{sub iso}) was verified with the EPID (EPID{sub iso}) and an ionization chamber (IC{sub iso}). The average ratio, <EPID{sub iso}/IC{sub iso}>, was 1.00 (0.01 SD). Both measurements were systematically lower than planned, with <EPID{sub iso}/plan{sub iso}> and <IC{sub iso}/plan{sub iso}>=0.99 (0.01 SD). EPID mid-plane dose images 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 <{gamma}{sub avg}>=0.39, {gamma}{sub max}=2.52, and <P{sub {gamma}}{sub lt1}>=98.7%. 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 <{gamma}{sub max}> 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 EPID dosimetry, 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.
- OSTI ID:
- 20853607
- Journal Information:
- Medical Physics, Vol. 33, Issue 10; Other Information: DOI: 10.1118/1.2230810; (c) 2006 American Association of Physicists in Medicine; Country of input: International Atomic Energy Agency (IAEA); ISSN 0094-2405
- Country of Publication:
- United States
- Language:
- English
Similar Records
In vivo dosimetry by an aSi-based EPID
Accurate two-dimensional IMRT verification using a back-projection EPID dosimetry method