Novel dosimetric phantom for quality assurance of volumetric modulated arc therapy
(a) Prototype of the ArcCHECK QA dosimeter along with a helical CT slice of the phantom showing the hollow cylindrical wall and the apparent density of the diode assemblies (b). A schematic of the dosimeter system (not to scale) is shown in (c).
(a) Comparison of Farmer ion chamber measurements, performed in a replica of the new dosimeter system, to the dose calculated with the TPS at the location of the sensitive volume of the chamber. The comparison was done for a 6 MV static beam at different gantry angles. The dosimeter system measurement (one ring of diodes) for a 6 MV static beam at gantry angle 0° is compared in (b) with the corresponding TPS dose calculation. All curves were normalized at 0° of gantry angle.
(a) Histogram of the relative number of diodes which agree with the TPS calculation within 3%/2 mm tolerance. The comparison was performed for the HN, sarcoma, and prostate plans with 90–360 CPs. The symbols in (a) represent the diode pass rate when the measurements for the 90 CP plans are alternately compared to the calculated the 90, 180, and 360 CP plans. The variation of pass rate for the 180 CP plans for different tolerance criteria is shown in (b).
(a) Calculated dose map along the cylindrical measurement surface for the head and neck VMAT plan with 180 control points. The red and blue dots represent the out of tolerance diodes with a dose superior and inferior to the TPS results, respectively. (b) The calculated dose profile (full line) at the level of the dashed line in (a) is compared to the dosimeter system measurement [● symbols, the color legend is the same as in (a)]. The relative diode pass rate for this plan was 86.6% for 3%/2 mm tolerance.
Increase in the number of diodes, which did not satisfy the tolerance of 3% or 2 mm for the HN, sarcoma, and prostate plans as a function of (a) gantry rotation offset and (b) scaling error. The dashed line represents the estimated reproducibility of the dosimeter system’s response and the treatment unit for the 180 CP head and neck plan (two SDs represent a variation of 2.4%).
Increase in the number of diodes, which did not satisfy the tolerance of 3% or 2 mm for the HN, sarcoma, and prostate plans as a function of phantom setup error in (a) RL and (b) AP directions. The dashed line represents the estimated reproducibility of the dosimeter system’s response and the treatment unit for the 180 CP head and neck plan (two SDs represent a variation of 2.4%).
Summary of tumor histology and dose prescription for the cases planned with VMAT in this study (fx: fraction; : 95% of prescribed dose; PTV: planning target volume).
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