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SU‐E‐T‐92: The Field Size Dependence of the ArcCheck 4D Diode Array and Its Correction Strategy
Author(s) -
Li J,
Yan G,
Lu B,
Liu C
Publication year - 2011
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 - 10.1118/1.3612043
Subject(s) - field size , linear particle accelerator , diode , ionization chamber , field (mathematics) , physics , beam (structure) , optics , square (algebra) , computational physics , mathematics , nuclear medicine , geometry , medicine , optoelectronics , ion , ionization , quantum mechanics , pure mathematics
Purpose: The ArcCheck 4D diode array was designed specifically for rotational therapy QA and records data every 50 milliseconds. This work quantifies its field size dependence and designs strategy for its correction. Methods: The field size dependence was measured using a 6 MV photon beam of a linac and compared with ion chamber measurements in the same geometry. Square field sizes of 4×4 cm2 to 30×30 cm2 as well as rectangular fields of 6×4 to 6×30 cm2 elongated in both the X and Y directions were measured. To correct for the field size dependence, the field size for each signal update was automatically detected and a correction factor that corresponded to the equivalent square was applied to the raw data. Five fixed‐gantry IMRT plans (18 fields) that covered a variety of sites were used to test the correction strategy. Treatment plans were generated using the Pinnacle3 (v9.0) with a density override of 1.05 g/cc for the entire cylinder. Only beams that did not go through the treatment couch were used for the comparison to eliminate the uncertainty associated with couch transmission. Results: Normalized to a field size of 10×10 cm2, the diodes over responded by 2.1% at 30×30 cm2 and under responded by 2.0% at 4×4 cm2. The biggest improvement in passing rates was seen in the head and neck plan which exhibited the largest treatment fields, with the average passing rates improved from 79.5% and 90.4% for 2%/2mm and 3%/3mm, respectively, when no field size dependence was corrected, to 95.7% and 98.7%, respectively, with field size dependence correction. Conclusions: The ArcCheck diodes exhibit a measurable field size dependence which can be corrected using its 4D raw data. This correction is important for large and small treatment fields which allows for the use of more stringent criteria.