Evaluation of a Commercial 4-D Diode Array for Helical Tomotherapy Plan Verification

The performance of a commercial 4-D diode array, the ArcCHECK (Sun Nuclear, FL, USA) has been tested for helical tomotherapy patient-specific quality assurance (QA). Seventeen helical tomotherapy plans were selected representing a variety of tumour sites. Agreement between measured and calculated dose distributions was assessed by evaluating the gamma index (γ) with criteria ranging from 2% dose differences or 2 mm distance to agreement (2%/2 mm) to 3%/3 mm. For a subset of plans, some delivery errors were voluntarily introduced in all the four rotational-translational directions to investigate the identification ability of the device. For a single plan point, dose comparison was carried out in the low dose gradient region in order to quantify the amount of dose deviation as a function of the absolute dose delivered. QA plans were also calculated and delivered to radiochromic films (GafChromic EBT2, ISP Corp., USA) within a cylindrical solid water phantom for comparison. For the 16 plans delivered, the mean percentage value of points with γ < 1 was 97.2% ± 2.2%, 94.2% ± 4.6%, 95.1% ± 3.8% and 90.8% ± 6.2% calculated with (3%/3 mm), (3%/2 mm), (2%/3 mm) and (2%/2 mm) criteria, respectively, demonstrating that the 4-D diode array can be used for rotational IMRT QA. Four of the 'wrong' measurements still had a good QA response showing a number of points >90% fulfilling γ < 1 calculated with the 3%/3 mm criterion. The number of 'false positives' decreased with tighter γ calculations (3%/2 mm and 2%/3 mm) and disappeared with 2%/2 mm. Point to point comparison showed a mean dose deviation of 1.2% ± 1.1%, 1.9% ± 1.6% and 2.1% ± 1.3% for low, medium and high dose regions, respectively. The mean percentage value of points with γ < 1 was 96.3% ± 3.3% and 91.4% ± 2.4% calculated with (3%/3 mm) for diode array and film dosimetry showing a consistent superiority of the device with respect to GafChromic film (p < 0.05). (author)