A feasibility study of Dynamic Phantom scanner for quality assurance of photon beam profiles at various gantry angles

Abstract

Although the availability of the multileaf collimator (MLC) has been a definite advance in the delivery of radiation therapy, the effect of gantry rotation on beam profiles and output factors during treatment with the MLC is an essential quality assurance issue. To address the variation of profiles at different gantry angles, a Dynamic Phantom scanner consisting of a 20x12x6 cm3 scanning Lucite block was designed as a cross-beam-profile scanner by Advanced Radiation Measurements Inc. (ARM). To our knowledge, differences between scanned profiles at different gantry angles acquired with a finite size of Lucite and full-size (60x60x50 cm3) water phantom had not previously been investigated. We, therefore, performed a feasibility study on a first prototype Dynamic Phantom scanner without the gantry attachment mount. Quantitative comparison of scanned profiles with the Dynamic Phantom and a full-size water phantom were performed at a 00 gantry angle. Radiation fields used in this preliminary study were defined by collimator jaws instead of the MLC to reduce the beam edge uncertainty to less than1 mm. As a constancy check, additional profiles with the gantry at 900 and 2700 with the same field sizes at 1 and 5 cm depth in Lucite were performed with both photon beam energies. Results obtained by comparing scanned profiles just inside the field edges for both 6 and 18 MV at 5cm depth of Lucite gave good agreement (less than 1% variation). Additionally, use of the Dynamic Phantom resulted in reduced penumbra width (about 0.5 mm out of 5-8 mm) and reduced (1-2%) scatter dose beyond the field edges for both 6 and 18 MV beams, compared with a standard water phantom scanner. In a constancy check, profiles scanned at 00 and 900/2700 gantry angles with the Dynamic Phantom revealed minimal variation in measured percentage profile and penumbra width. To examine the use of a Dynamic Phantom scanner on electron beams, similar profiles scanned with 6, 12, and 20 MeV electron beams were acquired with both scanners. Observed disagreements in profiles (3-4%) and penumbra width (3-4mm out of 12 mm) were larger for the electron beam than for photon beam. We conclude that the Dynamic Phantom scanner has the potential to be a useful device for the routine quality assurance on beam profiles of photon beams with jaws or MLC at various gantry angles.