Development and characterization of an interferometer for calorimeter-based absorbed dose to water measurements in a medical linear accelerator

The quantity of relevance for external beam radiotherapy is absorbeddose to water (ADW). Aninterferometer was built, characterized, and tested to measure ADW within thedose range of interest for external beam radiotherapy using the temperature dependence of therefractive index of water. Theinterferometer was used to measure radiation-induced phase shifts of alaser beam passing through a (10 × 10 × 10) cm3 water-filled glass phantom,irradiated with a 6 MV photon beam from a medicallinear accelerator. Thefield size was (7 × 7) cm2 and thedose was measured at a depth of 5 cm in the water phantom. The intensity of the interference pattern was measured with aphotodiode and was used to calculate the time-dependent phase shift curve. The system was thermally insulated to achieve temperature drifts of less than 1.5 mK/min. Data were acquired 60 s before and after theirradiation. The radiation-induced phase shifts were calculated by taking the difference in the pre- and post-irradiation drifts extrapolated to the midpoint of theirradiation. For 200, 300, and 400 monitor units, the measureddoses were 1.6 ± 0.3, 2.6 ± 0.3, and 3.1 ± 0.3 Gy, respectively. Measurements agreed within theuncertainty withdose calculations performed with a treatment planning system. The estimated type-A,k = 1uncertainty in the measureddoses was 0.3 Gy which is an order of magnitude lower than previously published interferometer-based ADW measurements.
Source: Review of Scientific Instruments - Category: Physics Authors: Source Type: research