Correction factor measurements for multiple detectors used in small field dosimetry on the Varian Edge radiosurgery system.

PURPOSE

Accurate dosimetry of small fields remains a challenge to the clinical physicist. Choosing the appropriate detector and determination of kQclin,Qmsr (fclin,fmsr) factors continue to be an area of active research. The purpose of this study is to evaluate the output factors for a dedicated stereotactic accelerator using multiple dosimeters designed for use in small fields and evaluate published kQclin,Qmsr (fclin,fmsr) factors relative to measured values using a commercial scintillating fiber.

METHODS

Four microionization chambers, a commercial plastic scintillation detector, and a semiconducting diode were used to measure output factors for a linear accelerator. Field sizes ranging from 6 × 6 to 0.6 × 0.6 cm(2) were measured in a water phantom at 10 cm depth for 100 cm SSD. All microionization chambers were mounted in both vertical and horizontal configurations. Fields were normalized to the output at 5 × 5 cm(2). Output correction factors, kQclin,Qmsr (fclin,fmsr), were calculated as the ratio of a detector response relative to the scintillating fiber response for a given clinical field size, fclin.

RESULTS

Ionization chambers consistently under-responded for small fields relative to the scintillating fiber. Variations in response between horizontal and vertical mounting were most notable for the microchambers, with the vertical mounting which reduced the magnitude of the necessary correction factor, kQclin,Qmsr (fclin,fmsr), for the microionization chambers ranging from 1.1 to 1.2 for the smallest field size at all energies. The semiconducting diode over-responded by 7% for the smallest field size across all energies, resulting in a kQclin,Qmsr (fclin,fmsr) of ∼ 0.93.

CONCLUSIONS

The commercial scintillating fiber, which produces accurate and consistent ratios of dose to water for nonstandard fields, can be used to measure correction factors for various detectors used in a clinical setting. This can allow for comparison of measured correction factors to previously published values.