Dosimetry of small fields has become increasingly important with the rise in popularity of intensity-modulated radiotherapy (IMRT), volumetric-modulated arc therapy (VMAT) and stereotactic body radiotherapy (SBRT). However, conventional point dose measurements are time-consuming to set up and subject to several sources of uncertainty.

First proposed in 2010, the dose-area-product ratio (DAPR) is the ratio of DAP at two depths and is analogous to the tissue phantom ratio (TPR). The DAP is the product of the absorbed dose measured by the chamber and its active cross-sectional area. Measured using large-area plane-parallel chambers (LAC) with a cross-sectional area wider than the radiation field, the DAPR eliminates positioning and volume averaging uncertainties. As part of routine dosimetry QA, as well as acceptance and commissioning testing, measurement of the parameter could be used to trace doses measured in the clinic to absolute doses measured at a standards laboratory.

Detector measurements

In the latest study of DAPR, collaborators from Turku University Hospital (Tyks), Tampere University Hospital (Tays) and the Radiation and Nuclear Safety Authority of Finland (STUK) measured DAP values with two commercial chambers and performed Monte Carlo simulations. The detectors had active area diameters of 81.6 and 39.6 mm. They were used to measure the DAP in water of 6 MV photon beams collimated with 50 × 50 mm fixed jaws combined with cones ranging in diameter from 4 to 40 mm. Measurements and simulations at 10 and 20 cm deep were used to calculate DAPR20,10.

The experiments demonstrated that the set-up of the chambers was easier and quicker than that needed for the point measurement of TPRs, and that measurement of DAPR20,10 was feasible. For the first time, DAPR20,10 was also shown to have a field size dependence. Measured and simulated values dropped sharply with increasing field size then plateaued at field diameters greater than 20 mm. Beam collimation is thought to be the key underlying cause, with the dependency resulting from differences in collimator scatter between the different cones, first author Jarkko Niemelä of Tyks told medicalphysicsweb.

Measurement and simulation results also provided the first evidence of DAPR20,10 dependence on chamber size. Across all field sizes, DAPR20,10 values measured using the larger chamber were an average of 5.2% greater and up to 6.8% greater than for the smaller chamber. The researchers attribute this dependence to variations in dose contributions from low-energy scattered photons. The contributions at 20-cm deep increase with the distance from the beam central axis, relative to those at 10-cm deep, resulting in higher DAPR20,10 values. Simulations provided evidence consistent with this explanation, showing a significant relative increase in the off-axis ratio at 20-cm deep with distance from the beam central axis. Based on the finding, chambers used to make DAPR20,10 measurements must be reported along with their measurements, concluded the researchers.

In a broader conclusion, the two detectors used in the study are promising candidates for DAPR20,10 measurements in small fields, Niemelä told medicalphysicsweb. However, significant work is needed before DAPR20,10 can be recommended as a replacement for TPR20,10. "There should be a thorough investigation and comparison of the whole dosimetry processes, including the comparison of uncertainties, reproducibilities, costs and reliabilities of these methods," said Niemelä.

Following up their study, the collaboration is looking to identify an optimal chamber size that exhibits minimal variation in DAPR20,10 across field sizes and with beam energy. The researchers are also investigating the behaviour of DAPR20,10 in different types of small fields, including those defined using collimator jaws and multileaf collimators.

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