An experimental method to calculate the on-axis dose in small field for stereotactic radiotherapy
Purpose: The use of small fields in advanced radiotherapy techniques has increased, in particular in stereotactic treatments. However, measuring on-axis dose in such fields is challenging. In this study, we developed an analytic model to accurately estimate the on-axis dose in small fields.
Methods: Our study was carried out using 6 MV photon beams from four linear accelerators and with three dosimeters placed in a water tank: EBT3 Gafchromic films, a 31016 PinPoint ionization chamber and a 60017 E diode. The out-of-field leakage factor defined as the ratio of the central axis dose to the off-axis dose was modeled. On-axis doses estimated from out-of-field measurements were compared with the measured ones.
Results: The experimental validation of the present method was performed for square and rectangular fields with sizes ranging from 0.5 ⨯ 0.5 cm2 to 10 ⨯ 10 cm2. We found the leakage factor exhibits an exponential decrease independent of the accelerator. This behavior can be integrated in the model to estimate the on-axis dose with an agreement better than 2% compared to EBT3 film measurements at a 10 cm depth and an 8 cm cross-plane off-axis distance.
Conclusion: We have developed an analytic model to estimate the on-axis dose in small fields based on the out-of-field leakage measurement. This model can be used to validate dose and output factor measurements.
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