SU-E-T-544: An Empirical Formula to Obtain Tissue-Phantom Ratios From Percentage Depth Dose Curves for Small Fields

Abstract

Purpose: For small photon fields, accurate values of tissue-phantom ratios (TPR) are difficult to obtain either by direct measurement or by the conventional method of converting from measured percentage-depth doses (%dd). This study aims to develop an empirical method to accurately obtain TPRs from %dd curves for small radiosurgery beams. Methods: The Monte Carlo simulation codes BEAMnrc/DOSXYZnrc were used to simulate the accelerator head and accessory. The Monte Carlo directly calculated TPR values as a function of depth were compared with TPRs converted from %dd curves in a water phantom for field sizes ranging from 4 mm diameter to 10×10 cm2 fields. Direct measurements of TPRs were performed with the detector remaining fixed at an SAD of 100 cm and increasing the detector depth by adding water. Results: Using the Monte Carlo values, we developed an empirical formula to obtain TPRs from %dd. The conventional method of obtaining TPRs from %dd underestimate TPR by 3.4% and 0.6% at a depth 1.5 cm and overestimate TPR by 6.2% and 1.7% at a depth of 25 cm for 4 mm and 30 mm diameter circular fields, respectively. The empirical formula is derived from realistic Monte Carlo simulations using field sizes ranging from 4 mm to 30 mm and depth ranging from 1.5 cm to 25 cm. TPRs calculated using this function deviate from TPRs directly calculated from Monte Carlo by less than 0.5%. The accuracy of this empirical formula is validated against the directly measured TPRs in water. Conclusion: The developed empirical method has the potential to greatly simply the work in obtaining TPRs from measured %dd curves for small fields. By using this developed empirical formula the uncertainties between directly measured TPRs and converted TPRs from measured %dd curves are within 1%.