Abstract
Purpose: We have developed a fast algorithm to calculate patient specific 2D distribution of absolute dose per MU (D/MU) for megavoltage photon beams. Method and Materials: To calculate D/MU correctly, one has to perform the scatter‐integration with proper weights to account for the primary fluence distribution. In our PC‐based MU‐calculation program, the ratio of scatter‐to‐primary dose, Ds/Dp, for a 2‐D non‐uniform field is calculated as: urn:x-wiley:00942405:media:mp1880:mp1880-math-0001 with and is the 2D intensity profile within the IMRT field, pi is 1 inside field and 0 outside field collimation for segment field i of the IMRT field. This convolution is calculated using a Fast Fourier Transform. The parameters a, w, and d0 characterize the phantom scatter properties and can be determined from measured PDD. Results: SPR can be calculated on an Intel Pentium 2.8GHz PC for 0.5‐cm grid size in less than 0.1 second for a 80×80 grid covering 40 × 40 cm2. For IMRT patients examined, the standard deviation of all comparisons is 1.5±3.9%. Conclusion: This algorithm is ideally suitable for calculating patient‐specific MU for quality assurance of patient specific IMRT fields for photon beams. The major attraction is the fast speed, which allows potential real‐time applications.
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