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Comparison of Two Methods for Minimizing the Effect of Delayed Charge On the Dose Delivered with a Synchrotron Based Discrete Spot Scanning Proton Beam

T Whitaker

T Whitaker1*, C Beltran1, N Remmes1, J Kruse1, M Bues2, E Tryggestad1, A Tasson1, M Herman1, (1) Mayo Clinic, Rochester, MN, (2) Mayo Clinic Arizona, Phoenix, AZ

SU-E-T-402 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: This study compares two methods for minimizing the effect of delayed charge on the dose delivered with a discrete-spot-scanning-proton beam. Delayed charge (DC) is the charge received after the planned MU are delivered.

Methods: The delivery of phantom and patient were simulated by applying normally distributed values for DC, with a mean of 0.001(SD 0.00025) MU, to each spot. Two correction methods (CM1, CM2) were used account for the DC. CM1 accounts for the DC by setting the beam monitor based on the cumulative received MU. CM2 reduces the beam monitor setting by a predicted value for the DC in addition to CM1. Each fraction was simulated and then recomputed in the treatment planning system. The dose difference between the original plan and the simulated treatment was evaluated. Both methods were tested while varying the dose per fraction from 0.5 to 2 Gy and the fractions per plan from 1 to 25.

Results: While varying the dose per fraction, the maximum dose difference was 0.37-0.39 Gy and 0.03-0.05 Gy for CM1 and CM2 respectively. While changing the total number of fractions, the maximum dose difference grew at a rate of 0.015 and 0.0018 Gy per fraction for CM1 and CM2 respectively. For CM1, the maximum dose difference corresponded to the first spot in each layer. For CM2, dose variations were only related to the variation in DC. CM1 and CM2 showed the same characteristics for each beam in the patient plans. The area of excess dose for each beam overlapped in the patients.

Conclusion: CM1 did not minimize DC effects for the first spot delivered in a layer. CM2 minimized the effect of the DC for all spots. DC effects and dose per fraction are not correlated. DC effects are proportional to the number of fractions and beams.

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