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The Correlation of Empirical Estimation of Second Hand Monitor Unit Calculation in Proton Pencil Beam Treatment

J Syh

J Syh1*, J Syh2 , H Wu3 , (1) Willis-Knighton Medical Center, Shreveport, LA, (2) Methodist Hospital, Houston, TX, (3) Willis-Knighton Medical Center, Shreveport, LA


SU-I-GPD-T-112 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: In order to obtain monitor unit (MU) calculations in proton pencil beam scanning system, a handful of parameters were collected from TPS. Total MU per individual pencil beam with its prescribed dose, planning PTV, evaluation PTV, total number of spots per beam, number of energy layers and treatment depth were all playing a particular role in the second MU calculations.

Methods: Utilize Proton Therapy unit, the Monitor Unit (MU) was defined as charges, that is, the number of protons collected on dose pad of Ion Chamber which is the primer dose counter for 1 cGy of dose. A particular parameter was defined as charge per MU. Such a parameter is predefined according to the type and revision of Ion Chamber (IC) used in the nozzle. For other ICs, their parameter is calibrated to match the primary reading and it is about known nano-coulombs. Formulated MU calculation was independent of collecting charges and predict directly from treatment planning separately.

Results: Calibration protocol provides absorbed dose determination was crucial to associate dose with MU’s. Compared to proton passive scattering beam, which include beam energy, field size, water equivalent depth and source to point of calibration distance, the MU for PBS can be computed by the components directly and estimate the total Monitor Units which has been formulated and illustrated. R square of a DOM factor was estimated 0.921 with deviation of 0.2 to 5.6% range in 22 beams.

Conclusion: A Singe Field Uniform Dose MU calculation formula was tested in 22 individual field used for patient prostate treatment with specific patient QA process for gamma index analysis additionally. Other treatment sites are also included to form the analytic equation to have second chance to be checked with similar calculation approaches for monitor units of proton pencil beams.

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