TH-CD-BRA-06: Proton Beam Entrance Dosimetry with the Exradin W1 Plastic Scintillation Detector.

PURPOSE: To assess the feasibility of using a commercial plastic scintillation detector, the Exradin W1, for precise, accurate entrance dosimetry in a passively scattered proton beam

METHODS: Measurements of passively scattered proton beam entrance dose were performed using an Exradin W1 calibrated on a Cobalt-60 unit, and a calibrated parallel plane ion chamber at energies from 140 to 250 MeV. A fixed spread out Bragg peak (SOBP) width of 8 cm was used at all energies except 140 and 225 MeV, where selected SOBP widths spanning all those available were used. Comparisons between the ion chamber and W1 measured dose were used to determine whether and how much the W1 under-responded in the presence of the high linear energy transfer radiation (a phenomenon called ionization quenching, well known to occur at depth in proton beams where LET reaches its maximum). The W1's precision was also evaluated by taking the coefficient of variation of repeated measurements.

RESULTS: From 200 MeV to 250 MeV the W1 under-responded by approximately 7% ±1%, and from 140 MeV to 180 MeV it under-responded by approximately 10% ±1%. The under-response was consistent within these bounds and negligibly affected by the SOBP width. An empirical correction factor with only two discrete values (one corresponding to energies greater than or equal to 200 MeV, and another for lower energies) was sufficient to produce better than 1% agreement between the W1 and ion chamber for all measurements. The W1 exhibited excellent precision, better than 1% at all energies but for the lowest where it was less than 2%.

CONCLUSION: The Exradin W1 was found to be capable of precise, accurate entrance dosimetry with the application of a simple correction factor based on proton beam energy. This project was supported in part by award number CA182450 from the National Cancer Institute.