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Characteristics of Microchambers for Dosimetry in Proton Beam Therapy

I Das

I Das1,2*, L Coutinho2, L Zhao2, M McEwen3, C Cheng1,2, (1) Indiana University- School of Medicine, Indianapolis, IN, (2) Indiana University Health Proton Therapy Center, Bloomington, IN, (3) National Research Council, Ottawa, ON

SU-E-T-81 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose:Small fields are frequently used in proton beam therapy for radiosurgery and patch-fields. Characterization of such fields requires small detectors. However, proper corrections such as ion recombination (Pion) and polarity (Ppol) need to be evaluated in proton beam which is the subject of this study.
Methods:Five microchambers from 3 different vendors (PTW: pinpoint; Standard imaging: A16, A14SL and IBA: CC01 and CC4) were evaluated. Proton reference condition (range-16 cm, SOBP 10 cm, 10x10cm2 field) was used for the measurements. All measurements were taken in a water phantom using dedicated cable and electrometers. Pion and Ppol were measured for various dose rates (beam current ranging from 0.2 nA to 8 nA).
Results:The readings versus voltage for most microchambers are relatively linear and do not change much. Our results indicate that all microchambers can be safely used in lower voltage range (100-300V) except A14SL where signal is abnormal. The data for Pion can be fitted for pulsed or continuous beam that are produced from Cyclotron at very high frequency. It is obvious that proton beam can be categorized by either of these beams and Pion calculated by either equation provides values that are within experimental uncertainty (0.4%). The magnitude of Pion is nearly identical to that of the photon beams. The dose rate change up to 16 fold does not have any effect on Pion for most chambers. The Ppol values are 1.017, 0.989, 1.005, 0.995 and 0.998 for pinpoint, A16, A14SL, CC01 and CC4 chambers respectively.
Conclusion:It is concluded that for microchambers 100-300V provide satisfactory reading with minimal (<0.5%) recombination effect. Polarity effect is also in the same order of magnitude as that of the other radiation beam. Except A14SL most microchambers are well suited for small field dosimetry in proton beam.

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