Passively scattered proton beam entrance dosimetry with a plastic scintillation detector

Abstract

We tested the feasibility of using plastic scintillation detectors (PSDs) for proton entrance dosimetry. A PSD built with BCF-12 scintillating fiber was used to measure the absolute entrance dose of a passively scattered proton beam for energies ranging from 140 MeV to 250 MeV, and for a range of spread out Bragg peak (SOBP) widths at 2 energies, to quantify the effect of ionization quenching on the response of the detector and to determine the necessity of Cerenkov radiation correction in proton beams. The overall accuracy and precision of the PSD was evaluated by measuring lateral beam profiles and comparing the results with profiles measured using film. The PSD under-responded owing to ionization quenching, exhibiting approximately a 7% loss of signal at the highest energy studied (250 MeV) and a 10% loss of signal at the lowest energy studied (140 MeV). For a given nominal energy, varying the SOBP width did not significantly alter the response of the PSD. Cerenkov radiation contributed negligibly to the PSD signal and can be safely ignored without introducing more than 1% error in the measured dose. Profiles measured with the PSD and film agreed to within the uncertainty of the detector, demonstrating good relative accuracy. Although correction factors were necessary to account for ionization quenching, the magnitude of the correction varied minimally over a broad range of energies; PSDs therefore represent a practical detector for proton entrance dosimetry.

Exradin W1 plastic scintillation detector for in vivo skin dosimetry in passive scattering proton therapy

“…We found that the average under-response of the PSD attributable to ionization quenching was 7.8% ± 1.0%, similar to the results published by Wootton et al [11]. The under-response for each field is shown in Figure 3.…”

“…The snout position was set at 30 cm. It is important to note that in a previous study by Wootton et al comparing a PSD with a plane-parallel ionization chamber, a snout position of 45 cm was used, whereas we used 30 cm, as required for patient treatment fields [11]. Thus, the air gap was 15 cm larger in the previous study than in the present study.…”

“…Typical distal and proximal margins for prostate patients are around 1.2 cm and 0.9 cm respectively (The distal margin is 3.5% of the range of the beam + 3 mm, and the proximal range is 3.5% of the (range-SOBP width) +3mm), which are much larger than the 0.28 cm thickness of the PSD. In evaluating the effect of differences in air gap/snout placement on the response of the PSD, we have shown that the quenching correction factor was similar to that used in the Wootton et al study when the PSD was used with small air gaps [11]. Using PSDs for general IVD in proton dosimetry (for example, measuring rectal wall dose) is a more difficult problem because the conditions would vary more from beam to beam and patient to patient and would be harder to determine (e.g.…”

“…The phantom used was a plastic water block (CIRS, Norfolk, VA, USA) with an ionization chamber slot. The PSD was attached in front of the ionization chamber and to its side outside of the active volume surface as demonstrated in figure 2 [11]. The snout position was set at 30 cm.…”

“…Therefore, a correction factor based on the patient’s skin temperature must be used. Ionization quenching can be corrected for by characterizing the PSD under-response compared with the response of an ionization chamber as a function of proton energy, as demonstrated in a previous study that characterized a PSD for use as a beam entrance dosimeter in a passively scattered proton beam [8,11].…”