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

Abstract

In vivo skin dosimetry is desirable in passive scattering proton therapy because of the possibility of high entrance dose with a small number of fields. However, suitable detectors are needed to determine skin dose in proton therapy. Plastic scintillation detectors (PSDs) are particularly well suited for applications in proton therapy because of their water equivalence, small size, and ease of use. We investigated the utility of the Exradin W1, a commercially available PSD, for in vivo skin dosimetry during passive scattering proton therapy. We evaluated the accuracy of the Exradin W1 in six patients undergoing proton therapy for prostate cancer, as part of an Institutional Review Board-approved protocol. Over 22 weeks, we compared in vivo PSD measurements with in-phantom ionization chamber measurements and doses from the treatment planning system, resulting in 96 in vivo measurements. Temperature and ionization quenching correction factors were applied on the basis of the dose response of the PSD in a phantom. The calibrated PSD exhibited an average 7.8% under-response (±1% standard deviation) owing to ionization quenching. We observed 4% under-response at 37 °C relative to the calibration-temperature response. After temperature and quenching corrections were applied, the overall PSD dose response was within ±1% of the expected dose for all patients. The dose differences between the PSD and ionization chamber measurements for all treatment fields were within ±2% (standard deviation 0.67%). The PSD was highly accurate for in vivo skin dosimetry in passively scattered proton beams and could be useful in verifying proton therapy delivery.

Keywords: In vivo dosimetry; Plastic scintillation detector; Proton therapy; Skin dose.

Figure 1

The Exradin W1 scintillator and SuperMAX Electrometer.

Figure 2. A sketch of the beams-eye view of the in-phantom setup

The ion chamber (dashed line) is placed in its slot inside the water-equivalent phantom. The center of the ion chamber is placed at the central axis. The Exradin W1 scintillator was placed in front of the ionization chamber and to its side outside of the active volume surface.

Figure 3. Percentage of plastic scintillation detector under-response that was attributable to ionization quenching in the phantom for each treatment field

Error bars represent the standard deviation of at least three repeated measurements.

Figure 4. ⁶⁰Co calibration results

A: Distribution of normalized baseline measurements. The distribution of the measurements was centered very close to zero and fell within a range of ±2%. B: Weekly ⁶⁰Co baseline measurements of plastic scintillation detector dose response normalized to the average dose delivered across 22 weeks and corrected for source decay. All measurements fell within ±1%. Error bars represent the standard deviation of repeated measurements.

Figure 5. Dose differences by field for each patient

Dose difference between the plastic scintillation detector and plane-parallel ionization chamber for each field (left and right lateral fields) by patient.

Figure 6

Dose differences between the plastic scintillation detector and plane-parallel ionization chamber for all patient skin dose measurements acquired.