Real-timein vivorectal wall dosimetry using plastic scintillation detectors for patients with prostate cancer

Abstract

We designed and constructed an in vivo dosimetry system using plastic scintillation detectors (PSDs) to monitor dose to the rectal wall in patients undergoing intensity-modulated radiation therapy for prostate cancer. Five patients were enrolled in an Institutional Review Board–approved protocol for twice weekly in vivo dose monitoring with our system, resulting in a total of 142 in vivo dose measurements. PSDs were attached to the surface of endorectal balloons used for prostate immobilization to place the PSDs in contact with the rectal wall. Absorbed dose was measured in real time and the total measured dose was compared with the dose calculated by the treatment planning system on the daily CT image dataset. The mean difference between measured and calculated doses for the entire patient population was −0.4% (standard deviation 2.8%). The mean difference between daily measured and calculated doses for each patient ranged from −3.3% to 3.3% (standard deviation ranged from 5.6% to 7.1% for 4 patients and was 14.0% for the last, for whom optimal positioning of the detector was difficult owing to the patient’s large size). Patients tolerated the detectors well and the treatment workflow was not compromised. Overall, PSDs performed well as in vivo dosimeters, providing excellent accuracy, real-time measurement, and reusability.

A method to correct for temperature dependence and measure simultaneously dose and temperature using a plastic scintillation detector

“…Variations of 0.1% to 0.5%/°C were reported depending on the type of scintillator used. This becomes particularly important when the PSD is calibrated at a temperature far from intended use conditions, as could be the case for a detector used in vivo (Wootton et al, 2014). One solution to this problem is to calibrate the PSD at the intended use temperature.…”

Passively scattered proton beam entrance dosimetry with a plastic scintillation detector

“…PSDs can therefore make measurements in a beam without significantly perturbing it (Beddar et al 2001). Finally, PSDs have been used for in vivo dosimetry in photon-based therapy already (Wootton et al 2014). Although there are some drawbacks to using PSDs, such as ionization quenching, which we address in the current study, PSDs are nonetheless promising candidates for in vivo entrance dosimetry in proton therapy.…”

In vivodosimetry: trends and prospects for brachytherapy

“…110,111 The balloons both immobilize the prostate during the treatment and allow for real-time IVD at well-defined points near the rectal wall. The dose rate to the rectal wall can be monitored in real time and compared with the expected dose rates from the TPS based on CT image data sets.…”

“…However, the dosimetric accuracy of IVD systems at non-equilibrium conditions should be further studied for BT. Alternatively for rectal wall dosimetry, the inflatable balloons discussed in this section could be filled with water, rather than air, in order to avoid non-equilibrium conditions, as was done for the endorectal balloon by Wootton et al 111 It is important to note that the positions of IVD probes inserted into BT source applicators, or embedded in the surface of inflatable balloons attached to intracavitary applicators, are correlated with the applicator positions, hence most dosemeters (with the exception of RADPOS) would not be able to reveal inter-or intrafraction organ-applicator movements, for example, caused by perineal oedema, 51 vaginal packing and/or source applicator clamping effects [47][48][49]52 or varying filling status of the rectum and/or bladder. 42,43,49 On the other hand, dosemeters inserted into catheters positioned in OARs are minimally correlated with source applicator movements, hence they could allow for the detection of organ-applicator movements.…”