Investigation of an implantable dosimeter for single‐point water equivalent path length verification in proton therapy

Abstract

Purpose:In vivo range verification in proton therapy is highly desirable. A recent study suggested that it was feasible to use point dose measurement for in vivo beam range verification in proton therapy, provided that the spread-out Bragg peak dose distribution is delivered in a different and rather unconventional manner. In this work, the authors investigate the possibility of using a commercial implantable dosimeter with wireless reading for this particular application. Methods: The traditional proton treatment technique delivers all the Bragg peaks required for a SOBP field in a single sequence, producing a constant dose plateau across the target volume. As a result, a point dose measurement anywhere in the target volume will produce the same value, thus providing no information regarding the water equivalent path length to the point of measurement. However, the same constant dose distribution can be achieved by splitting the field into a complementary pair of subfields, producing two oppositely "sloped" depth-dose distributions, respectively. The ratio between the two distributions can be a sensitive function of depth and measuring this ratio at a point inside the target volume can provide the water equivalent path length to the dosimeter location. Two types of field splits were used in the experiment, one achieved by the technique of beam current modulation and the other by manipulating the location and width of the beam pulse relative to the range modulator track. Eight MOSFET-based implantable dosimeters at four different depths in a water tank were used to measure the dose ratios for these field pairs. A method was developed to correct the effect of the well-known LET dependence of the MOSFET detectors on the depth-dose distributions using the columnar recombination model. The LET-corrected dose ratios were used to derive the water equivalent path lengths to the dosimeter locations to be compared to physical measurements. Results: The implantable dosimeters measured the dose ratios with a reasonable relative uncertainty of 1%-3% at all depths, except when the ratio itself becomes very small. In total, 55% of the individual measurements reproduced the water equivalent path lengths to the dosimeters within 1 mm. For three dosimeters, the difference was consistently less than 1 mm. Half of the standard deviations over the repeated measurements were equal or less than 1 mm. Conclusions: With a single fitting parameter, the LET-correction method worked remarkably well for the MOSFET detectors. The overall results were very encouraging for a potential method of in vivo beam range verification with millimeter accuracy. This is sufficient accuracy to expand range of clinical applications in which the authors could use the distal fall off of the proton depth dose for tight margins.

Dose detectors, sensors, and their applications

“…Two possible ways out are to use dose‐weighted correction factors for mixed fields or to correct the experimental dose distributions with beam data that match the measured depth‐dose curves …”

Noninvasive cardiac arrhythmia ablation with particle beams

“…The ultrasound image could be used for 4D‐imaging and subsequently to associate the detected emissions with a tissue location. An additional device like the Calypso would be needed to position the probe in a global coordinate system …”

Time‐resolved diode dosimetry calibration through Monte Carlo modeling for in vivo passive scattered proton therapy range verification

“…Sparing healthy tissue by reducing the applied margins provides motivation to develop millimeter accuracy in range verification techniques. The use of a small volume detector array for range measurements has been proposed,4, 5 which could provide the opportunity for in vivo range verification and be used on‐line to adapt the treatment plan and minimize these margins for passively scattered proton delivery.…”

“…Because the detector response is beam‐specific, experimental measurements in homogeneous media have been employed to establish a calibration curve of the response of the detector to WEPL for every SOBP that may be delivered for each given clinical case 4, 5, 6, 7, 8, 9, 10. This process is both tedious, as it necessitates a separate set of measurements for every new ‘scout’ beam (a 1 cm overshoot of the predicted detector depth with a dose of 4 cGy), as well as inconvenient due to the time constraints for access to the clinical beamline.…”