Dose evaluation of an NIPAM polymer gel dosimeter using gamma index

Polymer gel dosimeters for pretreatment radiotherapy verification using the three-dimensional gamma evaluation and pass rate maps

Polymer gel dosimeters (PGDs) have been widely studied for use in the pretreatment verification of clinical radiation therapy. However, the readability of PGDs in three-dimensional (3D) dosimetry remain unclear. In this study, the pretreatment verifications of clinical radiation therapy were performed using an N-isopropyl-acrylamide (NIPAM) PGD, and the results were used to evaluate the performance of the NIPAM PGD on 3D dose measurement. A gel phantom was used to measure the dose distribution of a clinical case of intensity-modulated radiation therapy. Magnetic resonance imaging scans were performed for dose readouts. The measured dose volumes were compared with the planned dose volume. The relative volume histograms showed that relative volumes with a negative percent dose difference decreased as time elapsed. Furthermore, the histograms revealed few changes after 24h postirradiation. For the 3%/3mm and 2%/2mm criteria, the pass rates of the 12- and 24-h dose volumes were higher than 95%, respectively. This study thus concludes that the pass rate map can be used to evaluate the dose-temporal readability of PGDs and that the NIPAM PGD can be used for clinical pretreatment verifications.

Evaluation of accuracy and precision in polymer gel dosimetry

PURPOSE

To assess the overall reproducibility and accuracy of an X-ray computed tomography (CT) polymer gel dosimetry (PGD) system and investigate what effects the use of generic, interbatch, and intrabatch gel calibration have on dosimetric and spatial accuracy.

METHODS

A N-isopropylacrylamide (NIPAM)-based gel formulation optimized for X-ray CT gel dosimetry was used, and the results over four different batches of gels were analyzed. All gels were irradiated with three 6 MV beams in a calibration pattern at both the bottom and top of the dosimeter. Postirradiation CT images of the gels were processed using background subtraction, image averaging, adaptive mean filtering, and remnant artifact removal. The gel dose distributions were calibrated using a Monte Carlo (Vancouver Island Monte Carlo system) calculated dose distribution of the calibration pattern. Using the calibration results from all gels, an average or "generic" calibration curve was calculated and this generic calibration curve was used to calibrate each of the gels within the sample. For each of the gels, the irradiation pattern at the bottom of the dosimeter was also calibrated using the irradiation pattern at the top of the dosimeter to evaluate intragel calibration.

RESULTS

Comparison of gel measurements with Monte Carlo dose calculations found excellent dosimetric accuracy when using an average (or generic) calibration with a mean dose discrepancy of 1.8% in the low-dose gradient region which compared to a "best-case scenario" self-calibration method with a mean dose discrepancy of 1.6%. The intragel calibration method investigated produced large dose discrepancies due to differences in dose response at the top and bottom of the dosimeter, but the use of a dose-dependent correction reduced these dose errors. Spatial accuracy was found to be excellent for the average calibration method with a mean distance-to-agreement (DTA) of 0.63 mm and 99.6% of points with a DTA < 2 mm in high-dose gradient regions. This compares favorably to the self-calibration method which produced a mean DTA of 0.61 mm and 99.8% of points with a DTA < 2 mm. Gamma analysis using a 3%/3 mm criterion also found good agreement between the gel measurement and Monte Carlo dose calculation when using either the average calibration or self-calibration methods (96.8% and 98.2%, respectively).

CONCLUSIONS

An X-ray CT PGD system was evaluated and found to have excellent dosimeteric and spatial accuracy when compared to Monte Carlo dose calculations and the use of generic and interbatch calibration methods were found to be effective. The establishment of the accuracy and reproducibility of this system provides important information for clinical implementation.

Small-Field Measurements of 3D Polymer Gel Dosimeters through Optical Computed Tomography

With advances in therapeutic instruments and techniques, three-dimensional dose delivery has been widely used in radiotherapy. The verification of dose distribution in a small field becomes critical because of the obvious dose gradient within the field. The study investigates the dose distributions of various field sizes by using NIPAM polymer gel dosimeter. The dosimeter consists of 5% gelatin, 5% monomers, 3% cross linkers, and 5 mM THPC. After irradiation, a 24 to 96 hour delay was applied, and the gel dosimeters were read by a cone beam optical computed tomography (optical CT) scanner. The dose distributions measured by the NIPAM gel dosimeter were compared to the outputs of the treatment planning system using gamma evaluation. For the criteria of 3%/3 mm, the pass rates for 5 × 5, 3 × 3, 2 × 2, 1 × 1, and 0.5 × 0.5 cm² were as high as 91.7%, 90.7%, 88.2%, 74.8%, and 37.3%, respectively. For the criteria of 5%/5 mm, the gamma pass rates of the 5 × 5, 3 × 3, and 2 × 2 cm² fields were over 99%. The NIPAM gel dosimeter provides high chemical stability. With cone-beam optical CT readouts, the NIPAM polymer gel dosimeter has potential for clinical dose verification of small-field irradiation.