Dosimetric characterization of a body‐conforming radiochromic sheet

Abstract Purpose A novel radiochromic PRESAGE sheet (Heuris Inc.) with 3 mm thickness has been developed as a measurement tool for 2D dosimetry. Its inherent ability to conform to irregular surfaces makes this dosimeter advantageous for patient surface dosimetry. This study is a comprehensive investigation into the PRESAGE sheet’s dosimetric characteristic, accuracy and its potential use as a dosimeter for clinical applications. Methods The characterization of the dosimeter included evaluation of the temporal stability of the dose linearity, reproducibility, measurement uncertainties, dose rate, energy, temperature and angular dependence, lateral response artifacts, percent depth dose curve, and 2D dose measurement. Dose distribution measurements were acquired for regular square fields on a flat and irregular surface and an irregular modulated field on the smooth surface. All measurements were performed using an Epson 11000XL high‐resolution scanner. Results The examined dosimeters exhibit stable linear response, standard error of repeated measurements within 2%, negligible dose rate, energy, and angular dependence. The same linear dose response was measured while the dosimeter was in contact with a heated water surface. Gamma test and histogram analysis of the dose difference between PRESAGE and EBT3 film, PRESAGE and the treatment planning system (TPS) were used to evaluate the measured dose distributions. The PRESAGE sheet dose distributions showed good agreement with EBT3 film and TPS. A discrepancy smaller than the statistical error of the two dosimeters was reported. Conclusions This study established a full dosimetric characterization of the PRESAGE sheets with the purpose of laying the foundation for future clinical uses. The results presented here for the comparison of this novel dosimeter with those currently in use reinforce the possibility of using this dosimeter as an alternative for irregular surface dose measurements.


| INTRODUCTION
The dosimetric characteristic of PRESAGE polyurethane dosimeters (Heuris Inc., Skillman, New Jersey, NJ, USA), formulated with a halogenated hydrocarbon free radical initiator and leuco dye has been thoroughly investigated and is well-characterized. 1 plan. 4 Additionally, PRESAGE was used to measure Gamma Knife Perfexion output factors in 2014. 5 The excellent agreement between the measurements acquired from PRESAGE and the manufacturer's Monte-Carlo based values indicates that PRESAGE is a suitable dosimeter for 3D dosimetry and for clinical research. It is important to mention that small discrepancies were observed between EBT3 film and PRESAGE measurement for low energy applications and small field dosimetry. 7 The main advantages of using PRESAGE over other types of dosimeters include its linear response to the absorbed dose over a wide dose range, its tissue-equivalency over a wide energy range, and its capability to provide 3D dose distribution with high resolution. Additionally, the dosimeter can be fabricated to any size or shape for customized clinical and research purposes.
Despite the positive outcomes of using PRESAGE as a 3D dosimeter, 3D dosimetry has not been commonly used in a clinical setting due to the need of specialized equipment to measure 3D dose distributions such as an optical computed tomography [3][4][5][6] scanner as well as the need of expert users to operate the equipment and analyze the data. Unlike the cylindrical PRESAGE dosimeters, PRESAGE sheets can be scanned with a flatbed scanner similar to the use of EBT3 radiochromic film (Ashland Inc.). The advantage of PRESAGE sheets over EBT3 film is its ability to conform to the patient's body contours.
For instance, the use of radiochromic films for in-vivo dosimetry has been questioned due to the problem of air gaps between the film and surrounding tissue. 9,10 To avoid the dosimetric errors caused by air gaps, EBT3 films are cut into small pieces and carefully taped when mounted on curved surfaces. Therefore, it cannot be used to measure in-vivo dose distribution over large areas. For dose calibration of the PRESAGE sheets, calibration curves are not necessary for relative dosimetry due to its well-known characteristic of having a linear response to the absorbed dose. Therefore, unlike radiochromic film, the accuracy of the measurements does not heavily depend on the calibration process. In 2015, a published study by Dumas et

2.C | Correction of lateral response artifact
In this study, the methodology using coefficient correction developed by Lewis et al. 13

2.E | 2D dose distribution measurements
For the verification of 2D dose measurements, measurements in three different scenarios were performed; regular square field measurement on flat surface, on irregular surface and irregular IMRT field on smooth surface without build up. In this study, EBT3 films were employed as the reference dosimeter for 2D dose measurement due to its capability of the stable dose response and fine resolution. 9 two dosimeters were exposed to 3 and 200 cGy. The readings are within 2% of the exposed dose level. In addition, the surface of the phantom is smooth so that a whole EBT3 film can be taped on the surface without air gaps between the film and the surface. Five   Fig. 4(b)].
In addition, the decay rate of the optical density is higher at the beginning and then gradually decreases.

3.B | Correction of lateral response artifact
In this section, the correction coefficients for the lateral response artifact are reported. It is important to mention that the obtained coefficients (A,B) are scanner-specific. The behavior of a certain scanner needs to be characterized at each institute. The study by Lewis et al. 13 shows that the coefficients are independent of the dose given for a specific lateral position, which corresponds to the results of our study. Figure 7 shows

3.D.3 | Irregular IMRT field on smooth surface
To evaluate the accuracy of relative dose measurement on the surface, the measured dose distribution of a modulated photon field using PRESAGE-LCV sheets was compared with those from the EBT3 film. Gamma test with 2%/2 mm criteria showed a passing rate of 99.1% (Fig. 12). In addition, the histogram of relative dose difference from pixel subtraction showed a nearly normal distribution with a mean of 0 and a standard deviation of 1.46%, which is within the statistical error of the two dosimeters (Fig. 13).
Additionally, the relative point dose measurements taken at the measured points from both OSLDs and PRESAGE-LCV sheets were compared to EBT3 films. To obtain the relative dose, the measured doses at five points from the OSLDs were normalized to the OSLD placed in the center. Table 2   In addition to the comparison of square fields, modulated dose measurements of PRESAGE-LCV sheets and OSLDs were compared with EBT3 films. From the results, OSLDs exhibit larger discrepancies than PRESAGE-LCV sheets due to their comparatively larger statistical and experimental uncertainty, the non-tissue-equivalence and changes in the OSLD response as a function of energy and dose. 16,17 The greater discrepancy in the relative dose measurements between OSLDs could be attributed to the uncertainty in the sensitivity of an individual dosimeter as well as the uncertainty in the performance of the reader used to measure the dose extracted from the OSLDs. 17 The difference in sensitivity between OSLDs may be ascribed to the slight differences in the amount of dosimetry material (Al 2 O 3 :C) contained in the dosimeter. 17 In addition, discrepancies may arise from the non-tissue-equivalent material (Al 2 O 3 :C

| CONCLUSIONS
This study investigates the dosimetric characteristics and the potential use of a novel dosimeter for in-vivo 2D dose measurement. The examined radiochromic sheets can be conformed to the patients' surface and therefore provide accurate relative dose measurement over large areas. Its dosimetric accuracy is comparable to EBT3 films with negligible energy, dose rate and angular dependence. Compared with OSLDs, the dosimeters exhibit less statistical error and higher resolution. Further investigation into its absolute dosimetry capability and other clinical and research applications, such as a replacement of bolus, will be conducted in the future.

CONFLI CT OF INTEREST
The dosimeters were provided by Heuris Inc. free of charge.