Date Published: July 13, 2017
Publisher: Public Library of Science
Author(s): Jong Min Park, So-Yeon Park, Chang Heon Choi, Minsoo Chun, Ji Hye Han, Jin Dong Cho, Jung-in Kim, Ganesh Chandra Jagetia.
To investigate the dosimetric characteristics of PRESAGEREU dosimeters.
Commercially available PRESAGEREU dosimeters (size of 10 mm × 10 mm × 45 mm) were divided into two groups, with one of the groups placed at room temperature of 22°C (RT group) and another group placed at low temperature of 10°C (LT group). A total of 3 dosimeters (set of dosimeters) were irradiated at a time, with doses of 1 Gy, 2 Gy, 4 Gy, 8 Gy, 12 Gy, 16 Gy, and 20 Gy, at a nominal dose rate of 400 MU/min at temperature of 22°C. The dosimeters were irradiated three additional times by delivering the same doses as those during the initial irradiations (4 irradiation cycles). Optical density (OD) was assessed using optical CT scanning.
Considering both linearity and sensitivity of the OD curves, R2 above 0.95 and sensitivity above 0.04 ΔOD/Gy were observed at the 1st irradiation (reading time ≤ 6 h) and 2nd irradiation (reading time = 0.5 h) for the RT group. For the LT group, those values were observed at the 1st irradiation (reading time ≤ 2 h), and the 3rd and 4th irradiations (both reading times = 0.5 h). Considering the reproducibility of signals in response to the same dose, dosimeters in the RT group showed average deviations among dosimeters less than 5% (the 1st and 2nd irradiations at the reading time of 0.5 h), while for dosimeters in the LT group showed average deviations among dosimeters less than 6% (the 3rd and 4th irradiations at the reading time of 0.5 h). For the rest, the OD curves were not linear, sensitivities of the dosimeters were lower than 0.04 ΔOD/Gy, and OD deviations at the same dose were larger than 6%.
At room temperature, PRESAGEREU dosimeters could be used for dose measurement only for up to two dose measurement sessions. At low temperatures, usage of PRESAGEREU dosimeters for dose measurement seems to be possible from the 3rd irradiation. When reusing PRESAGEREU dosimeters, the OD curve should be re-defined for every measurement session because the shape of this curve depends on the irradiation history.
Intensity modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT) can deliver prescription doses to target volumes while minimizing doses to radiosensitive organs at risk (OARs) located near the target volumes [1, 2]. By virtue of this capability, IMRT and VMAT can achieve superior tumor control while at the same time reducing radiotherapy-associated complications. To apply IMRT and VMAT in the clinical setting, careful verification of a treatment plan by measuring the delivered dose distribution is essential before proceeding to treating patients, because the inverse planning procedures of IMRT and VMAT are not intuitive [3, 4]. In addition, IMRT and VMAT are more susceptible to errors because those techniques generally generate steep dose gradients near the target volumes [3, 4]. In this respect, pre-treatment patient-specific quality assurance (QA) for both IMRT and VMAT is highly recommended and routinely performed in the clinical setting [3, 4]. Pre-treatment QA typically involves the measurement of a planar dose map of some kind, followed by two-dimensional (2D) gamma evaluation .
In this study, we investigated dosimetric characteristics of the reusable 3D radiochromic dosimetry material, PRESAGEREU. We investigated the effects of the number of reuses, the measurement time post-irradiation, the temperature, and the dose rate, on the linearity and sensitivity associated with PRESAGEREU. For the RT group, the temporal decay of the dosimeter signal following the initial irradiation was regular; however, the decay patterns following re-irradiations were irregular. For the 2nd irradiation of the RT group, PRESAGEREU signals reached the background level only at 6 h following irradiation, while for the 1st irradiation the signal reached the background level at 48 h following irradiation. The PRESAGEREU signals in response to the 3rd and 4th irradiations of dosimeters in the RT group did not decay, and were not fully cleared out. On the contrary, for dosimeters in the LT group, the temporal decay following the 3rd and 4th irradiations was more stable than that in response to the 1st and 2nd irradiations. In the case of the LT group, PRESAGEREU signals reached the background level at 48 h following irradiation, regardless of the irradiation history. Based on the decay patterns, we conclude that reusing PRESAGEREU at room temperature does not yield reliable results. Considering both linearity and sensitivity of the OD curves, R2 above 0.95 and sensitivity above 0.04 ΔOD/Gy were observed following the 1st irradiation (reading time ≤ 6 h) and 2nd irradiation (reading time = 0.5 h) for the RT group, while those values were observed for the LT group following the 1st irradiation (reading time ≤ 2 h), and the 3rd and 4th irradiations (both reading times = 0.5 h). In addition, considering the reproducibility of signals in response to the same dose, at the reading time 0.5 h following the 1st and 2nd irradiations dosimeters in the RT group yielded reliable values (average deviations among dosimeters < 5%), while for dosimeters in the LT group reliable values were obtained following the 3rd and 4th irradiations, for the same reading times of 0.5 h (average deviations among dosimeters < 6%). In these cases, the PRESAGEREU dosimeters exhibited no dependence on the dose rate, as shown in Figs 5 and 6. Thus, it seems that at room temperature PRESAGEREU dosimeters can be used up to two times. At lower temperatures (such as 10°C) PRESAGEREU dosimeters can likely be reused. The relationship between the signals (ΔOD) and doses, i.e., the OD curves, should be re-defined for every reuse because the OD curves depend on the irradiation history of PRESAGEREU. The reusability of PRESAGEREU at room temperature was not satisfactory, with PRESAGEREU usability extending only to two irradiations. The behavior of PRESAGEREU at low temperature (approximately 10°C) was different from that at room temperature, and it seems that at low temperature PRESAGEREU stabilized with increasing the number of reuses. Although PRESAGEREU reusability at low temperatures and short reading times post-irradiation seems feasible, the OD curve should be defined for every reuse because of its dependence on the number of reuses. In addition, for reliable PRESAGEREU dosimetry, the ΔOD values should be read at short reading times and the reading times must be consistent. Accurate PRESAGEREU–based dosimetry should be performed with care. Source: http://doi.org/10.1371/journal.pone.0180970