Hashimoto Shimpei, Fujita Yukio, Katayose Tetsurou, Mizuno Hideyuki, Saitoh Hidetoshi, Karasawa Katsuyuki
Department of Radiation Oncology, Tokyo Metropolitan Cancer and Infectious Diseases Center Komagome Hospital, 3-18-22 Honkomagome, Bunkyo-ku, Tokyo, Japan.
Department of Radiation Oncology, Tokai University School of Medicine, 143 Shimokasuya, Isehara-shi, Kanagawa, Japan.
Med Phys. 2018 Jan;45(1):382-390. doi: 10.1002/mp.12665. Epub 2017 Dec 5.
We evaluated the energy responses of a radiophotoluminescent glass dosimeter (RPLD) to variations in small-field and intensity-modulated radiation therapy (IMRT) conditions using experimental measurements and Monte Carlo simulation.
Several sizes of the jaw and multileaf collimator fields and various plan-class IMRT-beam measurements were performed using the RPLD and an ionization chamber. The field-size correction factor for the RPLD was determined for 6- and 10-MV x rays. This correction factor, together with the perturbation factor, was also calculated using Monte Carlo simulation with the EGSnrc/egs_chamber user code. In addition, to evaluate the response of the RPLD to clinical-class-specific reference fields, the field-size correction factor for the clinical IMRT plan was measured.
The calculated field-size correction factor ranged from 1.007 to 0.981 (for 6-MV x rays) and from 1.012 to 0.990 (for 10-MV x rays) as the jaw-field size ranged from 1 × 1 cm to 20 × 20 cm . The atomic composition perturbation factor for these jaw fields decreased by 3.2% and 1.9% for the 6- and 10-MV fields, respectively. The density perturbation factor was unity for field sizes ranging from 3 × 3 cm to 20 × 20 cm , whereas that for field sizes ranging from 3 × 3 cm to 1 × 1 cm decreased by 3.2% (for 6-MV x rays) and 4.3% (for 10-MV x rays). The volume-averaging factor rapidly increased for field sizes below 1.6 × 1.6 cm . The results for the MLC fields were similar to those for the jaw fields. For plan-class IMRT beams, the field-size correction and perturbation factors were almost unity. The difference between the doses measured using the RPLD and ionization chamber was within 1.2% for the clinical IMRT plan at the planning-target volume (PTV) region.
For small fields of size 1.6 × 1.6 cm or less, it was clarified that the volume averaging and density perturbation were the dominant effects responsible for the variation in the RPLD response. Moreover, perturbation correction is required when measuring a field size 1.0 × 1.0 cm or less. Under the IMRT conditions, the difference in the responses of the RPLD between the reference conditions and the PTV region calculated by Monte Carlo simulation did not exceed 0.8%. These results indicate that it is feasible to measure IMRT dosage using an RPLD at the PTV region.
我们通过实验测量和蒙特卡罗模拟，评估了放射性光致发光玻璃剂量计（RPLD）对小射野和调强放射治疗（IMRT）条件变化的能量响应。
使用RPLD和电离室对几种大小的准直器 jaws 和多叶准直器射野以及各种计划类IMRT射束进行了测量。确定了6兆伏和10兆伏X射线的RPLD射野大小校正因子。还使用带有EGSnrc/egs_chamber用户代码的蒙特卡罗模拟计算了该校正因子以及扰动因子。此外，为了评估RPLD对临床类特定参考射野的响应，测量了临床IMRT计划的射野大小校正因子。
随着准直器 jaws 射野大小从1×1厘米变化到20×20厘米，计算得到的射野大小校正因子范围为1.007至0.981（对于6兆伏X射线）和1.012至0.990（对于10兆伏X射线）。这些准直器 jaws 射野的原子组成扰动因子对于6兆伏和10兆伏射野分别降低了3.2%和1.9%。对于射野大小从3×3厘米到20×20厘米，密度扰动因子为1，而对于射野大小从3×3厘米到1×1厘米，密度扰动因子降低了3.2%（对于6兆伏X射线）和4.3%（对于10兆伏X射线）。对于射野大小小于1.6×1.6厘米，体积平均因子迅速增加。多叶准直器射野的结果与准直器 jaws 射野的结果相似。对于计划类IMRT射束，射野大小校正和扰动因子几乎为1。在计划靶区（PTV）区域，对于临床IMRT计划，使用RPLD和电离室测量的剂量差异在1.2%以内。
对于大小为1.6×1.6厘米或更小的小射野，明确了体积平均和密度扰动是导致RPLD响应变化的主要效应。此外，当测量大小为1.0×1.0厘米或更小的射野时需要进行扰动校正。在IMRT条件下，蒙特卡罗模拟计算的参考条件与PTV区域之间RPLD响应的差异不超过0.8%。这些结果表明在PTV区域使用RPLD测量IMRT剂量是可行的。
