Implementation of Constant Dose Rate and Constant Angular Spacing Intensity-modulated Arc Therapy for Cervical Cancer by Using a Conventional Linear Accelerator.

Background: Volumetric-modulated arc therapy (VMAT) can only be implemented on the new generation linacs such as the Varian Trilogy((R)) and Elekta Synergy((R)). This prevents most existing linacs from delivering VMAT. The purpose of this study was to investigate the feasibility of using a conventional linear accelerator delivering constant dose rate and constant angular spacing intensity-modulated arc therapy (CDR-CAS-IMAT) for treating cervical cancer. Methods: Twenty patients with cervical cancer previously treated with intensity-modulated radiation therapy (IMRT) using Varian Clinical 23EX were retreated using CDR-CAS-IMAT. The planning target volume (PTV) was set as 50.4 Gy in 28 fractions. Plans were evaluated based on the ability to meet the dose volume histogram. The homogeneity index (HI), target volume conformity index (CI), the dose to organs at risk, radiation delivery time, and monitor units (MUs) were also compared. The paired t-test was used to analyze the two data sets. All statistical analyses were performed using SPSS 19.0 software. Results: Compared to the IMRT group, the CDR-CAS-IMAT group showed better PTV CI (0.85 0.03 vs. 0.81 0.03, P = 0.001), clinical target volume CI (0.46 0.05 vs. 0.43 0.05, P = 0.001), HI (0.090.02 vs. 0.11 0.02, P = 0.005) and D95 (5196.33 28.24 cGy vs. 5162.63 31.12 cGy, P = 0.000), and cord D2 (3743.8 118.7 cGy vs. 3806.2 98.7 cGy, P = 0.017) and rectum V40 (41.9 6.1% vs. 44.2 4.8%, P = 0.026). Treatment time (422.7 46.7 s vs. 84.6 7.8 s, P = 0.000) and the total plan Mus (927.4 79.1 vs. 787.5 78.5, P = 0.000) decreased by a factor of 0.8 and 0.15, respectively. The IMRT group plans were superior to the CDR-CAS-IMAT group plans considering decreasing bladder V50 (17.4 4.5% vs. 16.6 4.2%, P = 0.049), bowel V30 (39.6 6.5% vs. 36.6 7.5%, P = 0.008), and low-dose irradiation volume; there were no significant differences in other statistical indexes. Conclusions: Patients with cervical cancer treated with CDR-CAS-IMAT using Varian Clinical 23EX can get equivalent or superior dose distribution compared to those treated with IMRT. CDR-CAS-IMAT has a less treatment time and MU, which can reduce the uncertainty factor and patient discomfort in treatment.