Robotic Stereotactic Radiation Therapy Compared to 3D MR Image Guided HDR Brachytherapy in the Treatment of Cervical Cancer

Abstract

target and organs at risk (OARs) during breathing are not considered in this three-dimensional (3-D) planning system, because doses are calculated at the end-exhale computed tomography (CT) images. On the other hand, the four-dimensional (4-D) plans using CT images from all respiratory phases seem to be similar to the actual doses. To know the difference between the calculated doses and actual doses delivered to the target and OARs, doses were calculated using 4-D plans and compared with those of the 3-D plans. Materials/Methods: Fourteen patients with lung cancer were treated at our institution from December 2012 to June 2013. 4-D CT scans were performed using multi slice CT and respiratory gating system. 4-D CT images were categorized into 10 respiratory phases. CT images were imported to the treatment planning system and structures were delineated on the endexhale phase CT images. Monte-Carlo algorithm was used for the dose calculation. 48 Gy in four fractions were prescribed at D95 of the PTV. Doses were calculated by both 3-D and 4-D plans in each patient. Maximum doses of the GTV (D-Max), GTV-D99, PTV-D95, Lung-V20, Conformity Index (CI) and Homogeneity index (HI) were compared in each patient. Furthermore, maximum doses of the liver, stomach, esophagus, trachea and spinal cord were also evaluated. Results: The actual doses of 4708.09 cGy 91.62 in PTV-D95 calculated by 4-D plans were significantly decreased in comparison with the prescribed doses of 4800 cGy by 3-D plans (p Z 0.02). The maximum differences of the calculated doses between 3-D and 4-D plans in GTV-D99 and PTV-D95 were 400.0 cGy and 266.7 cGy, respectively. In 3-D plans, doses were overestimated at the average of 2% (max 7.5%). The differences of maximum doses between 3-D and 4-D plans were not statistically significant in each OAR. However, the differences of calculated doses between 3-D and 4-D plans were relatively high in liver (-110.82 cGy 454.28 cGy), esophagus (-86.31 cGy 344.27 cGy) and trachea (-972.89 cGy 536.14 cGy), respectively. Conclusions: Although Monte-Carlo calculation has 2% uncertainty, and deformations of both target and OARs during respiration are inevitable, it was conceivable that the difference of the calculated doses between 3-D plan and 4-D plan to the target tumor were negligible. In contrast, the maximum doses of the OARs varied widely in each case. The accurate margins of OARs were affected by respiratory motions especially in organs such as liver, esophagus, and trachea. 4-D planning should be employed in the treatment planning of the cases with tumor located near the liver, esophagus and trachea to avoid the unexpected overdose-radiation to OARs. Author Disclosure: K. Okawa: None. M. Inoue: None. M. Taro: None. T. Koshi: None. S. Ota: None.