Evaluation of geometrical uncertainties on localized prostate radiotherapy of patients with bilateral metallic hip prostheses using 3D-CRT, IMRT and VMAT: A planning study

Objective Since most radiation treatment plans are based on computed tomography (CT) images, which makes it difficult to define the targeted tumor volume located near a metal implant, this study aims to evaluate and compare three treatment plans in order to optimally reduce geometrical uncertainty in external radiation treatment of localized prostate cancer. Methods Experimental subjects were three prostate patients with bilateral hip prosthesis who had undergone radical radiotherapy. The treatment plans were five-field three-dimensional conformal radiation therapy (3D-CRT), fixed 5-field intensity-modulated radiation therapy (IMRT) using similar gantry angles, and single-arc volumetric modulated arc therapy (VMAT). The monitor units (MUs), dose volume histograms (DVHs), the dose indices of planning target volume (PTV), clinical target volume (CTV) and rectum were compared among the three techniques. The geometrical uncertainties were evaluated by shifting the iso-center (2- 10 mm in the anterior, posterior, left, right, superior, and inferior directions). The CTV and rectum dose indexes with and without the iso-center shifts were compared in each plan. Results The Conformity Index of PTV were 1.35 in 3D-CRT, 1.12 in IMRT, and 1.04 in VMAT, respectively. The rectum doses in 3D-CRT are also higher than those in IMRT and VMAT. The iso-center shift little affected the CTV dose when smaller than the margin size. The rectum dose increased especially after a posterior shift. Additionally, this dose increase was larger in the VMAT plan than in the 3D- CRT plan. However, the VMAT achieved a superior rectum DVH to that of 3D- CRT, and this effect clearly exceeded the rectum-dose increase elicited by the iso-center shift. Conclusion For radiotherapy treatment of localized prostate cancer in patients with hip prosthesis, the dose distribution was better in the VMAT and Metal Artifact Reduction (MAR)-CT image methods than the conventional methods. Because the anatomical structure of the male pelvic region is relatively constant among individuals, we consider that VMAT is a valid treatment plan despite analyzing just three cases.