Speculation of Intrafractional Motion Errors in Patients with Prostate Cancer during Pelvis RapidArc Radiotherapy

Purpose: To evaluate the intrafractional motion errors in patient with prostate cancer received pelvis irradiation during RapidArc radiotherapy.Materials and Methods: A total of eighteen high risk group prostate cancer patients were treated prostate and pelvis lymph nodes by RapidArc radiotherapy then boost dose by Cyberknife. All patients had kV cone beam computerized tomography (CBCT) scans in their first three fractions of treatment. During these treatments, the CBCT scans were registered to planning CT simulation images as reference to perform registration procedure based on soft tissue windows matched with clinical tumor volume (CTV). The errors of isocenter position were corrected by couch shifted. The second and third CBCT images were immediately acquired before and after RapidArc treatment. These errors of isocenter position on the left-right (LR), superior–inferior (SI) and anterior–posterior (AP) directions were analyzed retrospectively.Results: Under RapidArc technique with a shortened treatment delivery time (about 3 min), the residual systemic and random errors in pre and post-radiation treatment revealed limited. Based on the paired 2^(nd) and 3^(rd) CBCT images, the intrafractional errors (mean ± SD) in LR-SI-AP directions were -0.4 ± 0.8, -0.2 ± 1.0, 0.1 ± 0.8 mm. No intrafractional error differences in three axes, except borderline significant in LR direction (p= 0.046). Isotropic planning margins created with the linear addition of internal margin to clinical tumor volume was respectively 3.6, 4.4, 4.5 mm in LR-SI-AP axes and 2.5, 3.2, 3.3 mm if generated with quadrature addition.Conclusion: Use of the faster RapidArc technique with accurate kV CBCT images online verification for prostate cancer pelvis radiotherapy, the intrafractional motion errors were limited. These speculated intrafractional errors could be applied to improve the accuracy of radiation delivery and a smaller PTV margin might be adopted.