Streamlining the image-guided radiotherapy process for proton beam therapy: a service evaluation

Abstract

Purpose or Objective Modern radiotherapy requires image-guidance to ensure precision and accuracy of treatment delivery. To implement image-guided proton beam therapy (IGPBT) at our Proton Beam Therapy (PBT) Centre, a 3-step process was developed. This study reports on an evaluation to assess the feasibility of reducing this to a 2-step imaging process, thereby reducing overall treatment time and unnecessary imaging doses. Material and Methods The first 20 consecutive patients treated at the Centre were included in the study. Patient characteristics are recorded in Table 1. The initial IGPBT workflow consisted of: (i) 2-dimensional kilo-voltage (2DkV) image pair acquisition for gross positioning assessment, (ii) 3- dimensional cone-beam computed tomography (CBCT) acquisition to assess target volumes and organs at risk, and (iii) repeat 2DkV pair to verify translation and rotation corrections before delivering treatment. Imaging data from fractions 1-5 and then weekly [biweekly for head and neck treatment sites] was collated to evaluate (i) initial 2DkV imaging dose using estimated delivered dose (μGy), (ii) elapsed time between 2DkV and CBCT acquisition, and (iii) concordance of online matched values for the 2DkV and CBCT image registrations, evaluated by registration on bony anatomy and soft tissue, using ARIA OIS (v13.7, Varian Medical Systems, USA). To assess the correlation between 2DkV and CBCT image registration, data was analysed using Pearson’s Correlation Coefficient and Bland-Altman analysis. Results 229 fractions were evaluated (per patient: range 8-19, median 10). 19 (8.3%) fractions required patient repositioning following the initial 2DkV. Using a 2-step imaging process would reduce the imaging dose by 3.4mGy on average for all patients over a whole treatment course. The use of the 3-step process required a mean additional time of 5.1 minutes (range: 3.3 to 9.9) compared to the 2- step process. Overall image results indicated that correspondence between the mean displacements from the initial 2DkV and CBCT images for all treatment sites was high, with R=0.94, 0.94 and 0.80 in the anterior-posterior, superior-inferior and right-left directions respectively. Bland-Altman analysis showed there was very little bias and narrow limits of agreement. However, discrepancies in rotation correction were observed: although there was only small bias, there were relatively wide limits of agreement (no less than +/-0.6o). Conclusion The results presented confirmed only a minority of fractions required patient repositioning after the initial 2DkV. Removing this from the standard 3-step IGPBT process and streamlining to a 2-step workflow (commencing with CBCT) would reduce imaging dose and treatment times, thus improving efficiency and overall service capacity. The 2-step IGPBT workflow has now been implemented at our PBT Centre and is standard verification protocol. For challenging cases (e.g. paediatric patients under GA) further investigations are required before the 3-step workflow can be modified.