Assessment of interfraction patient setup for head-and-neck cancer intensity modulated radiotherapy using multiple CT based image-guidance

Purposes: Image-guided radiotherapy is routinely used in conjunction with head-and-neck (H&N) IMRT. The purpose of this work is to assess interfraction patient localization using KV cone beam CT (KVCBCT), MV cone beam CT (MVCBCT) and MV fan beam CT (MVCT) for H&N cancer radiation. Methods and Materials: Three CT-based IGRT modalities used for H&N IMRT were considered in this study: kV conebeam (Synergy, Elekta), MV cone beam (MVision, Siemens), and MV fan beam (TomoTherapy). The daily variations in medialateral, craniocaudal and anteroposterior dimension were measured. The CTV-to-PTV margins were calculated using 2.0∑ +0.7σ, where ∑ and σ were systematic and random positioning errors, respectively. The influence of patient characteristics (i.e., weight, weight loss) on interfraction patient setup was also investigated. Results: A total of 3302 CT scans for 117 patients were retrospectively analyzed. Average inter-fraction displacements (±standard deviation) in the medialateral, craniocaudal and anteroposterior direction were 0.5±1.5, -0.3±2.0, 0.3±1.7 mm for KVCBCT, 0.2±1.9, -0.2±2.4 and 0.0±1.7 mm for MVCT and 0.0±1.8, 0.5±1.7 and 0.8±3.0 mm for MVCBCT. For MVCBCT, 30.2% of the patients had displacements greater than 3 mm in one translational direction, compared to 11.4% and 3.4% for MVCT and KVCBCT, respectively. On average, both systematic (in lateral and vertical direction) and random setup errors for MVCBCT were larger than KVCBCT and MVCT. Maximal uniform CTV-to-PTV margins were 3.0, 4.6 and 7.4 mm for KVCBCT, MVCT, and MVCBCT. No statistically significant difference of setup error with respect to translational direction was observed for the evaluated characteristics as well as during early, middle and late treatment courses. Conclusion: CTV-to-PTV margin in H&N IMRT may be a function of the imaging modality. These data indicate that larger uniform margins of 5 and 7 mm may be appropriate for MVCT and MVCBCT, respectively, compared to a smaller margin of 3 mm for KVCBCT.