Room: AAPM ePoster Library
Purpose: treatment planning for head and neck (H&N) cancer patients require non-coplanar proton beams, due to the complexity of the patient anatomy and target location. Hence, targeting accuracy for all beams needs to be carefully evaluated prior to delivering proton beam therapy (PBT), which adds additional time for the patients on the table and little extra imaging dose to the patients. In this study, we analyzed the couch shifts to evaluate whether imaging is required before delivering each field with different couch angles.
Methods: an IRB approval, a retrospective analysis was performed in 10 H&N patients who were treated with PBT in 2019. Each plan was made with 2-3 non-coplanar and 2 planar fields. As per our institutional treatment protocol, a CBCT was taken at the beginning of the setup followed by orthogonal kV images before delivering each field. The Cartesian (Longitudinal, Vertical, Lateral) and Angular (Pitch and Roll) shifts for each field were recorded from the treatment summary on the first two fractions and every fifth fractions thereafter. A net magnitude (R) of 3D shift in Cartesian coordinates was calculated as: R=sqrt(?Lng²+?Vrt²+?Lat²).
Results: were a total 915 Cartesian and 610 angular shift values recorded for 10 patients. The 305 net shifts values for all 10 patients ranged from 0 to 8.3mm. Out of 915 Cartesian shifts, 637 were zero and the remaining 278 were non-zero ranging from -5 to +8 mm. Similarly, of the 610 angular shifts, 448 were zero and the remaining 162 were ranged from -2.5° to +3.0°. Couch shifts were larger towards the end of the course of treatment.
Conclusion: This study confirms the need for orthogonal kV imaging before delivering each field of H&N proton beam therapy. Further analyses are warranted to assess the correlation of couch shifts with individual patient's treatment history.