Comparison of image-guidance in proton and photon radiation therapy: Preliminary clinical experience

Proton therapy (PT) has a unique depth dose profile (Bragg peak), where it is more superior than IMRT in target volume dose coverage and lower OARs doses. Proton is more sensitive to density and contour changes along the traversed beam as these would affect Bragg peak dose deposition and overall dosimetry. Geometrical uncertainties are crucial in affecting dosimetry for both IMRT and PT, however more detrimental for the latter as they affect proton's penetration range. To minimize these uncertainties, image-guidance in PT is essential. Hence, we aim to compare PT and IMRT image-guidance based on our early PT clinical experiences. From June 2023 to January 2024, 74 patients (7 brain, 32 head-and-neck (HN), 10 thorax-abdo, 18 prostate and 7 paediatrics) received PT in our centre. PT clinical imaging experiences were compared with departmental IMRT imaging protocols. IMRT (orthogonal kV and/or CBCT) used bone-based and soft-tissue-based registration. Soft-tissue-based registration is inevitable in PT (CBCT) due to additional need to focus on overall contours. Thus, low-dose contours acting as beam shape surrogates are also implemented in PT image registration for evaluation of overall contour match along individual beam path. In brain and HN cases, matching criteria and correction strategies are similar for IMRT and PT. However, additional attention is given to shoulder positions, sinuses filling and contour variations in PT HN image verification. Prostate and liver cases used CBCT matching for both treatment techniques. Furthermore in PT, additional real-time fiducial tracking is utilised to reduce intra-fractional motion. Image-guidance enables accurate target-alignment and monitoring of anatomical/contour changes to trigger adaptive replanning if required. To achieve proton's superior robust plan, surrounding anatomical structures must constantly be in the same position as planned in addition to precise target-alignment. Thus, 3D-volumetric imaging and 6D correction strategies are highly relevant in PT compared to IMRT.