Speaker
Description
Objective
Upright particle therapy systems introduce new beam angle freedom, but also new challenges: limited inter-fractional imaging, sensitivity to anatomical changes, and the need for robust panning strategies adapted to this novel setup. Head and neck treatments are particularly demanding, as anatomical variations during the treatment course frequently led to plan adaptation, increasing clinical burden. This work investigates whether shoot-through (ST) fields can improve plan robustness against real anatomical changes in a gantry-based configuration representative of upright delivery.
Methods
A growing cohort of H&N adenoid cystic carcinoma patients treated at CNAO with a fixed beamline was selected. All patients had required at least two plan adaptations during their clinical treatment course. Each patient was replanned in a gantry-based configuration using two strategies: a conventional gantry IMPT reference plan (3–6 coplanar beams, robust optimization 3mm/3.5%) and an ST-enhanced hybrid plan (1–4 IMPT fields combined with 6–10 ST fields at 227 MeV, all co-optimized, robustness pushed to the limit within clinical goal constraints). Both plan types were recalculated on all available clinical re-evaluation CTs (reCTs) without re-optimization, using the same scan that had triggered adaptation in the clinic.
Results
ST-enhanced plans achieved identical clinical goals to the gantry IMPT reference, at the cost of an average +1.0 Gy increase in low-dose bath, a predictable and manageable trade-off. Under simulated uncertainties (3mm/3.5%), ST-enhanced plans showed consistently smaller D99 deviations across all three CTVs (HR, IR, LR). On reCTs, gantry IMPT required adaptation in 4/8 cases; ST-enhanced plans reduced this to 1/8, pushing D99 below the 5% clinical replanning threshold across all targets.
Outlook & Discussion
These results demonstrate that integrating ST fields into gantry-based planning substantially reduces adaptation burden against real anatomical changes, without requiring new hardware or delivery infrastructure. Beyond robustness, ST beams offer a built-in per-fraction range monitoring opportunity: the exit signal from transmission beams can be detected by a fixed detector behind the patient, providing in-vivo range verification at every fraction without additional imaging. Preliminary phantom measurements at CNAO using an anthropomorphic head phantom with a pseudo nasal cavity show that range shifts due to cavity filling variations are detectable and consistent with TPS predictions, validating the concept for the anatomical changes most relevant to this cohort. Next steps include expanding the patient cohort, quantifying delivery time benefits, and integrating ST-enhanced planning together with the in-vivo range verification system into the full upright clinical workflow.