Room: AAPM ePoster Library
Purpose: The major concern/huddle of Halcyon’s clinical adoption is its radiation field size limitation of 28x28cm2 in treating large targets. This is the first feasibility study of treating a Total Lymph Node patient with Halcyon, where 3-isocenter plan (not supported natively) designed to treat large target. This plan quality is compared with protocol specified 3D plan.
Under Total Lymph node protocol of MDP-101-MLK, patient’s lymph nodes and spleen were contoured as CTV and 5mm margin added as PTV, 68x35x25cm3. Transplanted right kidney was shielded from radiation. protocol specified 3D plan was generated with Elekta Infinity for comparison purpose, where 3 fields technique with mantle, paraortic/spleen and pelvis adopted. IMRT plan was generated with Halcyon, where 3 isocenters with 3cm overlap between sup/mid and mid/inf fields placed, all isocenters were optimized together with dynamic feathering. The 3-isocenter plan was spitted to 3 different single-isocenter plans to facilitate delivery. Three single-isocenter plans were delivered separately with CBCT imaging alignment at each isocenter. Plan uncertainty analysis was performed by shifting sup/inf isocenters 5mm closer and 5mm further from mid isocenter. DVH were compared with Infinity 3D plan.
Halcyon IMRT plan achieved 90.2% PTV coverage and 111.9% hotspot; while Infinity 3D can achieve 61.0% coverage and 112.2% hotspot. For all critical structures except right kidney, Halcyon IMRT deliver less mean dose than Infinity 3D plan, especially in high dose region.
Even if there are shift error in patient SI direction, the PTV D90/Dmean and Body Dmax change are less than 20%; compared to ~200% hotspot from misaligned field matching in 3D planning,
In comparison, Halcyon IMRT plan with 3-isocenters can achieve better coverage and lower critical structure dose in treating large target. Halcyon IMRT plan is safer to delivery because it is more setup errors forgiving due to dynamic feathering feature.