SU-F-T-188: A Robust Treatment Planning Technique for Proton Pencil Beam Scanning Cranial Spinal Irradiation.

PURPOSE: To propose a proton pencil beam scanning (PBS) cranial spinal irradiation (CSI) treatment planning technique robust against patient roll, isocenter offset and proton range uncertainty.

METHOD: Proton PBS plans were created (Eclipse V11) for three previously treated CSI patients to 36 Gy (1.8 Gy/fractions). The target volume was separated into three regions: brain, upper spine and lower spine. One posterior-anterior (PA) beam was used for each spine region, and two posterior-oblique beams (15° apart from PA direction, denoted as 2PO_15) for the brain region. For comparison, another plan using one PA beam for the brain target (denoted as 1PA) was created. Using the same optimization objectives, 98% CTV was optimized to receive the prescription dose. To evaluate plan robustness against patient roll, the gantry angle was increased by 3° and dose was recalculated without changing the proton spot weights. On the re-calculated plan, doses were then calculated using 12 scenarios that are combinations of isocenter shift (±3mm in X, Y, and Z directions) and proton range variation (±3.5%). The worst-case-scenario (WCS) brain CTV dosimetric metrics were compared to the nominal plan.

RESULTS: For both beam arrangements, the brain field(s) and upper-spine field overlap in the T2-T5 region depending on patient anatomy. The maximum monitor unit per spot were 48.7%, 47.2%, and 40.0% higher for 1PA plans than 2PO_15 plans for the three patients. The 2PO_15 plans have better dose conformity. At the same level of CTV coverage, the 2PO_15 plans have lower maximum dose and higher minimum dose to the CTV. The 2PO_15 plans also showed lower WCS maximum dose to CTV, while the WCS minimum dose to CTV were comparable between the two techniques.

CONCLUSION: Our method of using two posterior-oblique beams for brain target provides improved dose conformity and homogeneity, and plan robustness including patient roll.