Room: Exhibit Hall | Forum 7
Purpose: When non-small cell lung tumors increase in size and/or are centrally located, achieving standard dose volume constraints in SBRT become a challenge. This study evaluated the potential benefit of pencil beam scanning proton plans by better dosimetry and enabling SBRT for challenging cases.
Methods: Two groups of 9 patients are used: one group has smaller tumor volumes ranging from 13.2-71.2cc receiving photon SBRT; the other group has larger volumes from 103.4-336.1cc receiving hypo-fractionation treatment except the smallest volume patient. Proton SBRT plans use 4-field robust IMPT and photon SBRT plans use 2 arcs. Patient-specific organ motion is used to generate the beam specific PTV in Eclipse for each proton field with 3-mm setup and 3.5% CT calibration uncertainties and robust scenarios. CBCT images are used to generate virtual CT for the validation of robust IMPT plans. IBA universal nozzle and movable range shifter are used in proton SBRT plans. All patients have target motion within 8 mm.
Results: For the first group, robust IMPT plans are inferior to photon SBRT for volume OAR constraints (lung, heart, and chest wall). However for the second group, robust IMPT plans are superior to photon SBRT for volume OAR constraints (lung and heart) and are comparable for chest wall constraints when tumors are extremely peripheral. Both proton and photon SBRT can meet the max dose requirement of spinal cord and esophagus generally. Multiple virtual CT analysis prove that IMPT plans are robust during delivery.
Conclusion: Proton-specific uncertainties in water equivalent thickness convert to larger geometric margins in proton SBRT than photon SBRT to ensure robust coverage of CTV. As a result, lung tumors with volume < 100 cc should use photon SBRT but those with volume > 100 cc should be considered for proton SBRT using motion mitigation and robust planning.