3168 - Dosimetric Improvement of Intensity Modulated Proton Therapy Planning for Spinal Tumors Using a Solid Water Bolus Instead of Range Shifter: A Case Study

Purpose/Objective(s): A range shifter (RS) is typically required for proton therapy of superficial tumors, however, using RS leads to larger beam penumbra and increased (sometimes unacceptable) doses to adjacent organs at risk (OARs). This study aimed to investigate the use of a bolus replacing RS in intensity modulated proton therapy (IMPT) planning for spinal tumors and its effect on dosimetric improvement.

Materials/Methods: A 4-year-old patient with thoracic malignant spinal tumor was treated with IMPT at our center. The treatment prescription included three phases: Phase 1 (craniospinal irradiation + primary spinal CTV: 36 Gy/20 fx), Phase 2 (boost to CTV: 18 Gy/10 fx), and Phase 3 (additional boost to spinal tumor bed CTV_TB: 5.4 Gy/3fx). Dose constraints for kidneys: Dmean (Left) < 15 Gy, Dmean (both) < 18Gy (acceptable 20Gy); for spinal cord: D0.1cc < 47 Gy. Phase 1 was planned using a RS with conventional simulation (Plan 1) and delivered. Due to unacceptable kidney dose with plans for phases 2 and 3 on initial simulation using a RS(Plan 2a and Plan 3a), re-simulation was performed for Phases 2 and 3 by placing 4 cm solid water plates beneath the vacuum bag under the tumor region, and new treatment plans were generated without RS (Plan 2b and Plan 3b). Dose summation was performed for Plan 1, Plan 2b and Plan 3b (Sum_Bolus), and compared to the dose summation of Plan 1, Plan 2a and Plan 3a (Sum_RS).

Results: Compared to the original plan with RS, target coverage was slightly better with a bolus: CTV V98% (Sum_Bolus vs Sum_RS): 91.06% vs 90.67%; CTV_TB: 84.1% vs 82.36%. In addition, OAR sparing was better for spinal cord: D0.1cc was 46.26 Gy vs 47.83 Gy, and for kidney Dmean: was13.46 Gy vs 18.02 Gy (left); 23.87 Gy vs 26.78 Gy (right); and18.52 Gy vs 22.55 Gy (both). The patient was treated with the new simulation using bolus without RS for phases 2 and 3 successfully.

Conclusion: For patients with superficial spinal and peripheral tumors, placing solid water under the vacuum cushion during simulation can increase the water-equivalent depth of the target volume and clinically practical. This approach allows IMPT planning without a RS, leading to large dose reduction to organs at risk (OARs) and surrounding normal tissues while maintaining similar or better target coverage of IMPT plans.