2468 - Dosimetry and Quality Assurance of a Phase I Trial of Proton GRID Therapy on a Compact Proton Therapy Delivery System

X. Zhao¹, J. Setianegara², Y. Hao³, T. R. Mazur⁴, W. Smith¹, J. Liu¹, T. Zhang⁴, S. M. Perkins⁵, A. J. Apicelli III⁶, M. T. Prusator¹, and T. Zhao⁴; ¹Washington University School of Medicine in St. Louis, St. Louis, MO, ²Department of Radiation Oncology, University of Kansas Medical Center, Kansas City, KS, ³Washington University in St. Louis, St. Louis, MO, ⁴Washington University School of Medicine in St. Louis, Department of Radiation Oncology, St. Louis, MO, ⁵Brain Tumor Center, Siteman Cancer Center, Washington University School of Medicine, St. Louis, MO, ⁶VA St. Louis Health Care System, Saint Louis, MO

Purpose/Objective(s): Proton palliative spatially fractionated (GRID) radiotherapy is a novel spatially-fractionated treatment technique for bulky invasive tumors with reduced toxicities that is being investigated in current clinical trials. However, treatment delivery on compact proton therapy machines with increased spot sizes is challenging. In this work, we developed planning techniques to meet constraints on skin and other critical OARs in spatially fractionated proton GRID treatment delivered with a proton therapy S250i proton unit. Materials/

Methods: Clinical proton GRID plans have been created and delivered to five patients with different treatment sites (Psoas, Pelvis, Abdomen etc.). For each patient, at least 4 proton GRID fields were created in addition to a concurrent boost and uncollimated proton field. Dose vertices were generated within the GTV through the use of proton beam interlaces created with 45° gantry angle separations and strategic isocenter shifts between each GRID field. Peak-to-valley dose ratio of 2:1 was achieved in GTV through modulated delivery of 60 Gy in dose vertices and 18 Gy proton dose to the rest of the PTV. A custom brass aperture was utilized in half of the GRID fields, designed to collimate the proton spot sizes of proton therapy S250i for a sharper lateral penumbra.
Results: PTV V_18Gy = 95% was met, and V60Gy (%GTV) was found to be = 0.7% in all five treatment plans. Average skin point maximum dose and D10cc decreased with more fields, with D0.035cc < 33 Gy and D10cc < 31 Gy for all plans. Colon, Spinal cord and other critical OAR dose fluctuated with increasing GRID fields, and GRID field directions. All patient-specific quality assurance (PSQA) of these five cases passed institutional criteria with better than 90% gamma passing rate with a 3mm/3% criterion.
Conclusion: All five treatment plans met dose coverage objectives. By adding more GRID fields with smaller angular separations, all the protocols OAR constraints were met. For targets close to critical OARs, properly selecting the GRID field gantry angle can decrease the dose significantly. Abstract 2433 – Table 1: QA results and dosimetry details of the five delivered proton GRID treatments

Cases	Treatment Site	Grid Fields MatriXX QA (3%/3mm)	Open Field MatriXX QA (3%/3mm)	Skin Max Dose [Gy]	Critical OAR	Critical OAR Constraint	Peak Valley Ratio
1st	Left Psoas	90.70%	96.80%	30	Spinal Cord	D0.03cc < 22.5 Gy	2.1
2nd	Left Psoas	90.70%	93.70%	28.6	Large Bowel	D0.03cc < 34.5 Gy	2.2
3rd	Right Pelvis	90.20%	98.70%	28.8	Lumbar Plexus	D0.1cc < 33 Gy	2.1
4th	Right Abdomen	92.50%	99.70%	31	Duodenum	D0.03cc < 22.2 Gy	1.9
5th	Right Thigh	91.20%	98.30%	24.8	Right Femur	V24Gy < 10 cc	2.2