2211 - Volumetric Modulated Arc Therapy Integrating Static Angle Ports and Dynamic Collimator Rotation: A New Approach to Spine SBRT Treatment

B. Archibald-Heeren¹, M. V. Byrne², M. Jennings³, R. M. Scheuermann⁴, and G. G. Y. Kim⁵; ¹Icon Cancer Centres, Sydney, NSW, Australia, ²RADIATION ONCOLOGY CENTRE, WAHROONGA, NSW, Australia, ³Department of Medical Physics, Townsville University Hospital, Douglas, QLD, QLD, Australia, ⁴Department of Radiation Oncology, University of Pennsylvania, Philadelphia, PA, ⁵Department of Radiation Oncology, University of California San Diego, San Diego, CA

Purpose/Objective(s): A new optimization algorithm was developed within a commercial system allowing for volumetric modulated arc therapy (VMAT) with integrated static modulated ports and dynamic collimator rotation (VMATp). This multi-institutional study undertook retrospective plan quality analysis for spine stereotactic body radiotherapy (SBRT) plans between current VMAT plans and VMATp. Two approaches were implemented to investigate if VMATp could improve plan quality, or if equivalent plan quality was possible with fewer delivery arcs. Materials/

Methods: 11 spine SBRT patients were replanned with a 24Gy/2Fx prescription and 2mm planning target volume (PTV) margins. The cohort included cervical, thoracic, and abdominal disease. For each case, 3 plans were created: a VMAT plan using 2-3 static collimator arcs, a VMATp plan with 2-3 dynamic arcs and =9 ports (VMATp_A), and a VMATp plan with a single dynamic arc and =9 ports (VMATp_B). Each case was planned by two planners to minimize the impacts of planner bias. The agreed optimum plan for each technique, judged by adherence to clinical goals (Table 1), was analysed for a total of 33 plans in the study. Plan quality was quantified between techniques by 14 spine SBRT clinical goals. Intentionally challenging goals were determined through review by 4 international centres prior to the commencement of the study. Statistical analysis was performed with the Wilcoxon ranked t-test.
Results: All three techniques demonstrated high plan quality. Over 80% of clinical goals were met in all 33 plans across the study. VMATp_A plans resulted in the highest average ratio of passing clinical goals (VMAT: 89%, VMATp_A: 92%, VMATp_B: 87%). No significant variation in target or organ at risk objectives were recorded, though VMATp_A and VMATp_B plans trended to slightly higher PTV coverage (average V100% increase of 0.41% and 0.17%, respectively) whilst maintaining spinal cord dose. A statistically significant increase in the volume receiving 50% dose was found for VMATp_B plans relative to the VMAT plans (4.45 and 4.15 respectively, p= 0.014) however all plans met the determined clinical goals. Plan monitor units were 16% (780MU) less for VMATp_B plans than VMAT deliveries.
Conclusion: For spine SBRT, a new volumetric modulated arc therapy optimization algorithm incorporating static ports and dynamic collimator rotation allowed for equivalent plan quality to current VMAT optimization whilst reducing the number of required arcs from three to one. Abstract 2211 - Table 1: Multi-institutional agreed clinical goals for the retrospective study

Region of Interest	Metrics	Targets
PTV	V100%, D90%, dMax	>95%, >24Gy, <150%
GTV	V100%	99.90%
Esophagus	dMax	<20Gy
Spinal Cord	d0.03cc, d0.35cc, d1.20cc	<15Gy, <12.6Gy, <10.4Gy
Spinal Cord PRV	dMax	<17Gy
Cauda Equina	dMax	<17Gy
Trachea	dMax	<20Gy
PTV	Gradient Measure, Conformity Index	<2.0cm, 1.0 < > 1.2
BODY	R50	< 4.5