Explore the Potential Clinical and Dosimetric Improvements for Brain Cancer Proton Beam Treatment Based on the New Delivery and Planning Technique Developments - Collimator Based Intensity Modulated Proton Therapy and Spot-Scanning Proton Arc Therapy

Abstract

Utilizing Intensity Modulated Proton Therapy (IMPT) for brain cancer patient has been reaching its dosimetric limitation due to the large lateral penumbra of the pencil beam spots and secondary proton scattering in the air and tissue. To improve the plan quality of using IMPT, new merging techniques were under development including collimator base IMPT (co-IMPT) and Spot-Scanning Proton Arc (SPArc) therapy. This is the first study to quantitatively investigate the dosimetric outcome and benefits among these three different treatment modalities (IMPT, co-IMPT and SPArc) for brain cancer treatment. A glioblastoma (GBM) patient CT and structure data set was used in this study whose clinical target volume (CTV) is abutting the brainstem. 3mm expansion from CTV to PTV is used. PTV is prescribed to 66 Gy with acceptable minor deviation 95% PTV covered by 95% of prescription dose (6270 Gy) due to the brainstem location. 3-field IMPT uses single field optimization technique to ensure the uniform PTV coverage through all three beams (PA; LT-LAT; Vertex directions); 3-field co-IMPT uses the same beam arrangement but adding a block margin 5mm for PTV to reduce the lateral penumbra; SPArc used two non-coplanar arcs. All theplans use the same objective functions for the optimization. Final objective values of each plan were analyzed. The lower of objective value means a better agreement to the plan optimization goal. Plan qualities were evaluated via Homogeneity Index (GI = D5%/D95%); Comformity Index (CI = Target volume covered by the prescription/ total volume received prescription dose); Brainstem maximum dose (0.03cc); Brainstem mean dose; mean brain dose. Treatment delivery time is estimated based on 2ms spot switching time, gantry rotation speed = 6 degree per second. Proton therapy systems with different energy layer switching time (ELST) from 0.1s to 4s were simulated as well. Comparing SPArc, co-IMPT and IMPT plans, the brainstem maximum dose received 49.99Gy, 53.41Gy and 54.10Gy; The brainstem mean dose received 16.99Gy, 23.19Gy and 25.18Gy and the mean brain dose received 6.12Gy, 8.12Gy and 9.11Gy respectively. Furthermore, CI of PTV is 0.97 for SPArc, 0.86 for co-IMPT and 0.83 for IMPT respectively. HI of PTV is 0.76 for SPArc; 0.79 for co-IMPT and 0.78 for IMPT. SPArc treatment delivery time is close to 3 field IMPT or co-IMPT when the ELST is close to 0.1s (Table 1). The final plan objective value is 0.1857 for SPArc; 0.2927 for co-IMPT; 0.3041 for IMPT.Abstract TU_26_3150; Table 1Estimated treatment delivery time vs. ELST using IMPT; co-IMPT and SPArcEnergy Layer Switching Time (s)IMPT (s)co-IMPT (s)SPArc (s)0.17878950.295851270.5108108222114514638122202216993295298101743603741335 Open table in a new tab The study shows both co-IMPT and SPArc could improve the plan quality and spare the dose to organs abutting the target. SPArc demonstrated a significant potential to further improve the plan quality. More developments are needed in order to deliver SPArc therapy in a clinical machine.