ESTRO 2020 Abstract book

S836 ESTRO 2020

Purpose or Objective Due to the concerns over large lateral penumbra from proton Pencil Beam Scanning(PBS) technique, PBS is not considered in the single fraction brain stereotactic radiosurgery (SRS) as of today. There is an immediate need to clarify such misunderstanding in the era of rapid technique development especially with the advancement of the Spot-scanning Arc therapy(SPArc). This study addressed the challenges of plan quality in the management of single brain SRS target with different sizes and locations via Intensity Modulated Proton Therapy (IMPT), SPArc and Volumetric Modulated Arc Therapy (VMAT) with a goal to provide a reference in the selection of an optimal treatment modalities in future. Material and Methods To simulate the different target locations, a Gross Tumor Volume (GTV)(0.3cc) was inserted in the deep central and peripheral region of a head CT set. To simulate the different target sizes, the GTV was expanded with a uniform margin every 2mm increment, corresponding to a different target volume (from 0.3cc to 26.05cc) (Fig. 1). Three planning groups: IMPT, SPArc and VMAT were generated in RayStation ver. 9A using the same planning objective function and robust optimization parameters (2mm setup and 3.5% range uncertainty for proton planning and 2mm setup uncertainty for VMAT planning) based on the clinical proton beam model (3.3mm 1-sigma @ 227MeV at iso). Prescription was 18Gy in 1 fx with at least 96% of GTV received full prescription dose in the worst-case-scenario robustness evaluation (Fig. 2). Multiple dosimetric metrics were analyzed to assess the plan quality such as dose Conformity Index (CI) (Volume of target/Volume covered by the prescription dose); R50 (Volume of the target/Volume covered by 50% of the prescription dose); V12Gy; and mean dose of brain.

Results In comparison with IMPT, VMAT clearly showed its advantage in the CI and R50 in any target locations and size where a sharp dose fall-off is clinically desired. However, IMPT showed advantage over VMAT in large peripheral target in terms of normal brain tissue dose sparing especially V12 of normal brain tissue where single fraction SRS might not be considered clinical feasible via VMAT. In comparison with VMAT, SPArc has an equivalent or better CI in any size of peripheral targets and deep centrally located targets which were bigger than 4.74cc. In addition, SPArc significantly reduced the normal brain tissue dose over VMAT. For the deep centrally located tumor smaller than 4.74cc, VMAT plan still offered better dose CI (Fig. 3).

Conclusion At the current technique stage, VMAT holds the dosimetric advantage in the single brain lesion SRS over IMPT. But IMPT could be used as an alternative approach when dose to the brain tissue exceeds constraints in VMAT plan. With the new technical development, SPArc showed its potential advantage to offer better dose sparing to the brain tissue over VMAT with an equivalent or higher CI in the peripheral brain lesion and deep centrally located lesion bigger than 4.74cc PO-1469 A new method for automated planning of prostate plans using geometrical ideal dose distribution M. Kusters 1 , M. Kentaro 2 , P. Van Kollenburg 1 , R.J. Smeenk 1 , R. Monshouwer 1 , Y. Nagata 2 1 Radboud university medical center, Radiation Oncology, Nijmegen, The Netherlands ; 2 Hirsohima University hospital, Radiation Oncology, Hiroshima, Japan Purpose or Objective Currently most of our radiotherapy plans are automatically generated with a single treatment technique template in the Auto-Planning module in Pinnacle 16.0.2 (Philips Healthcare, Fitchburg, WI, USA).