Abstract Book

S1060

ESTRO 37

EP-1950 Prescription isodose implications on SBRT lung treatments with VMAT. G. Pozo 1 , A. Prado 2 , M. Manzano 2 1 Hospital Universitario 12 de Octubre, Servicio de Oncología Radioterápica. Sección de Radiofísica., Madrid, Spain 2 Hospital Universitario 12 de Octubre, Radiofísica y Protección Radiológica, Madrid, Spain Purpose or Objective To compare lung SBRT plans with the same PTV locations and dose prescription if the prescription isodose is set to 95% or to 67%. Material and Methods Treatment plans were generated using Eclipse v.11 TPS (Varian Medical Systems, Palo Alto. California) utilizing a Varian Clinac iX with 6 and 15 MV energies. AAA (Analytical Anisotropic Algorithm) algorithm with 2.5 mm grid size were used. PRO v11.0.31 optimizer was employed for VMAT. All parameters related to the optimization cost functions were kept at same values when changing the prescription isodose. Three non- coplanar partial arcs were used from 30° to 180° with collimation angles were set to 30°, 330° and 30°, respectively. Couch angles were set to -10°, 0° and +10° so as to limit low dose extension. A cohort of 8 patients with laterally located right lung tumours was selected. 60 Gy in fractions of 7.5 Gy/fr was prescribed to every patient. Patient immobilization was achieved by a thermoplastic body mask. For plan evaluation several indexes described in ICRU 91 were used. Paddick conformity index (CI) and D 2% for PTV were considered, and D max and D avg were obtained for the main OARs. Gradient index (GI) was calculated so as to provide dose fall-off information. Average values were obtained for evaluated parameters. A two-tailed Student t-test was performed to elucidate whether the discrepancies are statistically significant or not. Results As a result of changing the prescription isodose from 95% to 67%, D avg of every studied OAR diminished as is shown in table 1. Variations were normalized to 95% prescription isodose value. Consequently, a negative value implies a decrement in the index considered with respect to 95% prescription isodose. Only for lung, bronchial tube and chest wall these differences were statistically significant (p<0.008). D max increased for lungs and chest wall (p<0.04) while it diminished for bronchial tube (p<0.005).

to high dose per fraction schemes. A proper balance should be achieved between high doses and toxicity to obtain the best clinical outcome for the patient. EP-1951 Evaluation of repainting for moving targets treated with continuous or pulsed scanned proton beams D. Dumont 1 , C. Ribeiro 2 , G. Janssens 3 , X. Geets 1 , A. Knopf 2 , E. Sterpin 4 , A. Meijers 2 1 Cliniques Universitaires Saint-Luc, radiotherapy, woluwe-saint-lambert, Belgium 2 University Medical Center Groningen UMCG, Department of Radiation Oncology, Groningen, The Netherlands 3 Ion Beam Applications IBA, Advanced Technology Group, Louvain-la-Neuve, Belgium 4 KU Leuven, Department Of Oncology- Laboratory of Experimental Radiotherapy, Leuven, Belgium Purpose or Objective Treatment of lung tumours with pencil beam scanned proton therapy (PBS-PT) entail specific challenges due to the high sensitivity of protons to density variations and the interplay between spot delivery and tumour motion. Repainting has been demonstrated as an effective strategy to mitigate the interplay effect. Proton facilities based on isochronous cyclotrons (IBA, ProteusPlus(PPlus)) with continouse beam extraction can deliver scaled repainting (further referred to as “controlled repainting”). Facilities based on synchrocyclotrons (IBA, ProteusOne (POne)) feature “intrinsic repainting” (uncontrolled) because their delivery is performed in multiple pulses. The aim of this study is to assess the intrinsic repainting and compare its performance to standard controlled repainting. Material and Methods For five non-small cell lung cancer (NSCLC) patients plans were created using RayStation. The prescription dose was 60 Gy. All plans were 4D robustly optimized based on the full inhale and full exhale phase, accounting for set-up errors of 5 mm and range uncertainties of ±3%. Three sets of plans were obtained per patient: (P1) PPlus without repainting, (P2) PPlus with 5 controlled repaintings, (P3) POne with intrinsic repainting. The treatment plans were reconstructed based on log file data and subplans, were each subplan corresponds to a specific 4DCT phase. For the splitting into subplans 5 s breathing cycle was assumed. To evaluate the plans, the voxel-wise minimum of the target (minimum dose obtained from all the scenarios in each voxel of this volume) was calculated with a dedicated tool [1], that simulates 14 scenarios of setup and range errors. As a result, effects of breathing motion, interplay, setup errors and range uncertainty were evaluated by analyzing the V 95 of the target. Results Target coverage was sufficient for all nominal plans (V95 > 99.5%). Results from the robustness evaluation are shown in Table 1 and Figure 1. All 4D plans recalculated for PPlus showed lower V 95 values than the initial plan. POne obtained adequate coverage for all patients just employing intrinsic repainting. The use of a range shifter, which increase the spot size in POne plans may explain partially these results .

and

ΔD avg

results for

Table 1: Evaluation of ΔD max

the main OARs.

Table 2: Relative variations of several indexes evaluated. An improvement in CI value was achieved by utilizing a 67% prescription isodose, hence creating a sharper dose distribution nearby the PTV (a GI reduction). These results are statistically significant for CI (p=0.008) and for GI (p=0.003). Conclusion The use of lower prescription isodose levels in laterally located lung tumours improves conformation to PTV, steepens the dose fall-off and significantly diminishes average dose in lungs, chest wall and bronchial tubes. However, maximum doses are increased in lung and chest wall. A good management of heterogeneities in GTV is desirable due to the stimulation of immunological response and tumour vascular epithelium damage linked