ESTRO 36 Abstract Book

S856 ESTRO 36 _______________________________________________________________________________________________

EP-1589 A novel integrated biological optimization strategy for cervical carcinoma C. Tao 1 , Z. Feng 2 , J. Zhu 1 , J. Lu 1 , Y. Yin 1 1 Shandong Cancer Hospital Affiliated to Shandong University- Shandong Academy of Medical Sciences, Radiation Oncology, Jinan, China 2 School of Physics and Electronics- Shandong Normal University, Shandong Province Key Laboratory of Medical Physics and Image Processing Technology, Jinan, China Purpose or Objective The purpose of this study was to evaluate the quality of intensity-modulated radiation therapy (IMRT) plans optimized by biological constraints for cervical carcinoma. Furthermore, a new integrated strategy in biological planning module was proposed and verified. Material and Methods In this study, twenty patients of advanced stage cervical carcinoma were enrolled. For each patient, dose volume based optimization (DVBO), biological model based optimization (BMBO) and integrated strategy based optimization (ISBO) plans were produced with the same treatment parameters. Different biological models, including LKB and Poisson model, were also used for organ at risk (OAR) respectively. To evaluate the plan quality of BMBO plans, the dosimetry differences between BMBO plans and their corresponding DVBO plans were evaluated. And ISBO plans were compared with DVBO and BMBO plans respectively to verify the performance of the integrated strategy proposed in this study. Results Compared with DVBO plans, BMBO plans produced slight inhomogeneity and worse coverage of planning target volume (PTV) (V95 ＝ 96.787, HI=0.098: p<0.01). However, the tumor control probability (TCP) value were comparable. And BMBO plans produced lower normal tissue complication probability (NTCP) of rectum (NTCP=0.108) and bladder (NTCP=0.144) compared with corresponding DVBO plans (NTCP=0.186 and 0.178 for rectum and bladder, respectively) with significant differences. Better results of V95, D98, CI and HI values could be found in ISBO plans (V95=98.307, D98=54.181Gy, CI=0.762, HI=0.087) compared with BMBO plans (V95=96.787, D98=53.419Gy, CI=0.707, HI=0.098) with significant differences. Furthermore, ISBO plans produced lower NTCP values of rectum (NTCP=0.14) and bladder (NTCP=0.159) than DVBO plans (NTCP=0.186 and 0.178 for rectum and bladder, respectively; with p<0.01 and p<0.01 for rectum and bladder, respectively). Conclusion For cervical carcinoma cases, BMBO plans produced lower NTCP values of OARs than DVBO plans with a little worse target coverage and homogeneity. And the integrated strategy could produce better coverage, conformity and homogeneity of PTV than BMBO plans, and reduce the NTCP values of OARs compared with DVBO plans. EP-1590 Can bolus range shifting improve plan quality in the IMPT of head and neck cancer? S. Michiels 1 , A. Barragán 2 , K. Souris 2 , K. Poels 3 , W. Crijns 3 , J. Lee 2 , E. Sterpin 1,2 , S. Nuyts 1,3 , K. Haustermans 1,3 , T. Depuydt 1,3 1 KULeuven - University of Leuven, Department of Oncology, Leuven, Belgium 2 Université catholique de Louvain, Center of Molecular Imaging- Radiotherapy and Oncology - Institut de Recherche Experimentale et Clinique, Woluwe-Saint- Lambert, Belgium 3 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium

Purpose or Objective In intensity-modulated proton therapy (IMPT) of head and neck cancer (HNC), range shifter (RS) air gap is known to widen the pencil beams reaching the patient. The actual effect on dose distributions however remained unassessed. Moreover, emerging technologies such as 3D printing enable to implement RS as bolus, hereby removing the air gap and reducing the risk of collision compared to nozzle-mounted RS. In this study, we assess the impact of clinically applied air gaps on IMPT plan quality, the potential of Monte Carlo (MC) dose calculation plan optimization to mitigate air gap effects, and the potential advantage of bolus RS compared to currently used RS solutions. Material and Methods Oropharyngeal cancer patients were selected for IMPT based on potential reduction of normal tissue complication probability (NTCP) for xerostomia and dysphagia. Prescription was 54 Gy to the elective neck and 66 Gy to the primary tumor in 30 fractions. Table 1 shows the considered organs-at-risk (OAR). For the treatment planning, a 5 mm CTV-to-PTV-margin was used, and gantry angles 50°, 180° and 310°, with 4 cm of PMMA as RS. Pencil beams from clinical beam data were placed with 5 mm spot spacing and target margin. Beamlets were calculated with a 2x2x2 mm³ voxel size using the MCsquare fast-MC dose engine. Beamlet weight optimization was performed using the IPOPT non-linear solver. For each patient, 3 different RS cases were compared (Figure 1): applied as bolus, mounted on a 25 cm snout accessory or on a 40x30 cm² nozzle. Firstly, the air gap effect was determined by a MC recalculation of the optimal bolus plan (=baseline) on the snout and nozzle case. This represents the scenario in which the air gap is disregarded during optimization. Secondly, the snout and nozzle case were optimized to re-establish the initial clinical goals in the presence of air gap. Finally, differences in OAR dose and NTCP between all optimized RS cases were calculated. Results Results for 3 patients were compiled in Table 1. Suboptimal planning by disregarding the air gap yielded underdosage ranging from 3.4 Gy to 7.7 Gy for PTV 66Gy (D 98% ) and from 3.8 Gy to 10.1 Gy for PTV 54Gy . Case-specific optimization restored PTV coverage, but increased the mean dose to most OARs due to the compromised lateral penumbra of the pencil beams. Compared to the bolus plan, these OAR dose increments translated into xerostomia NTCP increases between 3.9% and 7.1% and dysphagia NTCP increases between 1.5% and 5.0%. Conclusion The actual effect of RS air gaps in clinical circumstances was quantified for IMPT in HNC. Including the air gap in the plan optimization is essential, but cannot cancel out the impact of spot degradation on OAR dose. In this regard, bolus RS can considerably improve plan quality compared to snout/nozzle mounted RS. NTCP reductions for xerostomia and dysphagia achieved with bolus were not negligible compared to the 10% threshold proposed by the model-based selection of patients for IMPT (Langendijk et al. Radiother Oncol 2013).