Abstract Book

S109

ESTRO 37

PV-0201 Automated proton planning by mimicking the reference photon dose for patient selection R.G.J. Kierkels 1 , A. Fredriksson 2 , S. Both 1 , J.A. Langendijk 1 , D. Scandurra 1 , E.W. Korevaar 1 1 University Medical Center Groningen, Department of Radiation Oncology, Groningen, The Netherlands 2 RaySearch Laboratories AB, Stockholm, Sweden Purpose or Objective In the Netherlands, the model-based approach is introduced to select patients for proton therapy using normal tissue complication probability (NTCP) models. Patient selection is based on ΔNTCP values derived from a proton and a photon dose distribution. For planning efficiency, fully automated treatment planning is required. In this study, we developed a dose mimicking and reduce (DMR) algorithm to automatically generate a robust multi-field optimized proton plan given a reference photon dose distribution and target and organs at risk (OAR) delineations. The DMR plans were compared against manually optimized (reference) robust proton plans. Material and Methods The DMR algorithm was evaluated using clinically accepted photon dose distributions of five head and neck cancer patients. The first step of the DMR algorithm comprised of DVH-based mimicking of the photon dose distribution in the clinical target volumes (CTVs) and OARs, accounting for setup and range uncertainties of 5.0 mm and ±3%, respectively. Plan robustness was achieved by mimicking the nominal photon dose in 21 perturbed scenarios. The use of protons provides leeway to improve the dose to the non-target tissues. The second step of the optimization therefore aims to reduce OAR doses as much as possible while retaining the target coverage and uniformity achieved in the first step. We evaluated each DMR plan against the photon plan and reference proton plan in terms of plan robustness and NTCPs of xerostomia, dysphagia and tube feeding dependence. Plans were considered sufficiently robust if CTV V95%≥98% in the voxel-wise minimum dose, which was derived from 14 perturbed dose calculations with 5.0 mm shifts and ±3% range uncertainty (28 scenarios in total). To avoid any bias from proton beam angle selection the same beam configuration was used in the DMR plans and the reference proton plans. Results The quality of the DMR proton plans was very similar to the reference proton plans in terms of dose values and robustness. All proton plans showed a primary and elective CTV coverage in the voxel-wise minimum dose of V95%>98%. Figure 1 illustrates a transversal view of a photon, DMR and reference proton plan. Within the studied cases, the sum of the ΔNTCPs (difference between photon and proton plans) ranged from 11.4 – 42.0% and 4/5 patients were selected for proton therapy based on either the DMR or reference proton plan. The mean dose difference between the DMR and reference proton plans of the parotid glands, swallowing muscles, and supraglottic larynx ranged from -4.1 to 4.5 Gy. This translated into differences in ΔNTCPs of -1.0 to 2.8%.

institution remained compliant with the dose coverage criteria. PTV and gold standard-PTV V100% were on average 93.6 %(70.5-99.3) and 83.3% (54.2-93.9), respectively, indicating that delineation variations have a larger influence on PTV coverage than variations in planning and irradiation techniques.

Conclusion Clinical guidelines and radiotherapy protocols should be further improved and are not a substitute for timely RTQA procedures. Furthermore, delineation remains the main source of BC rejection. Plan review without first reviewing delineation may not be efficient as contour variations significantly deteriorate dose coverage.