ESTRO 36 Abstract Book

S77 ESTRO 36 _______________________________________________________________________________________________

Conclusion The systematic sensitivity study revealed the capability of the PGI slit camera to detect range shifts under clinical conditions. In both treatment modalities, global range shifts can be detected. Additionally, in PBS a spot-wise comparison allows also the determination of interfractional local range shifts. Moreover, a still ongoing evaluation of PBS measured and simulated spot-wise profiles for absolute range verification will be presented. OC-0154 Proton therapy patient selection for oropharyngeal cancer patients: the impact of treatment accuracy M. Hoogeman 1 , S. Breedveld 1 , M. De Jong 2 , E. Astreinidou 2 , L. Tans 1 , F. Keskin-Cambay 1 , R. Bijman 1 , S. Krol 2 , S. Van de Water 1 , T. Arts 1 1 Erasmus MC Cancer Institute, Radiation Oncology, Rotterdam, The Netherlands 2 Leids University Medical Center, Radiation Oncology, Leiden, The Netherlands Purpose or Objective Comparative treatment planning including Normal Tissue Complication Probability (NTCP) evaluation has been proposed to select patients for proton therapy. NTCP, however, does not only depend on the type of radiation used, but also on the size of the safety margins or degree of robustness needed to account for treatment-related uncertainties. In this study, for the first time to our knowledge, the impact of margins and robustness settings to the selection of oropharyngeal cancer patients is investigated using fully automated comparative treatment planning. Material and Methods CT and contour data of 78 consecutive oropharyngeal patients were imported in our in-house developed system for automated treatment planning for Intensity-Modulated photon (IMRT) and proton radiotherapy (IMPT). Treatment plans were generated fully automatically for a simultaneously integrated boost scheme prescribing 70 Gy RBE to the primary tumor and pathological lymph nodes and 54.25 Gy RBE to the elective nodal areas in 35 fractions. IMRT treatment plans were generated with a 0, 3, or 5mm margin. IMPT 'minimax” robust optimized treatment plans were generated for five different setup and range robustness settings. Five validated NTCP models (see Fig. 1) proposed for IMPT patient selection were used in this study. Following Dutch consensus guidelines, patients were selected for IMPT if IMPT reduced NTCP by 10% or 5% for a grade II or a grade III complication, respectively. Results In total 624 treatment plans were generated automatically and approved by the authors. Figure 1 shows that the percentage of patients selected for IMPT decreases with increasing robustness setting for a given margin and also decreases with decreasing margin for a given robustness setting. In contrast to the size of the margin, the degree of robustness has little impact on patient selection for tube feeding dependence, which is the only grade III complication. For the other complications the impact of the degree of robustness setting is noticeably higher. For patient-rated sticky saliva, nearly no patient is selected for IMPT if robustness is included. If we consider high- precision IMRT using a 3mm margin and high-precision IMPT using a robustness setting of 3mm for setup and 3% for range errors, most patients are selected for proton therapy based on problems swallowing solid food (51.3%), followed by tube feeding dependence (37.2%) and decreased parotid flow (29.5%). Patient-rated sticky saliva and patient-rated xerostomia contributed only with 1.3% and 7.7% respectively.

One beam (1 GyE) was applied to a CIRS head phantom and monitored with the PGI slit camera. To investigate the influence of the spot dose, the same beam with 5 GyE was also delivered and measured. Global and local (5 cm in diameter) range shifts were introduced and the PGI profiles (prompt-γ counts over depth) with and without shifts were compared. Sum profiles containing prompt-γ counts over the entire fraction were used for the comparison of DS and PBS. Moreover, PGI profiles measured in PBS were analyzed spot-wise and will also be compared with simulated profiles for absolute range determination. Results A good agreement between introduced and measured global shifts was found in the sum profile evaluation for both modalities, PBS and DS (Table 1). Relative differences were below 2, 7 and 12 % for the 10, 7 and 4 mm shifts, respectively. Local shifts are not detectable using sum profiles. For the applied local shifts, a spot-wise comparison of PGI profiles in PBS allows the detection and localization of global and local shifts (Figure 1). For interpretation, neighboured spots should be clustered, as shifts detected for single spots are less reliable due to low statistics. Higher doses (5 vs. 1 GyE) allow the detection of smaller shifts as shown in Figure 1 for the 4 mm local shift. Table 1: Measured global shifts between sum profiles in DS and PBS with 1 and 5 GyE.

Figure 1: Spot-wise analysis of the determined range shifts: The points represent PBS spots of one energy layer, the size corresponds to the dose per spot, the color to the detected shift between two PGI profiles. Spots influenced by the local shifts (black ring) are highlighted with a black edge. Global and local shifts with 1 and 5 GyE were measured.