ESTRO 2020 Abstract Book

S886 ESTRO 2020

Results The results obtained for CTV are reported in Table 1. Isocenter shifts in the anterior and lateral directions lead to the maximum disagreement in the dosimetry of perturbed vs reference plans. Significant absolute differences were registered for isocenter shifts of 10 mm with a mean decrease for D95, D98, and Dm, of 7.2 Gy (range 1.7 13.5), 9.4 Gy (range 2.2 17.6), and 2.1 Gy (range 1.3 3.2), respectively. For 5 mm isocenter shifts, these mean values decreased to 1.9 Gy (range 0.9 3.8), 3.0 Gy (range 1.5 5.7) and < 0.8 Gy, respectively. Negligible differences resulted in 3 mm isocenter shifts. For the OARs only isocenter shifts in the left, posterior and inferior directions worsen the plan dosimetry; for the left lung mean V20, V40, and Dm of +8.4%, 7.6%, 3.6 Gy and +2.3 %, +2.1%, and +1.1 Gy, were registered for 10 mm and 3 mm shifts, respectively. For the heart, the higher difference was registered for isocenter shifts in the posterior direction with mean V25 and Dm of +7.0%, 3.0 Gy and +1.6 % and +0.8 Gy, respectively, for 10 mm and 3 mm isocenter shifts.

compared with thermoplastic masks (TM). 3DPrIm, however, cannot be created in the physical presence of the patient and thus requires a new RT workflow. In addition, tolerability and comfort cannot be assumed, given the novelty of the proposed 3DPrIm device. In this pilot study, the feasibility of 3DPrIm was evaluated on patients with HNC in terms of workflow, patient comfort and setup accuracy. Being the first application on actual patients, the study focused on 3DPrIm of the head. Material and Methods 3D PrIm was achieved for three HNC patients and was compared with the clinically used TM (5-points, closed face, generic headrest) using repeated CBCT on a linac. For the creation of the 3DPrIm, an additional planning CT was acquired of the patients immobilized without a TM (Fig. 1a). The segmented pristine body contour was used to design a closed posterior shell with cranial back-stop and an anterior shell with open face (Fig. 1b) in the software 3-matic (Materialise, BE). Weekly study CBCT’s (3DPrIm) and clinical CBCT’s (TM) of the same day were rigidly registered with the respective planning CT, based on a global, composite ROI consisting of the occiput and mandible. Residual setup errors were determined for five sub-ROI’s (occiput, mandible, C2, C6 and larynx) by subsequent rigid registration based on each individual sub- ROI and quantification of the difference relative to the global registration. At the end of the RT course, the patients and RTT’s completed a questionnaire polling their experience in terms of comfort and immobilization usability.

Conclusion Inaccuracy of patient positioning due to isocenter shifts of 5 mm or more, can lead to important inaccuracy in the dosimetry of VMAT left-sided PMRT plans. Perturbations of 3 mm around the isocenter do not affect the target coverage but the overdosage of the left lung and of the heart should be evaluated considering the dosimetric reference values obtained during the treatment plan. PO-1623 3D printed immobilization for head and neck cancer radiotherapy: A pilot study on patients S. Michiels 1,2 , K. Poels 1,2 , S. Nuyts 1,2 , T. Depuydt 1,2 1 University Hospitals Leuven, Department of Radiation Oncology, Leuven, Belgium ; 2 University of Leuven, Department of Oncology, Leuven, Belgium Purpose or Objective Due to the large flexibility in object shaping, 3D printing could enhance patient immobilization in head & neck cancer (HNC) radiotherapy (RT). In intensity-modulated proton therapy (IMPT), for instance, 3D printed immobilization (3DPrIm) may allow the integration of range shifter (RS) with minimal air gap, hereby reducing spot size and hardware collision risk compared with nozzle-mounted RS. In addition, the potentially improved patient-specific approximation of the body outline with 3DPrIm may reduce inter-fractional setup variations