ESTRO 2020 Abstract book

S937 ESTRO 2020

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.

may lead to under/overdosage of the target/organs-at-risk (OARs), compromising the patient outcome. The research aims to evaluate the impact of positioning errors in the dosimetry of volumetric modulated arc therapy (VMAT) for left-sided PMRT. Material and Methods Eleven VMAT treatment plans clinically delivered were included in this study. For each plan, eighteen perturbations were introduced shifting the isocenter from its reference position of 3, 5, 10 mm, in the inferior, superior, anterior, posterior, left and right directions. The dose-volume histogram (DVHs) of the target, left lung and heart were obtained for each perturbation introduced and case studied. A total of 209 DVHs were analysed. The absolute difference of the mean dose (Dm) and of the DVH endpoints Vx and Dy (percentage volume receiving x Gy, and dose covering y% of the volume, respectively), were used to compare the dosimetry of reference versus perturbed plans. Particularly Dm, V25, and V40 for the heart, Dm, V20, and V40 for the left lung, and D95, D98, Dm, and D1 for the clinical target volume (CTV). In Figure 1, the DVHs obtained for isocenter shifts of 10, 5, 3 mm, for a representative patient were shown.

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 compared with thermoplastic masks (TM). 3DPrIm,


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