ESTRO 2021 Abstract Book

S430

ESTRO 2021

patients received loco-regional irradiation with a contralateral CIED. Range shifters of 57 mm WED were used for all plans. SNE was evaluated for breast plans both with 25 fractions of 2 Gy RBE and 15 fractions of 2.67 Gy RBE and for H&N plans with 66 to 68 Gy RBE to CTV high risk, 60 Gy RBE to CTV intermediate risk and 50 Gy RBE to CTV3 low risk in 33 or 34 fractions. SNE was calculated as ambient dose equivalent H*(10) using Monte Carlo simulations in TOPAS v3.5. The CIED was isotropically expanded with 5 mm to define an area that accounted for treatment uncertainties. Results For breast cancer patients with contralateral CIED, SNE of the CIED+5mm was below 7 mSv per fraction for CTV (CTVp+CTVn) < 1500 cc in 2 Gy RBE fractions and CTV < 1000 cc in 2.67 Gy RBE fractions (see Figure 1c). For breast cancer patients with ipsilateral CIED, SNE of the CIED+5mm was between 5.6 and 9.6 mSv. SNE of the CIED+5mm was below 7 mSv per fraction in all H&N cancer patients (see Figure 1d). Based on these results, we created a clinical algorithm that can be used to decide whether it is possible, may be possible or not possible to treat a patient with CIED close to the target with proton therapy (see Figure 2).