ESTRO 2021 Abstract Book

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ESTRO 2021

interlaced geometry. The MRT peak dose was matched with the conventional plan to D 99.5%

= 37.5 Gy in the

PTV.

Results The target volumes received a higher mean dose with MRT compared to the conventional plan, see figure 2. The different PTV DVH shapes presumably resulted from the MRT algorithm adaption to lung tissue. Both treatment plans fulfilled all clinical constraints for OAR for lung tumor RT. The OAR with the highest doses (trachea, esophagus, ipsilateral and contralateral lung) received a lower maximum dose with MRT than with the conventional plan, whereas the heart and the myelon received a higher maximum dose with MRT. Dose metrics of interest for MRT (and the conventional plan) were ipsilateral lung – PTV: V 20Gy = 0.0% (2.3%), V 10Gy = 2.7% (5.9%); 20 mm-shell around PTV: D mean = 13.7 Gy (23.5 Gy), D mean (Peak) = 52.0 Gy; esophagus: D max = 4.3 Gy (6.4 Gy); heart: D mean = 1.7 Gy (0.1 Gy), D max = 5.9 Gy (2.1 Gy).

Conclusion We built a framework to compare MRT with conventional RT plans for clinical geometries. First results demonstrated the potential to reduce the dose to OAR for lung tumor MRT with kV x-rays. For clinical trials, it is essential to ensure a homogeneous target dose or to use the equivalent uniform dose (EUD) to account for the micrometer-scaled dose-volume effect. Especially the region near the PTV needs further investigation, ideally with the EUD, due to a complex dose distribution instead of a clear peak-valley profile. Further, the lung microstructure should be considered in an advanced dose calculation algorithm as it cannot be resolved in

clinical CT. References [1] Donzelli et al. Phys Med Biol 2018;63(4):045013.