ESTRO 2021 Abstract Book

S1358

ESTRO 2021

Results The combination of the 12 MeV energy together with the 12 cm applicator shows an irregular 90% isodose volume in water, moving away from the surface outside the central axis (fig 1). As the bevel angle is increased, the 90% isodose is reduced. When the 45º bevel is applied, the latter isodose is very small compared to the size of the applicator (9cm in one of the directions at zmax depth). This effect is less dramatic when decreasing the energy of the beam. The field factors show a downtrend similar to the other applicators as can be seen in figure 2. When selecting the 12 MeV energy, the FF of the 12 cm applicator is 12% and 42% lower than those of the 10 cm and 5 cm applicators respectively.

Fig 1: Transversal plane isodoses in water calculated with PENELOPE 2018 MC code. 12 cm applicator – 0º bevel vs. 10 cm applicator – 0º bevel, 12 MeV nominal energy.

Fig 2: 12 MeV output factors in water measured at the depth of maximum dose.

Conclusion The 12 cm applicator on some energy/bevel combinations does not meet what it is expected from an IORT accelerator. When the energy is high and the bevel angle is increased, the 90% isodose does no longer cover the surface of the tissue. If the prescription dose is adjusted to mitigate this effect, then hot spots would be added to the dose distribution. Regarding field factors, it is important to bear in mind that a treatment with the 12 cm applicator involves