Session Item

The Liac 12 MeV accelerator (SIT) is supplied with cylindrical applicators with diameters ranging from 3 to 10 cm. It is however possible to purchase a 12 cm diameter applicator which is indicated for intraoperative radiotherapy (IORT) of large lesions, such as sarcomas of the extremities.

The objective of this work is to provide a full dosimetric characterization of the 12 cm applicator with its four possible bevel endings and compare it with the regular applicators.

This study was carried out on a SIT accelerator model Liac 12 MeV with nominal energies of 6, 8, 10 and 12 MeV. The distal part of all applicators ends in 4 possible bevel angles: 0, 15, 30 and 45º.
All measurements were performed in water with an IBA Blue Phantom 2 beam analyzer and the IBA PPC05 and PTW 60019 microDiamond detectors.
The measurements performed comprise PDDs, OARs at maximum dose and R90 depths and field factors (FF) at maximum dose depth for the four accelerator energies.
In addition to that, a Monte Carlo modeling of the accelerator/applicator assembly has been carried out with the code PENELOPE 2018 (Nuclear Energy Agency) that completes the previous modeling of the other applicators.

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

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 longer irradiation times compared to the other applicators.