Background and Aims

Intra Operative Radiation Therapy (IORT) may represent one of the first clinical modalities of a Flash clinical treatment. Within IORT, whenever needed and possible, temporarily beam modifiers (such as the protection disc for breast carcinoma treatment) are used to protect the underlying healthy tissues during the irradiation. In this contribution we investigate the efficiency achievable in IORT-FLASH treatment using a GPU-based fast Monte Carlo called FRED (Fast particle thErapy Dose evaluator), as a tool for dose calculation and treatment optimization.

Methods

The FRED MC has been developed to allow a fast optimization of the Treatment Planning System in Particle Therapy (simulation time reduced by a factor of 1000), while keeping the dose release accuracy typical of a MC tool. We have simulated in detail the geometry and the material of the applicator coupled with the linac, provided by the SIT company (Aprilia, Italy). We have then combined the FRED simulation with a simple modelling of the FLASH effect and compared it with a conventional IORT treatment.

Results

The tumour coverage and the dose absorbed by the organs at risk have been compared, carrying out a quantitative analysis comparing the obtained Dose Volume Histograms, with a standard IORT treatment.

Conclusions

The results demonstrate the potential of FLASH effect in IORT and of FRED as a tool for treatment planning and dose-report calculations.

Background and Aims

Ionization chambers (IC) represent the standard for performing the commissioning of medical linacs. Nevertheless, their use in the UHDR range is not currently possible, due to the amount of charge produced by each pulse.

For dose-per-pulse (dpp) above 0.5 cGy/p, the approach implemented by international protocols for modelling ion recombination failed, because the free electron fraction p contribution must be considered. We modify the approach of Di Martino (2005) in order to obtain p by means of ionometric measurements only.

Methods

According to the proposed model:

where

q^col is the charge collected by IC;

V is the voltage applied to IC;

q^gen is the charge generated by the pulse;

A and λ are parameters depending on the IC..

By varying the voltage applied V, such function can be determined versus the unknown parameters (q^gen, A and λ ); then, such parameters can be determined by means of the non-linear regression method.

Once all parameters are known, p is calculated and and then k_sat is determined as:

being α = A / V .

Results

The method has been adopted for estimating k_sat for the Adv. Markus, both with the beam produced by ElectronFlash and by LIAC HWL.The fit provided an agreement better than 1% within IORT range and better than 5% with 0.6 Gy/p.

Conclusions

Once k_sat is known, ionometric measurements at the larger distance might become the central element of a reliable Quality Assurance program for any Flash linac.