Presenter of 2 Presentations
A NEW MODEL OF GAS CHAMBER FOR UHDR RANGE
A NOVEL METHOD FOR DETERMINING IC SATURATION FACTOR (UP TO 0.5 GY/P FOR ADV. MARKUS)
Abstract
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
qcol is the charge collected by IC;
V is the voltage applied to IC;
qgen 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 (qgen, 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 ksat is determined as:
being α = A / V .
Results
The method has been adopted for estimating ksat 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 ksat is known, ionometric measurements at the larger distance might become the central element of a reliable Quality Assurance program for any Flash linac.
Author Of 5 Presentations
A NEW MODEL OF GAS CHAMBER FOR UHDR RANGE
CHARACTERIZE THE ELF: THE NOVEL ELECTRON FLASH IRRADIATION SYSTEM UNVEILED WITH STANDARD DOSIMETRIC TOOLS_
Abstract
Background and Aims
As the development of suitable detectors in ultra high Dose Per Pulse (DPP) beams is on-going, temporary protocols solving limitations with standard tools are required for the acceptance of new dedicated machines. We present a protocol for system commissioning using clinically available dosimeters, optimizing the use of the detectors. Extra attention was given to beam characterization when varying the DPP.
Methods
Measurements were performed on the ELF, a dedicated research linac for Flash radiotherapy with electrons (ElectronFlash, SIT, Italy) at 7 and 9 MeV using standard dosimetric tools: a PTW Advanced Markus (AM) plane-parallel ion chamber, Gafchromic EBT-XD films, and alanine pellets. The system is highly tunable for pulse length, pulse repetition frequency and number of pulses. Collaboration with the developer allowed further tuning. The DPP was varied using pulse length, distance from the linac and e-gun current.
Results
Performance tests assessing dose, dose stability, PDD, profiles, and output linearity to system settings were measured cross-referencing at least two detectors (comparable within 5%). Energy and dose outputs, monitored with films and AM, were stable within 5% in several months. The AM was used for daily QA for doses up to 0.5 Gy/pulse (model proposed in abstract #211). Changing the DPP using the above-mentioned parameters resulted in a linear output change and minimal variation (<1 MeV) of the spectrum.
Conclusions
We characterized the ELF using standard dosimetric tools. We propose the combination of alanine and Gafchromic films as absolute dosimetric standard. The AM can be employed as real-time tool for daily QA.
A FEASIBILITY STUDY OF IORT-FLASH USING A GPU-BASED FAST MONTE CARLO
Abstract
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.
INORGANIC SCINTILLATORS FOR FLASH-IORT DOSIMETRY: DEVELOPMENT AND TEST OF A LYSO DETECTOR PROTOTYPE
Abstract
Background and Aims
Fast and accurate active dosimeters are the key component to perform quantitative measurements in FLASH radiotherapy. No reference active dosimeters are currently available since most detectors show non recoverable saturation effects for dose-per-pulse (DPP) values typical of FLASH (1Gy/p or higher). Aim of this study is to develop and test a detector prototype based on inorganic scintillators for FLASH-IORT active dosimetry.
Methods
The detector prototype is composed of a LYSO scintillating crystal (2x2x10 mm^3) wrapped in 5 Teflon layers and coupled to an optical fiber 1.2 m long (0.980 mm diameter, PMMA core). The other end of the fiber is connected to a Photodiode (PD, Thorlabs - SM05PD7A) read out by a multimeter (Keithley 617) that integrates the PD photocurrent. The detector was placed in a PMMA support and covered with solid water slabs. The phantom was irradiated with a 7 MeV IORT electron LINAC (NOVAC7 from SIT, Aprilia, Italy) at different SSDs and depths in solid water to vary DPP at the detector position. The DPP spanned from conventional (3 cGy/pulse) to FLASH values (250 cGy/pulse). The DPP was evaluated by means of calibrated Gafchromic films.
Results
The PD integrated charge was linear in the whole range of DPP values explored and 0.46 nC/Gy in sensitivity was measured.
Conclusions
Further measurements are planned to fully characterize the detector such as extend the DPP upper limit and investigate the dependency on the instantaneous DPP, but these first results indicate LYSO based detectors as promising candidates for FLASH-IORT active dosimetry.
A NOVEL METHOD FOR DETERMINING IC SATURATION FACTOR (UP TO 0.5 GY/P FOR ADV. MARKUS)
Abstract
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
qcol is the charge collected by IC;
V is the voltage applied to IC;
qgen 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 (qgen, 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 ksat is determined as:
being α = A / V .
Results
The method has been adopted for estimating ksat 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 ksat is known, ionometric measurements at the larger distance might become the central element of a reliable Quality Assurance program for any Flash linac.