Presenter of 1 Presentation
COMBINING PROTON CONFORMAL FLASH WITH TARGET LET OR RBE DOSE MAXIMIZATION – BEST OF TWO WORLDS?
Abstract
Background and Aims
We explore potential benefits of combining proton conformal FLASH delivery with LET optimization to enhance high LET energy depositions in the target volume. While high dose rate reduces damage to healthy tissue elevated LET increases the therapeutic effect. Increasing target LET comes at the price of higher dose in the portal volumes. The goal of this work was to seek a sweat spot where an overall gain is achieved.
Methods
Oppose beam prostate MFO conformal FLASH plans with single energy layer beams (214 MeV) and beam specific 3D range modulators were produced by Monte Carlo based optimization. LET enhanced plans were created by adding LET maximizing functions and a dose limiting penalty on the portal dose.
Results
For one exemplar plan (α/β=3, 3 Gy(RBE)/fx, 200 nA beam current) target LET increased from 2 keV/μm to 3.4 keV/μm (RBE 1.14 to 1.17) while portal dose increased by 14%. For the LET enhanced plan characteristic high LET horns at the target periphery were reduced from 4 keV/μm to 2.5 keV/μm. About 80% of the portal dose was delivered at >40 Gy/s evaluated over a 100 ms sliding window.
Conclusions
To yield an RBE gain the fraction dose had to be kept modest which worked against onset of the FLASH effect. For the case studied no overall therapeutic gain is expected. The complete elimination of the peripheral LET horns is advantageous for robustness and for risk organ protection. This feature is independent of fraction dose or dose rate and should be considered for future clinical application.
Author Of 3 Presentations
TREATMENT PLANNING TOOLS TO EVALUATE THE FEASIBILITY OF FLASH THERAPY WITH PROTONS
Abstract
Background and Aims
To exemplify the potential and limitations of two approaches to FLASH treatment planning with protons by testing them on two clinically realistic scenarios and different FLASH-specific parameters
Methods
We selected two planning approaches to be delivered with a cyclotron: 3D range modulator (3DRM) and transmission beams(TB). (See Table below for details on the beam delivery parameters.) We associated each planning technique with a disease site and a clinically applied hypofractionation protocol ( 3DRM - liver - 3x25Gy, TB - lung - 3x20Gy). We evaluated the resulting dose distributions for different beam currents (200nA and 800nA at isocentre), two dose rate definitions (dose-averaged dose rate (DADR) and a sliding time window), two minimum dose thresholds and two dose rate thresholds for the FLASH effect (4Gy and 8Gy, 40Gy/s and 100Gy/s, respectively).
Results
Both techniques achieved acceptable dose distributions with a limited number of fields (liver - 1 field, lung - 3 fields) for FLASH proton plans. All combinations of beam intensity, dose rate definition, dose and dose rate threshold we investigated were associated with some level of FLASH dose, suggesting that these disease sites and dosimetric protocols are reasonable candidates for FLASH proton therapy. The figure below shows an example of the results for 200nA and 4Gy and 40Gy/s thresholds.
Conclusions
Treatment planning studies are a useful tool to test candidate disease sites, protocols and planning techniques for proton FLASH. The next step will be to include additional combinations of beam production systems, planning techniques, and patient anatomies.
COMBINING PROTON CONFORMAL FLASH WITH TARGET LET OR RBE DOSE MAXIMIZATION – BEST OF TWO WORLDS?
Abstract
Background and Aims
We explore potential benefits of combining proton conformal FLASH delivery with LET optimization to enhance high LET energy depositions in the target volume. While high dose rate reduces damage to healthy tissue elevated LET increases the therapeutic effect. Increasing target LET comes at the price of higher dose in the portal volumes. The goal of this work was to seek a sweat spot where an overall gain is achieved.
Methods
Oppose beam prostate MFO conformal FLASH plans with single energy layer beams (214 MeV) and beam specific 3D range modulators were produced by Monte Carlo based optimization. LET enhanced plans were created by adding LET maximizing functions and a dose limiting penalty on the portal dose.
Results
For one exemplar plan (α/β=3, 3 Gy(RBE)/fx, 200 nA beam current) target LET increased from 2 keV/μm to 3.4 keV/μm (RBE 1.14 to 1.17) while portal dose increased by 14%. For the LET enhanced plan characteristic high LET horns at the target periphery were reduced from 4 keV/μm to 2.5 keV/μm. About 80% of the portal dose was delivered at >40 Gy/s evaluated over a 100 ms sliding window.
Conclusions
To yield an RBE gain the fraction dose had to be kept modest which worked against onset of the FLASH effect. For the case studied no overall therapeutic gain is expected. The complete elimination of the peripheral LET horns is advantageous for robustness and for risk organ protection. This feature is independent of fraction dose or dose rate and should be considered for future clinical application.
CAN UHDR VHEE DEVICES WITH ONLY A FEW FIXED BEAMS PROVIDE COMPETITIVE TREATMENT PLANS COMPARED TO VMAT ?
Abstract
Background and Aims
Future RT devices using very-high energy electrons (VHEE) (50-250MeV) may produce suitable beams to treat deep-seated tumours conformally and at ultra-high dose rates (UHDR) capable of triggering the FLASH effect. The FLASH effect has been observed for large doses delivered with overall treatment times less than 200ms. Such treatment durations do not allow the use of a movable gantry and multiple fixed beam lines (FBL) become mandatory. This treatment planning study evaluates VHEE dose distributions in patients using a varying number of FBL with different energies and source-axis-distances (SAD). The minimum requirements for delivering conformal VHEE RT comparable to conventional VMAT plans and trade-offs between plan quality and number of beam lines are assessed.
Methods
We performed VHEE and VMAT treatment planning for multiple indications (glioblastoma, mediastinum, lung, prostate) using RayStation (research version) and compared the dosimetric quality of VHEE plans to VMAT while assessing the impact of arrangement and number of FBL, beam energies, and SAD.
Results
Most substantial coverage and conformity improvement is achieved when increasing the beam energy from 50 to 100MeV. Further improvement is obtained specifically for deep-seated targets (>10-15cm) when increasing energies further to 200MeV. While VHEE plans using 16 coplanar beams outperform VMAT plans, we found that VHEE plans using only 3-5 beams have DVH metrics that are comparable to VMAT plans. Beams with SAD>1m are preferable for treatments using few beams.
Conclusions
UHDR VHEE devices with only a few FBL may provide competitive dosimetric conformity that may be additionally enhanced by the FLASH effect.