UF has had a proton facility operational in the last 2 years.
Reviewed the hyperbole associated with press releases around proton facilities. Particular claims: Precise 3D treatment with "millimeter" accuracy acheivable, it is superior to other options.
Bragg Peak: A phenomena of a heavy charged particle - Entrance dose is at a steady low level, and dose is largely deposited at the end of path with no exit. Platter to peak ratios is 1:3-5 for protons. For carbon ions this ratio is much better 1:10-12
Protons do scatter from a pencil beam, and increases with increased depth. Proton penumbra is 5-8mm, which is similar to a 6MeV photon ~6mm.
20% of protons interact with atomic nuclei producing fragmentation. Effect is to smear out the bragg peak slightly, however more significantly results in increased neutron doses. This is much greater than with photon therapy - and have theoretical radiobiologic risks.
Usually setup includes a cyclotron (15-20 million$ alone) and multiple treatment rooms/gantries to maximize efficiency. Synchrotron is an alternative to the cyclotron. Fortunately, protons can be steered quite ably with a magnetic field. Gantrys are ~100 tons due to the mass required to bend the particles at speed into a useable angle.
Treatment abilities: 50x50x50cc treatable volume. +/- 3 degree pitch and roll.
Pristine Proton Bragg Peaks:
100Mev range 4.3cm
250Mev range 28.5cm
Need either lateral scanning or a scanning beam to get a spread in the 2 dimension X and Y in the Beams Eye View. Also need some sort of range modulator / variable range shifter to treat the Z depth of the tumor within the patient. This results in the "Spread Out Bragg Peak." This results in a good depth coverage, however results in an increased entrance dose. Fall off remains 2-4mm.
Thus the primary advantage is not in the reduction in entrance dose, which dissappears with the use of the SOBP, but rather the reduction in the exit dose.
High dose area extends proximally. Thus the confromality is acheiveable with only the distal edge of the target, unless one uses IMPT or a scanning beam. A disadvantage to the scanning beam is an increase in the 'interplay' effect - target motion in the middle of the treatment delivery resulting in inhomogeneities in the target.
Comparing protons and IMRT:
Primarily the dosimetric comparisons have been using treatment planning software primarily. With IMRT dose homogeneity is inversely related to normal tissue sparing. IMPT may be able to improve this relationship, and reduce the number of fields required.
"The promise of protons is that they stop. The peril of protons is that we don't always know where..."
This is relavent with minor setup changes and misalignment. The exactness of the treatment is dependent upon the exactness of setup. Whereas with photons, dose distribution is smeared in these scenarios. With Protons this creates a binary effect of dose or no dose on a given day with setup variations.
Carbon -> Carbon 11, which decays with a positron. This may be imaged with a conventional PET scanner shortly after treatment. Dosimetry can be estimated and the variation in rectal DVHs with prostate treatment is on the order of 50-90%.
This can also be affected by tumors regressing during treatment. Frequent/scheduled replanning is a matter of ongoing research.
Challenges:
- CT number (HU) conversion to stopping power, effect is much greater on proton therapy than photon therapy. Needs much better calibration than with photons.
- Dose heterogeneity correction
- Inter and Intra fractional motion of targets and normal tissue
- Uncertainties in immobilization devices - these may affect proton ranges
- PTVs need to be more precisely defined. Dr. Palta reccomends a physician and physicist sitting down to discuss every individual treatment plan.
- Error bars may need to be considered on dose distributions
For prostate treatments, lateral beams are used. Due to the uncertainties, a 1cm margin was used - much greated that that needed with IMRT.
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