Part 3

Practical aspects– utilize uniform

scanning & discrete energy stacking protons for treatments

Maglev train at China with maximum speed of 431 km/h (268 mph)

Virtual source distances for various scanning magnets

Combined scanning magnet» single virtual source distance

Dual scanning magnets» two virtual source distances» effective source size and distance are

same for both axesParallel scanning

» single "infinite“ virtual source distance

Requirements of clinical performance

Dose Reference Volume (DRV)Transverse – lateral extentInside 2-times penumbraPenumbra width

Depth –longitudinal extentWithin modulation widthRequirements for lateral extent at above are only applied for depths with the longitudinal extent.

Scan Pattern Optimization -spot shape & size in air

Spot shape in air, i.e. proton fluence distribution at the entrance of patient.Large section near patient is under vacuum for IBA delicated nozzle at Essen. Measured spot sizes for this nozzle are ~4 and 6mm for beam ranges of 32 and 20 cm in circular shape. However, when a beam-position monitor located at entrance of gantry was inserted as extra material into beamline, elongated beam spot was observed as shown at most left of top panel.

Spot sizes in air also measured for a universal nozzle at MPRI are 6 to 14mm for various ranges as shown at bottom panel. Larger spot size for universal nozzle is due to no applied vacuum from scanning magnet to patient.

Scan Pattern Optimization -spot size in patient

Spot sizes in air σair measured at MPRI was fit as a function of range R; shown dash line.

Spot sizes σpt due to scatters in patient is calculated by 0.02275R + 0.12085E-4R2 as in Hong et al publication as open circles.

Total spot size including initial spot size and scatters in patient as close circles is calculated by for each beam rangeσtot (R) = (σair

2+ σpt2)1/2

Scan Pattern Optimization -Path spacing & over-scan inside field

Optimization based path spacing & over-scan inside field

Effects of collimation -Over-scan beyond field edge

Over-scan distance beyond edges of beam-limiting devices - upstream trimming collimators & patient aperture

Scan Pattern Optimization -various depths

Scan pattern optimization of spot size and path spacing focus on depths within longitudinal extent and the center of modulated protons at beamline isocenter.

Energy Stacking Optimization -Depth Spacing

Widths of non-modulated Bragg-Peak (BP) depth dosesvaries with proton energy (i.e. beam range) and delivery system

3 options for stacking energy layers to form required flatness over longitudinal extent are needed for MPRI TR2. For options with ranges of 12-20cm and 20-27m, depth spacing between consequent layers is 0.6 cm. However, 0.3 cm depth spacing is required for ranges of 4-12 cm when the width is only 0.5 mm for 4 cm range. For widths from 3cm to 1.5cm, 4 options are needed for OKC-IBA US beam-line with ~0.6cm depth spacing for all beam ranges.

Energy Stacking Optimization -Weights of energy layers

After depth-spacing is chosen for each option, original weights of energy layers were obtained for standard range of 16cm by theoretical model as shown blue points. During commission, depth doses with original weight were measured as shown blue points at bottom panel. A correcting algorithm was used to adjust ~8% title. Optimized weights of energy layers were then obtained as shown red points in both panels.

Because widths of non-modulated protons varies significant between options, using optimized weights from standard options results significant tilt on depth doses for non-standard options. The correcting algorithm described above is used for obtaining optimized weights for various options during commission. Similar procedure has been internally performed by IBA vendor.

Energy Stacking Optimization -between proton delivery systems

Weights of energy layers Energy options – range-shifter thickness and MU courting per proton Scattering generated by materials used in energy stacking

Weights of optimized energy-stacking layers vary betweenenergy options; depend on range-shifter thickness and MU courtingper proton. Although the trend ofweights used energy layers issimilar between different protonbeam-lines, subtle difference canbe related scattering generated byenergy layers in different beam-lines.

Depth Dose Measurements

Use of a single chamber to measure the dose at all depths requires repeating the whole delivery sequence for each depth.

Use of a MLIC (multiple layer ionization chamber) allows measurement of dose at many depths using a single delivery.

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R 27cm Scan 3cmCircular Pt-by-Pt 3cm circular

MLIC F.S. 10cm

Pt-by-Pt, F.S. 10cm

MLIC F.S. 10.cm

Dmitri et. al. 2007

Lateral Dose Profile Measurements

FilmStep-by-step method with mini-chamberMPIC (multiple pad ionization chamber) - 1D

or 2D configuration

In-plane Profiles

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Water PhantomMPIC

Treatment failure recover

Partial treatment delivery

Routine QA -Range-shifter of energy layer

Check beam range at off-axis positions to verify constancy of range-shifter material thickness used for energy stacking.

Routine Modulation QA

Ensure that files storing weights of energy layers for various modulations of each option are not corrupted.

» Check weights of energy layers for standard condition daily.» measure depth dose distributions monthly.» compare routinely used files with secure master files

annually.

For a proton system that weight and scanning aptitude of each energy layer are determined by an algorithm as function of beam range and modulation, ensure that the calculation algorithm is not corrupted

» Check weights of energy layers for standard condition daily» measure depth dose distributions and lateral profiles monthly.

Summary of practical aspects for utilizing uniform-scanning protons

Optimizations of scanning pattern and energy-stacking need be performed to satisfy clinic requirements on lateral and longitudinal extent for providing good treatments.

Specific dosimeters for depth doses and lateral profiles are needed for commissioning a US proton beamline efficiently.

Recovery treatment for partial delivery is required.

Beam range verify at off-axis positions is necessary when large area of range-shifter is used for energy stacking during commission.

Routine QA for off-axis range constancy as hardware and modulation as software should be performed to assure correct dose delivery.

References

1. Moyers, M. F. “Proton Therapy” The Modern Technology of Radiation Oncology: A Compendium for Medical Physicists and Radiation Oncologists ed. van Dyk, J. (Wisconsin: Medical Physics Publishing, 1999) p. 823 - 869.

2. Meyn, R. E. Peters, L. J. Mills, M. D. Moyers, M. F. Fields, R. S. Withers, H. R. Mason, K. A. "Radiobiological aspects of electron beams" Frontiers of Radiation Therapy and Oncology 25 eds. Vaeth, J. M. and Meyer, J. L. (S. Karger AG Basel, Switzerland, 1991) p. 53 - 60.

3. Moyers, M. F. "LLUPTF: eleven years and beyond" Nuclear Physics in the 21st Century (New York: American Institute of Physics, 2002) p. 305 - 309.

4. Moyers, M. F. Vatnitsky, S. M. Practical Implementation of Light Ion Beam Treatments (Wisconsin: Medical Physics Publishing, 2011).

5. V. Anferov, “Scan pattern optimization for uniform proton beam scanning”, Med. Phys. 36(8), 3560 (2009).

6. J.B. Farr et al., “Clinical characterization of a proton beam continuous uniform scanning system with dose layer stacking”, Med. Phys. 35, 4945 (2008).

7. Das, I. J. et al., "Dosimetric problems at low monitor unit settings for scanned and scattering foil electron beams" Med. Phys. 21(6), 821 (1994).

8. Nichiporova, D et al, “Multichannel detectors for profile measurements in clinical proton fields”, Med. Phys. 34(7), 2683 (2007).