Physics of Novel Radiation Modalities Particles and Isotopes. Todd Pawlicki, Ph.D. UC San Diego

Transcription

1 Physics of Novel Radiation Modalities Particles and Isotopes Todd Pawlicki, Ph.D. UC San Diego

2 Disclosure I have no conflicts of interest to disclose.

3 Learning Objectives Understand the physics of proton therapy Describe proton dose deposition List components of creating a proton beam Describe aspects of proton beam planning Compare proton and conventional plans

4 Some Proton History 1930 Cyclotron invented Lawrence EO, Livingston MS. The production of high speed protons without the use of high voltages. Physical Review Suggested for medical use Wilson RR. Radiological use of fast protons. Radiology First patients treated Tobias CA et al. Pituitary irradiation with high-energy proton beams a preliminary report. Cancer Research In 1961, the Harvard Cyclotron Laboratory started treating intracranial lesions st hospital-based system at the LLUMC Slater JM et al. The proton treatment center at Loma Linda University Medical Center: rational for and description of its development. IJROBP 1991.

5 Proton Facilities In Operation (accessed 3/2015) USA Japan 5 Germany Russia China 9 France Others

6 Depth Dose Photons Bragg Peak SOBP Photons Electrons Protons Schulz-Ertner et al. Semin Radiat Oncol, 2006.

7 Cobalt MV X-rays 160 MeV Protons Koehler and Preston. Radiology(104) , 1972.

8 Particle Properties Particle Symbol Charge Rest Mass e, Electron MeV e, Positron MeV p, 1 1 H Proton MeV n, 1 0 n Neutron MeV E = mc 2

9 Proton (charged particle) Interactions Electromagnetic interactions Excitation Ionization Bethe-Block formula S 1/v 2 Bragg peak p p p p e

10 Proton (charged particle) Interactions Nuclear interactions (i) p p I. Multiple Coulomb scattering Small q II. Elastic nuclear collision p Large q (ii) p III. Inelastic nuclear interaction nucleus (iii) p e, n p nucleus

11 Ionization Density 10.0 MeV Proton 0.5 MeV Proton 1.0 MeV Electron MeV Electron Hall. Radiology for the Radiologist. 4 th ed

12 Linear Energy Transfer (LET) Energy transferred per unit track length LET de dl kev μm Useful as a simple way to indicate radiation quality and biological effectiveness

13 Radiation LET (kev/ m) Cobalt-60 -rays kev x-rays MeV protons MeV protons 0.5 Hall. Radiology for the Radiologist. 4 th ed

14 Relative Biological Effectiveness Equal doses of difference types of radiation do not produce equal biological effects RBE D x ray D test RBE depends on Biological system (cell type) Clinical endpoint (early or late effects) Energy deposition characteristics Dose Hall. Radiology for the Radiologist. 4 th ed

15 RBE for Protons RBE is a function of LET RBE is not constant with depth Careful at distal end of targets and near critical structures Clinical RBE for protons Gy proton dose 1.1 Gy Cobalt dose A single value might not be sufficient Carabe et al. Phys Med Biol

16 Relative dose RBE MeV high Clinical RBE low Modulated beam Depth [cm] Source: S.M. Seltzer, NISTIIR 5221

17 Creating Proton Beams Energy should be variable starting at 70 MeV Maximum energy should be about 250 MeV F ele = q E F mag = q ( v B) F = m v2 r mv = qbr

18 Proton Beams Two basic proton accelerator options Cyclotron Protons revolve at the same frequency regardless of energy or orbit radius Synchrotron The magnetic field strength is increased in synchrony with the increase in beam energy mv = qbr

19 Cyclotron Magnet RF Magnetic Field Proton Source Proton Beam r = mv qb

20 Clinically Useful Proton Beams There are two main approaches Passive scattering systems Fixed depth of penetration Fixed modulation Active scanning systems Irradiation the target using a narrow beam Beam controlled in three dimensions

21 Passive Scattering Goitein et al. Physics Today

22 Active Scanning Goitein et al. Physics Today

23 Treatment Planning Acquisition of imaging data (CT, MRI) Delineation of regions of interest Selection of plan properties Beam directions Energies Conversion of CT values into stopping power Paganetti. Phys Med Biol

24 Range Uncertainty Dose calculation CT Imaging and calibration CT conversion to tissue CT grid size Inhomogeneities Other sources Commissioning measurement uncertainty Compensator design Beam reproducibility Patient setup Total range uncertainty 2 4% of proton range mm Paganetti. Phys Med Biol

25 Dose Distributions Photons Protons MacDonald et al. Cancer Investigation

26 Dose Distributions Greco & Wolden. Cancer

27 Dose Distributions Suit et al. Acta Oncologica

28 Proton superior to Photons

29 Proton superior to Photons

30 Proton superior to Photons LT Femoral Head PTV Bladder Rectum LT Kidney

31 Protons similar to Photons

32 Protons similar to Photons Brainstem PTV LT Optic Nerve LT Cochlea

33 Summary Proton physics differs considerably from photon and electron physics Scattering and active scanning are two methods of creating a proton beam Proton and conventional plans must be compared carefully proton plans are not always superior