The Physics of Medical Therapy Accelerators

A three part lecture series byGeorge Coutrakon, PhD

Asst. Professor in School of MedicineLoma Linda University Medical Center

Dept. of Radiation MedicineLoma Linda, California

Course Outline Feb 1st , 2008

• Introduction to Therapy Machines• What are the technical goals for hitting the target

with high doses of radiation?• What are the beam delivery systems that give the

most conformal dose to the target? • How do we shape the beam to hit the target with

good conformity both for protons and for X-rays?• How do these clinical requirements translate into

accelerator requirements?

Course Outline Feb. 2nd ( in 2 parts)

• Physics of electron and proton medical accelerators

• The physics of 3D intensity modulated (x-ray) radiation therapy (IMRT)

• Theory of operation of cyclotrons and synchrotrons for therapy

• Examples of commercial proton systems• Heavy Ion Accelerators and the push for Heavy

Ion Therapy and the Heidelberg Ion Therapy project

Electron linear accelerator mounted on a gantry

X-ray and electron treatment head

Requirements for Radiation Therapy

• Beams must penetrate the body deep enough for shallow and deep tumors ( 0 to 35 cm in water equivalent material

• Field sizes should be adjustable from 1 cm diameter up to 30 cm x 30 cm

• Dose should be uniform to +/- 3% over the tumor volume• Dose fall off at boundary of tumor volume should be

sharp, typically 90% of maximum dose to 10% of maximum dose in less than 1 cm.

• Need to treat 1 to 2 Gy/minute for a typical daily dose of 2 Gy.

• The national average for full course of treatments is about 30 daily fractions with 5 fractions per week.

Patient Treatment set-up for cranial tumors

Dose vs. Depth for typical clinical x-ray and electron beams

Dose vs. Depth in Tissue for P+, e-

and X-Rays

Dose = (Np/A) (1/ρ) dE/dx

dE/dx = 4π z2 e4 NA ρ (Z/A) (Log f(β) - β2)/mec2 β2

Energy Loss of Charged Particles in matter vs. particle momentum

Energy Loss of charged particles in matter vs. particle momentum

∫=0

0

)//(E

dxdEdER

50 to 250 50 to 250 MeVMeV is clinical range of is clinical range of interest interest

Eye Tumors at 50 Eye Tumors at 50 MeVMeV; Prostate Tumors at 250 ; Prostate Tumors at 250 Proton Energy vs Range in Tissue

05

10152025303540

0 100 200 300

Energy (MeV)

Ran

ge (c

m)

Series1

Proton intensity requirements for therapy (derived from energy loss tables, Janni, 1982)

Result: need about 1 x 109 protons/cm for 1 Gy

Requirements for Therapy Medical Accelerators

• Create beams that give the highest dose conformity to tumor and spare nearby healthy tissue. Maximize ratio of tumor control/side effects

• Reliability for high throughput--- 95% uptime required• a) Linear accelerators can treat up to 35 patients per day at 20

minute intervals• b) Proton accelerators do not fit on gantries and feed beams to 4 or 5

rooms that each can treat 40 patients per day• Energy range – Need variable energy depending on tumor depth • a) Electrons 6 – 25 MeV• b) Protons 70 – 250 MeV• c) Carbon ions 100 to 430 MeV/amu• Beam Current for 1 to 2 Gray/minute @ 1 – 2 min./treatment• a) Electrons: 300 μA to make Xrays, 1 μA for e- treatments• b) protons: 3-5 nAmps or 1 x 1012 protons/ minute• c) carbon: 3 x 109 C6+ per minute

Accelerator Requirements (con’t)

• Beam energies must be changed frequently throughout the day

• Good intensity uniformity over the treatment duration• Compact to fit in hospital environment

Accelerator requirements (con’t)

• Beam energies must be changed frequently throughout the day with good stability

• Good intensity uniformity over the treatment duration• Compact to fit in hospital environment• To achieve these goals, patients should have rotatable beam

lines called gantries that can shoot beams from any angle to a common intersection point ( called isocenter)

• The beam energy should be variable to achieve targeting at a variety of depths in the patient

• The beam delivery system will dictate the accelerator requirements, to some extent

Beam Delivery Systems (X-rays)Two modes of beam delivery in X-ray treatment are generally used

1. Conventional X-ray treatment (CRT) which uses 2 to 6 portals with fixed collimators

2. Intensity Modulated Radiation Treatment (IMRT)a) use 10 to 30 portals in 50 gantry increments called “step and shoot”b) within each step, use multiple collimator openings to modulate the beam intensity across the field. Beam energy and intensity from the accelerator are held constant.

Beam delivery systems for Hadron TherapyBoth need fewer portals per target due to the increased dose sparing of

the Bragg Peak. 1. Deliver uniform or equal SOBP along any ray trajectory throughout the

target. This fixed modulation usually accomplished by passive scattering and passive range modulation techniques

Hadron Beam Delivery Systems (con’t)

2. Conform the SOBP to the shape of the target both on the distal side and on the entry ( proximal side). This is called variable modulation since the width of the SOBP is not the same for all “rays” through the targetThis is usually accomplished using beam scanning techniques and “painting” each layer of the target with a unique energy. The energy variation is produced at the acceleraorrather than in the nozzle

Superposition of Bragg Peaks

Passive beam spreading vs. dynamic beam spreading techniques

Lateral Beam Spreading using Pb

Beam Spreading using Magnetic Deflection of Pencil Beams

Fast and Slow Scanning Magnets at LLUMC

Patient in Treatment Position in Proton Gantry

Dose distributions to prostate gland using 6 field x-rays (CRT) and 2 field protons

Propellor for Range Modulation

LLUMC Beam Delivery System

Example of spot scanning technique for Intensity Modulated Proton Therapy (IMPT)

Additional Accelerator Requirements for Hadron Beam Scanning

• Programmed sequence of energies in small (1 – 2 MeV steps) to deliver SOBP.

• Ability to change energies at the accelerator in several seconds

• Increased accuracy of accelerated energy (+/- 0.2 mm of range corresponding to about +/- 0.2 MeV.

• Programmed sequence of intensities from the accelerator with dynamic range approx. 10:1