Khan Outline: Ch 26 Proton Therapy

Front 1

Describe how high and low Z materials affect the stopping power of protons.

Back 1

Mass stopping power for protons is greater in low-atomic-number (Z) materials. Therefore, low-Z materials are more effective in slowing down protons.

High-Z materials scatter protons through larger angles than the low-Z materials. Thus, if one wants to scatter a beam with minimal loss of energy, one should use high-Z materials, and if one wants to decrease proton energy with minimum scattering, we should us low-Z materials.

Front 2

Define stopping power.

Back 2

The average rate loss of a particle per unit path length in a medium.

Front 3

What is linear stopping power also known as? Units?

Back 3

Stopping Power

MeV cm-1

Front 4

What is Bragg Peak?

Back 4

The sharp increase or peak in dose deposition at the end of the particle range.

Front 5

What is SOBP?

Back 5

Spread Out Bragg Peak.

To provide wider depth coverage, the Bragg peak can be spread out by superposition of several beams of different energies.

Front 6

Define RBE.

Back 6

Relative Biological Effectiveness is defined as the ratio of the dose of 250 kVp x-rays to produce a specified biologic effect to the dose of the given radiation to produce the same effect.

Front 7

What is the RBE for protons? When is it the greatest?

Back 7

The RBE for protons is 1.1

The RBE continuously increases with depth

Front 8

What are the three Proton accelerators.

Back 8

1. Linear Accelerator

2. Cyclotron

3. Synchrotron

Front 9

Describe the Linear Accelerator for Protons

Back 9

Conventional linear accelerators are not suitable for accelerating protons or heavier charged particles to high energies required for radiotherapy.

Front 10

Describe the Cyclotron for Protons

Back 10

- Cyclotron used in radiotherapy is a fixed-energy machine, designed to generate proton beams of a maximum energy of approximately 250 MeV

- This energy is sufficient to treat tumors at any depth by modulating the range and intensity of the beam.

- They cyclotron is isochronous- all the particles in the accelerator revolve at the same frequency regardless of their energy or obit radius. This means that the accelerator runs continuously during treatment and can deliver high dose rates as needed.

Cyclotron operates at a fixed maximum energy and requires energy degraders to treat more superficial tumors and to create SOBP beams at any depth.

Front 11

Describe the Synchrotron for Protons

Back 11

- In a synchrotron, the frequency of the accelerating potential is adjusted to compensate for the decrease in particle velocity.

- The proton beam of 3 to 7 MeV, typically from a linear accelerator, is injected and circulated in a narrow vacuum tube ring by the action of magnets located along the circular path of the beam.

- The proton beam is accelerated repeatedly through the radiofrequency (RF) cavity, powered by a sinusoidal voltage with a frequency that matches the frequency of the circulating protons.

- Protons are kept within the tube ring by the bending action of the magnets.

- When the beam reaches the desired energy, it is extracted.

- Synchrotrons are operated to produce the SOBP beams at any desired depth without the use of energy degraders.

Front 12

What is passive beam spreading?

Back 12

The beam is scattered using thin sheets of high atomic number materials.

- Dual scattering foils are required to obtain large fields of acceptable cross-beam uniformity.

- Custom blocking is used to shape the fields.

- Range modulators (low Z) are used to spread the Bragg peak.

- Range shifters (low Z) are used to adjust the depth of the Bragg peak.

Front 13

What is pencil beam scanning?

Back 13

Uniform fields are produced without loss of range by sweeping magnets to specific static positions to deliver the dose.

- Spot scanning, in which a pencil beam is moved by sweeping magnets to specific static positions to deliver the dose.

- Raster scanning, in which a pencil beam scans the field in a raster.

Front 14

How is absorbed dose calibrated for protons?

Back 14

General formalism for the determination of absorbed dose to water for proton beams is the same as for the photon and electron beams.

Calibration is performed with an ionization chamber in a water phantom.

- Chamber is calibrated by the reference calibration laboratory (NIST) in terms of absorbed dose to water in a 60Co y-ray beam.

- Reference calibration is based on absolute dosimetry of 60 Co beam using a calorimeter.

Front 15

What is the residual range of protons?

Back 15

The residual range Res is obtained from the measured depth dose curve.

- The residual range Res is determined from the measurement of Rp and Zref: Res = Rp - Zref

Front 16

What is the quality correction factor?

Back 16

Beam quality factor Kq,q0 is defined as the ratio of calibration factors for the given ion chamber in terms of absorbed dose to water irradiated by beams of quality Q and Q0

Front 17

How do the dose distributions of protons look compared to photons and electrons?

Back 17

Protons give less integral dose by a factor of about 3

Front 18

What treatment planning principles must be used for proton therapy?

Back 18

Basic principle of radiotherapy treatment planning for protons are essentially the same as for photons. These include 3D imaging data sets, delineation of target volumes and organs at risk, selection of beam angles and energies, design of field apertures, optimization of treatment parameters through iterative or inverse planning, display of isodose distributions and dose-volume histograms.

Front 19

What is the nozzle?

Back 19

A modern nozzle consists of many components for creating and monitoring a clinically useful beam

Components:

- Rotating range-modulator wheel
- Range-shifter plates to bring the SOBP dose distribution to the desired location in the patient
- Scattering filters to spread and flatten the beam in the lateral dimensions
- Dose monitoring ion chambers
- An assembly to mount patient-specific field aperture and rand compensator

Front 20

What algorithms are used in proton therapy (3)?

Back 20

- Pencil Beam

- Convolution/superposition

- Monte Carlo

Front 21

What are the clinical applications for protons?

Back 21

Proton beam therapy has been used to treat almost all tumors that are traditionally with x-rays and electron.

Proton therapy provides an excellent option for tumors in proximity of critical structures such as tumors in the brain, eye, and spine.

Protons can be optimized by the use of intensity-modulated proton therapy, achieving dose conformity comparable to intensity-modulated radiation therapy but with much less integral dose.

Front 22

What QA is recommended for protons?

Back 22

1. Automatic procedures to ensure correct beam energy being transported in the treatment nozzle and subjected to correct beam modulation to create the desired SOBP.

2. For a pencil beam scanning system, QA procedures to monitor scanning patterns determined by the treatment-planning system.

3. QA procedures to monitor the functionality of various interlocks related to beam delivery and patient safety.

4. Daily check of monitor unit calibration.

5. Verification of each treatment portal and monitor units.

6. Verification of correct installation of auxiliary equipment and patient -specific treatment aids.

7. Checking of patient setup and positioning systems.

8. QA of treatment-planning system and imaging devices.