Chapter 26: Proton Beam Therapy
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What do the protons interact with when traveling through a medium and what force renders these interactions possible? Pg. 515
As protons travel through a medium, they interact with atomic electrons and atomic nuclei of the medium through Coulomb force. Rare collisions with atomic nuclei causing nuclear reactions are also possible.
What are proton interactions mediated by and what are they? Pg. 515
Proton interactions are mediated by the Columbic Force.
- 1.) inelastic collisions with atomic electrons in which protons lose part of their kinetic energy to produce ionization and excitation of atoms, thereby result in absorbed dose.
- 2.) elastic scattering without loss of energy.
Two important facts:
- 1.) Proton bremmstrahlung interactions with nuclei are possible but negligible due to their size (mass).
- 2.) Nuclear scattering is the main contributor to multiple Coulomb scattering of protons.
Is the mass stopping power for protons greater or lower in high-Z materials? Pg. 515
The mass stopping power for protons is greater in low-Z materials.
Which materials, high-Z or low-Z, have a greater impact on the scattering-angle of the proton? Pg. 515
High-Z materials scatter protons through larger angles than the low-Z materials.
If you want to scatter a proton beam with a minimum loss of energy, what type of materials should you use? Pg. 515
You should use high-Z materials.
If you want to decrease the energy of the proton beam what type of materials should you use? Pg. 515
You should use low-Z materials.
If you want to decrease the proton beam energy with minimum scattering, what type of materials should you use? Pg. 515
You should use low-Z materials.
What is the rate of energy loss of a charged particle due to ionization and excitation proportional to? Pg. 516
The rate of energy loss due to ionization and excitation caused by a charged particle traveling in a medium is proportional to the square of the particle charge and inversely proportional to the square of its velocity.
As the particle loses energy, it slows down and the rate of energy loss per unit path length increases. As the particle velocity approaches zero near the end of its range, the rate of energy loss becomes maximum.
What is the Bragg Peak? Pg. 516
The depth dose distribution follows the rate of energy loss in the medium. For a monoenergetic proton beam, there is a slow increase in dose with depth initially, followed by a sharp increase near the end of the range. This sharp increase or peak in dose deposition at the end of the particle range is called the Bragg Peak.
What is one significant problem with proton beams and its Bragg Peak when trying to treat target volumes and what is the solution to solve this problem? Pg. 516
As seen in Figure 26.1, the Bragg peak of a monoenergetic proton beam is too narrow to cover the extent of most target volumes.
In order to provide wider depth coverage, the Bragg peak can be spread out by superposition of several beams of different energies. These beams are called the spread-out Bragg Peak beams (SOBP).
What is the definition of the relative biologic effectiveness (RBE)? Pg. 516
RBE of any radiation is 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.
The specified biologic effect may consist of cell killing, tissue damage, mutations, or any other biologic endpoint.
What beam factors does the RBE depend on? Pg. 516
Although the RBE depends on,
- 1.) the type and quality of radiation
- 2.) dose fractionation
- 3.) biologic endpoint
- 4.) MOST IMPORTANT: LET.
The greater the LET, the greater is the RBE. Because charged particles, in general, have greater LET than the megavoltage x-rays, the RBE of charged particles is greater than or equal to 1.0. Neutrons also have RBE greater than 1.0, because of the higher LET caused by their interactions involving recoil protons.
What region of the depth of the particles travel within the medium is its RBE most greatest? Pg. 517
Because the LET of charged particles increases as the particles slow down near the end of their range, so does their RBE. Thus, the RBE of charged particles is greatest in the region of their Bragg peak.
What are the two main proton beam accelerators used clinically? pg. 518
Conventional linear accelerators are not suitable for accelerating protons or heavier charged particles to high energies required for radiotherapy. The electric field strength in the accelerator structure is not sufficient to build a compact machine for proton beam therapy. A linear acclerator would require a large amount of space to generate proton beams in the clinically useful range of energies.
Therefore, the two main proton beam accelerators used clinically are the
- 1.) cyclotron
- 2.) synchrotron.
Why are cyclotrons and synchrotons better suited for clinical application? Pg. 518
Cyclotrons and synchrotrons are suitable for use in clinical facilities because they produce proton beams of sufficiently high energy and intensity for radiotherapy.
However, they differ in several aspects of beam specification and space requirements. For example, cycltrons produce high-intensity beams but have limited energy variability and are quite heavy (~150-200 tons). Synchrotrons are relatively low in weight and produce protons beams of variable energy. Also, the beam current in synchrotrons is lower than in the cyclotrons.
What is the maximum energy that cyclotrons give to protons before releasing them for treatment? Pg. 518
A cyclotron used in radiotherapy is a fixed-energy machine, designed to generate proton beams of a maximum energy of about 250 MeV. This energy would be sufficient to treat tumors at any depth by modulating the range and intensity of the beam with energy degraders.
Do synchotrons use energy degraders like cyclotrons? Pg. 519
Protons are kept within the tube ring by the bending action of the magnets. The strength of the magnetic field and the RF frequency are increased in synchrony with the increase in beam energy, hence the name synchotron. When the beam reaches the desired energy, it is extracted.
Synchrotons have a distinct advantage over cyclotrons in that they accelerate the charged particles to precise energies needed for therapy. In other words, the synchrotron is operated to produce the SOBP beams at any desired depth without the use of energy degraders.
The cyclotron, on the other hand, operates at a fixed maximum energy and requires energy degraders to treat more superficial tumors and to create SOBP beams at any depth.
Why are energy degraders problematic when used? Pg. 520
Energy degraders are problmeatic in several respects: They produce greater neutron contamination, require more shielding around the beam-generating equipment, and show higher post-treatment radioactivity from the metal collimators in the energy-degrading system.
Just before the patient enters the treatment room, the beam is spread out to its required field cross section in the treatment head-the nozzle. The beam spreading is done in two ways: 1) passive scattering, in which the beam is scattered useing thin sheets of high-Z materials or 2) scanning, in which magnets are used to scan the beam over the volume to be treated. Of the two, which is most currently, commonly used? Pg. 520
Although most accelerators currently use passive systems, there is a trend toward scanning to spread the beam.
In passive beam spreading systems, what are used to compensate simultaneously for external patient surface irregularity, internal tissue heterogeneity, and the shape of the distal planning target volume (PTV) surface? Pg. 521
In passive beam spreading systems, range compensators of low-Z materials are used to compensate simultaneously for external patient surface irregularity, internal tissue heterogeneity, and the shape of distal planning target volume (PTV).
In general, why is pencil beam scanning better than passive-beam spreading? Pg. 521
Pencil beam scanning is better due to this: in this system, the tissue region of interest is divided into a three-dimensional grid of voxels. The scanning system delivers specific doses at the grid points by placing the Bragg peaks within the voxels. Fields of any size and shape can be generated by pencil beam scanning, thus obviating the need for a custom-designed field aperture for every treatment portal. Because pencil beams of any energy and intensity are available, range compensators are not required.
Despite the pencil beam scanning is better than passive-beam spreading, there is still disadvantages with it. What is the major one? Pg. 522
The disadvantage is that the pencil beam scanning for both the conventional and IMPT techniques has a higher sensitivity to organ motion than the passive methods of beam scattering. In other words, intensity modulation is temporally not synchronized with organ motion during beam delivery. It should also be mentioned that the photon IMRT suffers from the same problem.
Limitations of pencil beam scanning have been disscussed by several investigators. What are some of the strategies to counteract the organ motion problem? Pg. 522
- 1.) "repainting" the dose multiple times over the organ motion period in order to achieve a statistical averaging effect on the dose distribution.
- 2.) increasing the scanning speed and thereby increasing the number of repaintings over the target volume, which further reduce the motion error through a greater degree of randomization and better averaging statistics.
- 3.) synchronizing beam delivery with the patient's breathing cycle.
- 4.) tumor tracking during treatment.
The problem of intrafraction organ motion is common to both photon and proton IMRT. It needs further investigation before approrpriate solutions are found for either modality.
The IAEA protocol specifies proton beam quality by the effective energy? What is the definition of "effective energy"? Pg. 522
It is defined as the energy of a monoenergetic proton beam that has the same residual range R_es as that of the given clinical proton beam. The effective energy is close to the maximum energy in the proton energy spectrum at the reference depth.
How is the residual range of the proton beam measured? Pg. 522
The reference depth z_ref is at the midpoint of the SOBP. The practical range R_p is defined as the depth at which the dose byond the Bragg peak or SOBP falls to 10% of its maximum value. The residual range is determined from the difference between those two values.
Besides absorbed dose calibration under reference conditions, clinical dosimetry (e.g., acceptance testing, commissioning, treatment planning, and monitor unit calculations) requires many other measurements under non-reference conditions. What are they? Pg. 524
- 1.) check of equipment performance specifications
- 2.) beam alignment
- 3.) beam energies
- 4.) central axis depth dose distributions
- 5.) transverse beam profiles
- 6.) isodose distributions
- 7.) output factors
These measurements should be made for a sufficient number of energies, field sizes, and source to surface distances so that clinical dosimetry can be performed and applied to all possible radiotherapy treatments.
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