Protons are heavy charged particles which contain an abundance of energy and interact with matter over a distance, depositing their highest dose near their final point in tissue (known as Bragg Peaks).
To make proton beams clinically useful, they must be fitted with an aperture or block to conform with lateral and distal dose distribution as well as with a range compensator.
Dose Distribution
Proton beam radiation is a low linear energy transfer (LET) radiation form. To calculate an effective dose from it, multiplying physical dose in gray times its relative biological effectiveness (RBE). Protons generally have an RBE value of around 1.1 as compared with photons; however, its actual value varies based on factors like entrance LET to tissues, depth of penetration, type of tissue being targeted as well as clinical/treatment settings.
Proton beams must be tailored to conform with three-dimensional, arbitrarily-shaped tumor targets for maximum efficiency. This is accomplished using a nozzle composed of scatter foils, ridge filter/modulator wheel assembly, patient aperture and range compensator components.
The shape of a nozzle creates a flatter dose distribution across the Bragg peak and an easier depth dose profile than that produced by traditional radiation therapy, with its dose distribution also being affected by patient anatomy scattering patterns; special attention must be taken in ensuring that this dose distribution does not create significant volumes of normal tissue distal to tumor.
Although proton therapy offers distinct advantages over conventional radiotherapy on a large scale, it has yet to show clear superiority over its more established rival. This may be the result of still maturing technology, limited experience with proton treatments, uncertainties in delivered biologically effective doses due to intra- and inter-fraction motion and setup variations or assumptions made when computing doses, as well as approximations or assumptions made when computing doses using current methods of computation.
PTVs and ORVs for proton therapy present problems due to being determined solely from clinical images of dose distribution; such a system cannot accommodate for uncertainties associated with delivery dose. A more robust solution must be pursued.
As PSPT provides good low-dose sparing capabilities, its conformality performance lags behind that of IMRT in this respect; only IMPT can compete in this regard. Furthermore, PSPT’s lateral spread makes it hard to avoid creating hotspots within and around its target that may lead to increased toxicity.
Secondary Electrons
Protons, like photons, can be scattered by electrons in the medium they traverse. This scattering causes a loss of energy, but it is much less than the energy lost by ionization. The effect is known as Coulomb scattering. This effect can be accounted for when designing beamlines and treatment heads, determining the dose distribution in a phantom or patient, or by using computer simulation programs for treatment planning.
A proton beam in water consists of a narrow, nearly monoenergetic pencil beam of varying width, referred to as the full-width at half-maximum (FWHM). The FWHM is a function of the initial energies and field size of a proton beamlet, but is also dependent on the geometry of the patient if a scanning proton beam system is used. The design and number of RMWs, which are the modules that are mounted in a beam delivery carousel to modify the initial energies and field sizes of a proton beam, vary with proton therapy systems vendors. Several RMWs are available on the Hitachi system at MDACC, enabling the system to produce pencil beams with a FWHM in air ranging from approximately 18 mm for 222 MeV up to 30 mm for 72 MeV.
As the proton beam passes through a patient, the energy deposition decreases rapidly as a function of the incident particle energy up to 145 MeV. This is because the protons scatter from the atoms of their target, the organs they pass through. The resulting secondary neutrons, however, are more energetic and have greater impact on the dose distribution than do the primary protons themselves.
The physics of the protons and their interaction with the target and the surrounding tissues provides the key to achieving high tumoricidal doses for patients treated with proton beams. This ability, coupled with IMPT, offers the potential to optimize both tumoricidal and normal tissue doses.
As with photons, physicists develop many of the technologies necessary for proton therapy including accelerators, magnetically scanned beams, treatment planning systems, and computed tomographic imaging. In addition, a team of doctors helps position each patient for treatment by marking the areas that will be treated and creating immobilization devices such as masks that fit around the head or body to keep the patient from moving during treatments.
Nuclear Interactions
Protons interact with atomic nuclei within a patient body to produce smaller atomic energy particles (known as a-particles), known as nuclear interactions, which produces secondary particles with lower energy than original protons resulting in a spread out of proton energy spectrum near Bragg peak due to probabilistic interactions.
This effect is still poorly understood. However, it has been documented that proton beams outshone photon beams by an increase of one factor of 1.1 when it comes to biological effectiveness (RBE).
An increased relative beam energy (RBE) value is caused by interactions between protons and radiosensitizers such as gold or bismuth radiosensitizers used for proton therapy, and radiosensitizers made of these metals (gold and bismuth radiosensitizers), composition of target being irradiated, configuration of system as a whole and interactions of protons with matter such as bones or soft tissues irradiated as well as their energy spectrum, scattering coefficients or energy deposition profiles.
Proton beams differ from photon beams in that their energy gradually dissipates as they pass through tissue, producing doses that gradually decline with depth within a patient and creating what is known as the Bragg curve (pronounced brag-kur). Due to this behavior, protons produce much larger penumbrae than photons do, necessitating an increase in target size or the use of apertures to decrease beam spot size.
Proton beams offer unique depth-dose characteristics that enable significant decreases in normal tissue radiation doses both near and far from a target volume, providing for increased tumor control while protecting vital organs at risk.
Proton therapy offers more clinically useful treatment than photons due to its unique depth-dose behavior; however, due to their different physical properties and interactions with matter, designing and evaluating proton plans require unique considerations compared with photon therapy plans.
Proton therapy utilizes a machine known as a particle accelerator to produce high-energy beams of protons that are then directed toward tumor sites within an irradiation room. Patients lie on a table while the proton beam passes over them during treatment sessions that last anywhere from several minutes to weeks.
Radial Properties
Proton therapy typically falls between 70 to 250 MEV, well within the therapeutic window for most tumor types. Protons are injected into a linear accelerator where they gain enough energy to reach the treatment room within fractions of a second; then directed onto patients by means of magnets known as beam transport systems.
To address these limitations and provide more conformal doses proximally and distally, an active scattering system featuring scatter foils, ridge filters, apertures and range compensators are used to provide more conformal radiation doses.
To generate beams with multiple energies that can spread their Bragg peak across different depths, narrow beams entering a nozzle are magnetically scanned and proton energies are adjusted according to desired patterns of dose – this process is known as dynamic spot scanning; additionally it facilitates use of IMPT while decreasing need for scatter foils, apertures and compensators.
This method remains far from ideal, however. It produces substantial neutrons that could damage both the nozzle and beam-transport system; furthermore, this approach only has the capability of shaping the lateral penumbra for targets with limited depth such as pituitary glands.
Due to proton therapy’s limited lateral penumbra and steep dose decline as you move further from its target, proton treatment imposes more restrictions on its proximal and distal margins than photon therapy; many of the formalisms, algorithms and techniques used for photon planning, optimization and plan evaluation do not readily adapt for proton treatment planning and evaluation.
To overcome these limitations, proton beams are accelerated to therapeutic energies using cyclotrons or synchrotrons and then directed toward patients via nozzles and beam-transport systems into a treatment room for delivery to them. Patients sit positioned on an immobilization table with immobilization devices to keep them still during each treatment session while radiation therapy team will mark areas to be treated either permanently or temporarily with permanent markers.
