Authored by
DR. MOHAMMED RAEES TONSE
MD (Oncology)
Consultant Oncologist
External Beam Radiation Therapy
Linear accelerators produce X-rays with an energy range from 4 MeV to 25 MeV. The electrons are accelerated to high energy and allowed to either exit the machine as an electron beam or to strike a target that produces X-rays, which are directed at the target and their output is managed with computer controls.
They are equipped with multi-leaf collimators (MLCs) which can move into the radiation field and block part of it. A multi-leaf collimator consists of two sets of 40-80 leaves, each around the thickness of 5-10 mm. Each leaf in the MLC is aligned parallel to the radiation field and can be moved independently to block part of the field, thus minimizing the amount of healthy tissue being exposed to radiation.
+ Treatment Planning
Treatment planning includes careful patient immobilization and determination of the radiation field as well as the dose and schedule for treatment. Extreme care is required in treatment planning by the radiation oncology team. Failure to deliver the full planned dose of radiation to a tumour target can result in under dosage of the same. The treatment planning process involves several key steps:
1. Patient Immobilization
The administration of multiple doses of radiation to precisely the same region requires that the patient be reproducibly immobilized during the planning process and subsequent treatment. Immobilization requires special devices that allow for the best treatment geometry while maximizing the patient’s comfort and the reproducibility of patient positioning.
2. Imaging
After immobilization, a computed tomography (CT) & magnetic resonance imaging (MRI) scan, thin slices 3DFSPGR sequences along with T2/Flair sequences of the brain is obtained while the patient remains in the treatment position to allow for precise target delineation. Special fiducial markers can be utilized to facilitate daily localization.
3. Delineation of the Target Volumes
Clinicians use the imaging studies to delineate the target volumes, as well as the normal structures. The target volume will include some margin of seemingly normal tissue to ensure that no tumour is missed. Each case should be discussed in a multidisciplinary tumour board as it helps the clinicians to optimally treat individual patients based on evolving molecular information and better radiological parameters that give us a better understanding on the biology of the tumour.
4. Conformal Therapy
Conformal therapy is a term that describes a strategy for matching the high-dose radiation region to the target volume while minimizing the radiation dose to normal tissues.
5. Stereotactic Radiation Therapy Techniques
Stereotactic techniques typically utilize photons that are generated by a linear accelerator or by a cobalt-60 source. In stereotactic radiosurgery (SRS) delivered on a linear accelerator, micro multi leaf collimators are used to shape the treatment beams and treatment delivered using multiple non coplanar arcs/fields to treat an approximate target of <4 cm in diameter. Immobilization is even more critical for SRS than for routine external beam RT in order to achieve high reproducibility
and precision. Single-fraction SRS treatments may utilize rigid headframes, which are fixed to the head using pins or screws, or a frameless system with a head mask with or without bite-block systems.
6. Intensity-modulated Radiation Therapy (IMRT)
IMRT is an advanced form of 3D-CRT that changes the intensity of radiation in different parts of a single radiation beam while the treatment is delivered. It relies upon computer control capabilities aided by highly sophisticated inverse planning and computational engines, to maximize the delivery of radiation to the planned treatment volume while minimizing radiation to normal tissue outside the target. IMRT results in a larger volume of normal tissue receiving lower doses of radiation compared with conventional and 3D conformal techniques planned by relatively simpler forward planning algorithms.
7. Particle/Proton Beam Therapy
Particle therapy is a special form of external beam RT with protons being the most widely used. Special equipment is used to generate high-energy particles on devices that are large and costly to build and operate. Proton beam radiation therapy (PT) reduces the dose to normal tissues by allowing for more precise dose delivery because of the unique physical properties of protons, which penetrate tissue to variable depth, depending upon their energy, and then deposit that energy in the tissue in a sharp peak, known as a Bragg peak. In most treatments, protons of different energies with Bragg peaks at different depths are applied to treat the entire tumour.
Protons have been shown to have a unique and distinct advantage over photon RT.
– Dose Escalation
In disease sites that may respond well to higher doses of radiation proton shows definite advantage. For example, skull base and paraspinal tumours (chondrosarcoma and chordoma), uveal melanoma and challenging sarcomas and increasingly explored clinical conditions of atypical/anaplastic meningiomas, hemangiopericytomas.
– Childhood and Low grade/Benign Tumours
Protons help in the preservation of intelligence and endocrine functions by reducing the dose to the hippocampus and cochlea.
– Second Malignancy
Protons reduce the integral dose as compared to photon RT, thus reducing the incidence of second cancers.
– Re-irradiation
Protons have demonstrated an advantage over photon RT in cases of re irradiation as it is technically very challenging to give radiation again.
In a comparison of proton and photon radiotherapy for pediatric brain tumours, including low grade glioma, ependymoma, craniopharyngioma, and medulloblastoma, significantly superior outcomes have been demonstrated in several retrospective and prospective series. A relatively small critical normal organ, such as the cochlea and hypothalamus, can be preserved in PT when not adjacent to the primary tumour volume. These advantages can result in preservation of intelligence, endocrine function, and hearing.
