Dr. Swarupa Mitra Consultant, radiation Oncology. Rajiv Gandhi Cancer hospital and research Institute. Rohini. New DelhiRead more
A machine directs the high-energy rays or particles at the cancer and normal tissue around it. The radiation used in external radiation therapy can come from a variety of sources, including an x-ray, electron beam or gamma rays. The type of radiation the physician chooses depends on the type of cancer y and on how deep into the body the doctor wants the radiation to penetrate. High-energy radiation is used to treat many types of cancer, while low-energy x-rays are used to treat some kinds of skin diseases. External radiation therapy is usually given five days a week for six or seven weeks. When radiation is used for palliative care, the course of treatment lasts for two or three weeks.
a. Conventional external beam radiotherapy
Conventional external beam radiotherapy (2DXRT) is delivered via two-dimensional beams using linear accelerator machines. 2DXRT mainly consists of a single beam of radiation delivered to the patient from several directions: often front or back, and both sides. ''Conventional'' refers to the way the treatment is ''planned'' or ''simulated'' on a specially calibrated diagnostic x-ray machine known as a simulator because it recreates the linear accelerator actions (or sometimes by eye), and to the usually well-established arrangements of the radiation beams to achieve a desired ''plan''. The aim of simulation is to accurately target or localize the volume which is to be treated. This technique is well established and is generally quick and reliable.
The worry is that some high-dose treatments may be limited by the radiation toxicity capacity of healthy tissues which lay close to the target tumor volume.Prior to the invention of the CT, physicians and physicists had limited knowledge about the true radiation dosage delivered to both cancerous and healthy tissue. For this reason, 3-dimensional conformal radiotherapy is becoming the standard treatment for a number of tumor sites.
The planning of radiotherapy treatment has been revolutionized by the ability to delineate tumors and adjacent normal structures in three dimensions using specialized CT and/or MRI scanners and planning software.
Virtual simulation, the most basic form of planning, allows more accurate placement of radiation beams than is possible using conventional X-rays, where soft-tissue structures are often difficult to assess and normal tissues difficult to protect.
b. Three-Dimensional Conformal Radiation Therapy (3D-CRT)
This is an enhancement of virtual simulation in which the profile of each radiation beam is shaped to fit the profile of the target from a beam's eye view (BEV) using a multileaf collimator (MLC) and a variable number of beams. Tumours are not regular; they come in different shapes and sizes.Three-dimensional conformal radiation therapy, or 3D-CRT, uses computers and special imaging techniques such as CT, MR or PET scans to show the size, shape and location of the tumor as well as surrounding organs. The radiation oncologist can then precisely tailor the radiation beams to the size and shape of the tumor with multileaf collimators or custom fabricated field-shaping blocks. Because the radiation beams are very precisely directed, nearby normal tissue receives less radiation and is able to heal more quickly.
c. INTENSITY MODULATED RADIOTHERAPY (IMRT)
Intensity modulated radiation therapy, or IMRT, is a specialized form of 3D-CRT that allows radiation to be more exactly shaped to fit the tumor. Computer-controlled x-ray accelerators distribute precise radiation doses to malignant tumors or specific areas within the tumor. The pattern of radiation delivery is determined using highly-tailored computing applications to perform optimization and treatment simulation (Treatment Planning). With IMRT, the radiation beam can be broken up into many “beamlets,” and the intensity of each beamlet can be adjusted individually. Using IMRT, it may be possible to further limit the amount of radiation received by healthy tissue near the tumor. In some situations, this may also safely allow a higher dose of radiation to be delivered to the tumor, potentially increasing the chance of a cure.
Image showing multiple IMRT beams delivering high-dose radiation The physicians must manually delineate the tumors one CT image at a time through the entire disease site which can take much longer than 3DCRT preparation. Then, medical physicists and dosimetrists must be engaged to create a viable treatment plan. Also, the IMRT technology has only been used commercially since the late 1990s even at the most advanced cancer centers, so radiation oncologists who did not learn it as part of their residency program must find additional sources of education before implementing IMRT.
d. Image Guided Radiation Therapy (IGRT)
Image guided radiation therapy, or IGRT, help better delivery of radiation to the tumour, since tumours can move between treatments due to differences in organ filling or movements while breathing. IGRT involves conformal radiation treatment guided by imaging, such as CT scans, ultrasound or X-rays, taken in the treatment room just before the patient is given the radiation treatment on a daily basis.
All patients first undergo a CT scan as part of the planning process. The information from the CT scan is then transmitted to a computer in the treatment room to allow doctors to compare the earlier image with the images taken just before treatment. During IGRT, doctors compare these images to see if the treatment needs to be adjusted. This allows doctors to better target the cancer while avoiding nearby healthy tissue. In some cases, doctors will implant a tiny marker in or near the tumor to pinpoint it for IGRT. This helps to account for organ/tumor motion even if the body is immobilized by a casting device.
e. Stereotactic Radiation
Stereotactic radiation is a specialized type of external beam radiation therapy. It uses focused radiation beams targeting a well-defined tumor using extremely detailed imaging scans. Radiation oncologists perform stereotactic treatments, often with the help of a neurosurgeon for tumors in the brain or spine.
There are two types of stereotactic radiation. Stereotactic radiosurgery (SRS) is when doctors use a single or several stereotactic radiation treatments of the brain or spine. Stereotactic body radiation therapy (SBRT) refers to one or several stereotactic radiation treatments with the body, such as the lungs.
An advantage of stereotactic treatments are that they deliver the right amount of radiation to the cancer in a shorter time than traditional treatments and treatments are given with extreme accuracy, which should limit the effect of the radiation on healthy tissues. One problem with stereotactic treatments is that they are only suitable for certain small tumors.
Stereotactic treatments can be confusing because many hospitals call the treatments by the name of the manufacturer rather than calling it SRS or SBRT. Brand names for these treatments include Axesse, Cyberknife, Gamma Knife, Novalis, Primatom, Synergy, X-Knife, TomoTherapy and Trilogy. This list changes as equipment manufacturers continue to develop new, specialized technologies to treat cancers.
f. Particle Therapy
In particle therapy (Proton therapy), energetic ionizing particles (protons or carbon ions) are directed at the target tumor. The dose increases while the particle penetrates the tissue, up to a maximum (the Bragg peak) that occurs near the end of the particle's range, and it then drops to (almost) zero. The advantage of this energy deposition profile is that less energy is deposited into the healthy tissue surrounding the target tissue.
Brachytherapy is radiation at short distances. The source of radiation is made in the form of wires, seeds or plaques and are inserted into the tumor for delivering high doses of radiation. These radioactive sources are Cesium, Iridium and Iodine. This type of treatment is very effective in certain types of cancer, such as cancer of the cervix, certain forms of head and neck cancer, and lung cancer.