Radiation Oncology
Radiation Treatment Delivery
Possible Side Effects from Radiation Treatment
Radiation Treatment and Effects on Normal Tissue
Minimizing the Exposure of Normal Tissues 

Radiation Oncology

Radiation Oncology is the branch of medicine that specializes in the use of high-energy radiation as a cancer treatment modality. Radiation is directed in a carefully controlled fashion to a specific part of the body, with the goal of destroying cancer cells without significant injury to the surrounding normal tissue. Radiation works by damaging the genetic material (DNA) of the tumor cell so that it loses the ability to reproduce itself; upon attempted division, the cell dies. Radiation is similar to surgery in that it is a form of local therapy that only treats a specific area of the body. In contrast, chemotherapy and hormonal therapy, which by design spread throughout the entire body, are referred to as systemic therapy.

Back to Top  

Radiation Treatment Delivery

The workhorse of the modern Radiation Oncology department is the linear accelerator (linac), a large X-ray machine that bears some resemblance to the devices that produce chest and dental X-rays. The difference is that the linac generates X-rays that are much higher in energy than the more commonly known diagnostic X-rays. When the linac is turned on, a well-defined beam of X-rays flows out of the machine and passes through the targeted part of the body. The instant the linac is turned off, there is no more radiation in the machine, in the room, or in the patient. The linac machine is not radioactive and the patient does not become radioactive from treatment. The process described above is called external irradiation. Additionally, treatment can be administered in the form of small radioactive sources that are actually implanted inside a part of the body near the tumor. This latter form of internal irradiation, known as brachytherapy, is particularly important in the treatment of gynecologic and prostate cancers. Brachytherapy also is increasingly being used for the treatment of a wide variety of malignancies, either as the sole form of radiation or in conjunction with external irradiation. Regardless of how the radiation is delivered (by external therapy, brachytherapy, or a combination of the two), the goal of treatment is to kill cancer cells and spare non-cancerous (normal) cells. Radiation is sometimes used as the only treatment for cancer, but most frequently, it is used as a supplement to open surgery. In such cases, surgical removal of the tumor is the primary therapy, and radiation therapy is used pre-operatively, or more commonly post-operatively, to "mop up" any microscopic cancer cells that escape "the surgeon's knife". Its specific purpose is to improve the likelihood of local control of the cancer, and ultimately, to increase survival. When radiation is used to supplement surgery it is referred to as adjuvant therapy.

Back to Top  

Possible Side Effects from Radiation Treatment

Patients often become nervous at the idea of being treated with radiation. Typical concerns include nausea and vomiting, diarrhea and hair loss (often patients do not recognize the distinction between chemotherapy and radiation side effects), worries about being “burned”, fears that radiation can actually cause cancers, and numerous other less common fears. These concerns sometimes may contain a grain of truth, but more often are exaggerations or misinterpretations of the facts. In the vast majority of cases, treating cancers with radiation involves simply using X-rays. To be sure, these X-rays are much more powerful than the diagnostic (“regular”) X-rays used for a chest film or a mammogram, but the experience need not be too different. The key difference is that because of their high energy, the X-rays used to treat cancer are able to penetrate deep inside the body and deposit their energy within the target tissues. Radiation is a purely local form of treatment, with the radiation being directed only to a particular part of the body. By the manner in which it works, a radiation beam does not distinguish between a cancer cell and a normal cell. All tissues in the targeted field will receive the same amount of radiation from a given beam. Radiation side effects arise from damage, which is often transient (temporary), to normal functioning tissue in the treatment field. Potential complications depend on the specific location of the body that is being irradiated. Thus, unlike the case with systemic chemotherapy, radiation can potentially cause hair loss only if the radiation beam is directed to the head, it can cause nausea and vomiting only if the abdomen is treated, and it can cause diarrhea only if the bowel is treated.

Back to Top  

Radiation Treatment and Effects on Normal Tissue

The basic dilemma and challenge for the radiation oncologist is the fact that cancer never exists in a vacuum and is by definition always surrounded by normal tissue. Obviously, if radiation only damaged tumor cells, it would be simple to cure any cancer simply by giving a sufficiently high dose of radiation. But this is never the case, and instead radiation oncologists are constantly faced with the trade-off of inflicting the maximal damage to the tumor while keeping the risk of damage to normal tissue to an absolute minimum. The art and science of Radiation Oncology is to figure out ways to achieve local control at lower doses, so that the chance of cure is maximized, while the risk of complications is minimized. Radiation Oncology is really in many ways the branch of medicine that deals with the irradiation of normal tissue. While the objective is always to kill off the cancer, a radiation oncologist must always be concerned with minimizing risk to nearby normal tissue.

What is the risk to the nearby by normal tissue? The relatively high doses of radiation used in Radiation Oncology translate into only a minimally increased risk of cancer. Why is there a difference? Simply stated, repetitive low dose radiation may cause mutations in cells that are perpetuated - the cells may be damaged, but not lethally, and when they divide, their mutations are passed on. In contrast, repetitive high dose irradiation causes lethal damage. The cells die when they attempt to divide and cannot pass on any mutations to a new generation of cells. Practically speaking, there is little risk that a course of high dose irradiation will induce cancer, certainly far less than 1%, and the potential benefits far outweigh any long-term risks.

Back to Top  

Minimizing the Exposure of Normal Tissues

There are several strategies that radiation oncologists routinely utilize to keep the risk of damage to normal tissue as low as possible. These strategies include.

1. Use of high energy X-rays
2. Precise delivery of radiation
3. Use of multiple treatment fields
4. Fractionation

Use of high energy X-rays
The first important means of sparing normal tissue, which was already touched upon earlier, is the use of high energy X-rays. Whereas diagnostic X-rays lose most of their energy at the skin surface, the linac-generated high energy X-rays are able to penetrate deep within the body before depositing their energy. Most modern radiation oncology departments have several technologies with different X-ray energies at their disposal. The higher the energy used, the deeper the penetration into the tissue. Because the X-rays do not lose most of their energy at the surface of the body, the skin is spared. Obviously the surface does receive some radiation, but unless the skin is specifically targeted for treatment, radiation "skin burns" are now very uncommon.

Precise delivery of radiation
A second key concept of modern Radiation Oncology is one of precision. The region to be irradiated, termed the tumor volume, should be as well-defined as possible such that it receives the full brunt of the treatment, while as little normal tissue as is feasible is irradiated. This may sound rather obvious, but in practice the ideal balance is very difficult to achieve. Unless the tumor volume is on or just below the skin surface, rarely is any abnormality seen or felt, particularly if the cancer has already been removed and treatment is only for potential microscopic residual disease. Instead, radiation oncologists must take advantage of all available means to define the radiation site. This involves looking at X-rays, CT scans and the like, studying the surgeon's operative note and the pathologist's report for details, and even going to the operating room to personally visualize the tumor while it is still in place. Knowledge of internal anatomy is also critical, not only so that normal, non-cancerous structures can be spared, but also so that seemingly normal tissues that are at risk for microscopic tumor involvement will not be mistakenly left untreated. Precision also comes into play with regard to the reproducibility of the daily treatments. Many courses of therapy require 30 to 40 or more treatments, and the radiation must be precisely delivered to the same target site - within 1/2 to 1 centimeter daily variation - or else there is a risk of under-dosing the tumor and/or overdosing the surrounding normal tissue. Elaborate means are used to ensure that the patient is lying in nearly the same position every day.

Use of multiple treatment fields
A third major principle is the use of multiple fields. Except in very rare circumstances, radiation oncologists treat from at least two different directions (e.g. front-to-back and side-to-side). For locations deep in the body, multiple angles are routinely used. The idea here is that only the tumor volume receives radiation from all the angles, and most of the surrounding normal tissues are excluded from one or more of the X-ray beams. The benefit is that normal regions receive only a portion of the total dose.

Fractionation
Perhaps the most important basic principle of modern Radiation Oncology is that of fractionation. Since the radiation beam cannot distinguish between tumor and normal tissue, radiation oncologists must take advantage of any inherent differences between the two. Fortunately most types of cancer cells are irreversibly damaged and killed by radiation at much lower doses than are normal cells. In other words, cancer cells tend to be more sensitive and normal tissues more resistant to radiation. Therefore, a dose of radiation is chosen for each individual treatment (or fraction) that is high enough to cause some damage to the cancer, but sufficiently low that most, if not all, of the normal tissue will be spared permanent damage. By delivering multiple such “fractions” in rapid succession (usually daily), it is possible to administer a total dose of radiation that is adequate to kill off the tumor, yet only minimally damaging to the surrounding normal tissue.

Back to Top