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Cancer Radiation Therapy

Types of Radiation Therapy

External beam radiation therapy (EBRT). EBRT creates a radiation beam and aims it at the tumor. The radiation adequately covers the tumor but minimizes the dose to the non-tumor normal tissues. Radiation is given in fractions rather than as a single dose, and the use of this fractionated radiotherapy allows normal cells time to repair between each radiation session, protecting them from injury.

Conventional fractionation in the United States is 1.8 to 2 Gray (Gy) per day, administered five days a week for five to seven weeks, depending on the particular clinical situation. (Gray is a unit of measure of absorbed radiation dose.) While this schedule is strictly for the convenience of physicians trying to maintain a normal workweek, the relatively long intervals between doses of radiation may allow cancer cells (as well as normal cells) to recover and regrow.

A number of different radiotherapy schedules have been suggested to overcome this problem (Shah N et al. 2000). These include hyperfractionation, in which the time between fractions is reduced from 24 hours to 6 to 8 hours to enhance the toxic effects on tumor cells (Fu KK et al. 2000) while still preserving an adequate time interval for the recovery of normal cells. Continuous hyperfractionated accelerated radiation therapy (CHART) is an intense schedule of treatment, in which multiple daily fractions are administered within a short period of time. Clinical studies have shown benefits of altered fractionation over conventional treatment for several cancers, including head and neck cancer (Goodchild K et al. 1999) and non-operable lung cancer (Ghosh S et al. 2003).

Proton beam radiation therapy. This is one of the most precise and sophisticated forms of external beam radiation therapy available. The advantage of proton radiation therapy over x-rays is its ability to deliver higher doses of shaped beams of radiation directly into the tumor while minimizing the dose to normal tissues. This leads to reduced side effects and improved survival rates (Suit HD 2003). As of 2002, more than 32,000 patients around the world had received part or all of their radiation treatment by proton beams.

There are approximately 19 proton treatment centers worldwide. Two major hospital-based facilities in the United States that regularly treat patients with proton beams (often fractionated) are Loma Linda University Medical Center in southern California (LLUMC Proton Treatment Center) and the Northeast Proton Treatment Center at Massachusetts General Hospital in Boston. The IU Health Proton Therapy Center (formerly Midwest Proton Radiotherapy Institute) in Bloomington, Indiana (http://iuhealthprotontherapy.org/) treats children and adults with certain brain tumors, as well as those with tumors that are close to vital organs and therefore cannot be treated successfully using traditional methods.

The efficacy of proton beam radiation therapy has been clinically proven (Shipley WU et al. 1995) in prostate (Slater JD et al. 1999; Zietman AL et al. 2005), lung (Bush DA et al. 1999), hepatocellular (Matsuzaki Y et al. 1995), and uveal melanoma (Courdi A et al. 1999; Munzenrider JE 1999; Spatola C et al. 2003), sarcomas of the skull base and cervical spine (Munzenrider JE et al. 1999), optic pathway gliomas (Fuss M et al. 1999), astrocytomas (Habrand JL et al. 1999), benign meningioma (Gudjonsson O et al. 1999), non-resectable rectal, esophageal (Koyama S et al. 2003), and liver cancers (Ask A et al. 2005b), head and neck cancers, including thyroid cancer (Ask A et al. 2005a; Sugahara S et al. 2005), and more.

Intensity modulated radiation therapy (IMRT). IMRT creates a shaped radiation beam, delivering high doses of radiation to the tumor and significantly smaller doses of radiation to the surrounding normal tissues (Hurkmans CW et al. 2002; Nutting C et al. 2000). This may result in a higher cancer-control rate and a lower rate of side effects (Garden AS et al. 2004; Welsh JS et al. 2005).

IMRT has been used successfully in the treatment of several types of cancer, including prostate (De Meerleer G et al. 2004), cervical (Ahmed RS et al. 2004), nasopharyngeal (Kwong DL et al. 2004), and pediatric cancers (Penagaricano JA et al. 2004).

Brachytherapy. Brachytherapy can be used for many types of cancers, but it is most commonly used to treat prostate cancer (Woolsey J et al. 2003) and gynecologic cancers, such as cervical or uterine cancer (Nakano T et al. 2005). Brachytherapy usually involves the insertion of devices around or within the tumor to hold radioactive sources or seeds. Radioactive isotopes, such as cesium, are then inserted into the delivery device, either temporarily or permanently, allowing for the slow delivery of a high dose of radiation to the interior of the tumor (Fieler VK 1997).

Radioimmunotherapy (RIT). Radioimmunotherapy, one of the newest developments in the treatment of non-Hodgkin's lymphoma (Harris M 2004), has achieved a high tumor response rate (up to 80 percent) in several clinical trials (Witzig TE et al. 2002). Radioimmunotherapy uses drugs called monoclonal antibodies, which have a radioactive isotope attached to them. This is targeted to the surface of a cancer cell, destroying it. Radioimmunotherapy can be used (in a targeted fashion) to treat single cells that have spread around the body (Riley MB et al. 2004). Because the radiation does not concentrate in any one area of the body, radioimmunotherapy does not cause side effects commonly seen with external beam radiation therapy. The most significant side effect associated with radioimmunotherapy may be a temporary drop in white blood cell or platelet count (Witzig TE et al. 2003).

Stereotactic body radiation therapy (SBRT). SBRT is a standard form of treatment for primary and metastatic brain cancer (Phillips MH et al. 1994). It is delivered using a machine called a gamma knife, which uses converging beams of gamma radiation that meet at a central point within the tumor, where they add up to a very high, precisely focused dose of radiation in a single fraction. Due to this precision, the cancer can be located in an area of the brain or spinal cord that might normally be considered inoperable (Song DY et al. 2004).

CyberKnife®. CyberKnife® is a non-invasive, precise radiation technique that can deliver concentrated and accurate beams of radiation to any site in the body. This system combines robotics and advanced image guidance cameras to locate the tumor’s position in the body and deliver highly focused beams of radiation that converge at the tumor, avoiding normal tissue. It is a successful method used to treat spinal tumors (Gerszten PC et al. 2004b) or tumors at other critical locations that are not amenable to open surgery or radiation, as well as to treat medically inoperable patients (Gerszten PC et al. 2004a). It can also be used to treat benign tumors and lesions in a previously irradiated site, or to boost standard radiotherapy (Bhatnagar AK et al. 2005; Degen JW et al. 2005).

Three-dimensional conformal radiation therapy (3D-CRT). 3D-CRT is a technique that uses imaging computers to precisely map the location of a tumor (Symonds RP 2001). The patient is fitted with a plastic mold or cast to keep the body part still so that the radiation can be aimed more accurately from several directions. By aiming the radiation more precisely at the tumor, it is possible to reduce radiation damage to normal tissues surrounding the tumor by up to 50 percent (Perez CA et al. 2002).

Radiation Therapy versus Medical X-rays (Diagnostic Imaging)

Although diagnostic x-rays provide great benefits, including the earlier detection of cancers and the possibility of early treatment, their use is associated with small increases in cancer risk (Ron E 2003). One study estimated that cancer risk due to diagnostic x-rays varied from 0.6 percent to 3 percent in the 15 developed countries studied (Berrington de Gonzalez A et al. 2004).

Therefore, it is prudent to avoid unnecessary x-ray procedures. Up to 30 percent of chest x-rays may not be necessary (McCreath GT et al. 1999). Unnecessary computed tomography (CT) examinations may result in increased radiation exposure (Fleszler F et al. 2003; Frush DP 2004). The cumulative risk of cancer mortality from CT examinations in the United States is about 800 radiation-induced cancer deaths per 1 million examinations in children under the age of 15 (Brenner D et al. 2001).

Mammography (chest x-ray) uses low-dose x-rays to create a detailed image of the breasts. Although there is some controversy regarding mammography’s effectiveness in reducing breast cancer mortality, successful treatment is linked to early diagnosis, as mammography can often show changes in the breast before they can be detected by manual examination (Olsen O et al. 2001).

The effective radiation dose from a mammogram is about the same as the average person receives from background radiation over a three-month period (Sabel M et al. 2001).

At present, the consensus view is that the benefits of screening women over 50 years of age with yearly or twice-yearly mammograms substantially outweighs the associated risks due to radiation exposure (Beckett JR et al. 2003). However, there appears to be no significant benefit for women under the age of 40, and there may be harm for women under 30 due to the danger of cancer developing after exposure to radiation (Brenner DJ et al. 2002). Therefore, the main area of controversy concerns women between the ages of 40 and 49.

Typical effective doses from diagnostic medical exposures in the 1990s

Diagnostic procedure

Typical effective dose in millisieverts (mSv)

Equivalent number of chest x-rays

Approximate equivalent period of natural background radiation(1)

X-ray examinations:

Limbs and joints (except hip)

<0.01

<0.5

<1.5 days

Chest (single PA film)

0.02

1

3 days

Skull

0.07

3.5

11 days

Thoracic spine

0.7

35

4 months

Lumbar spine

1.3

65

7 months

Hip

0.3

15

7 weeks

Pelvis

0.7

35

4 months

Abdomen

1.0

50

6 months

Intravenous urogram (IVU)

2.5

125

14 months

Barium swallow

1.5

75

8 months

Barium meal

3

150

16 months

Barium follow-through

3

150

16 months

Barium enema

7

350

3.2 years

CT head

2.3

115

1 year

CT chest

8

400

3.6 years

CT abdomen or pelvis

10

500

4.5 years

Radionuclide studies:

Lung ventilation (Xe-133)

0.3

15

7 weeks

Lung perfusion (Tc-99m)

1

50

6 months

Kidney (Tc-99m)

1

50

6 months

Thyroid (Tc-99m)

1

50

6 months

Bone (Tc-99m)

4

200

1.8 years

Dynamic cardiac (Tc-99m)

6

300

2.7 years

PET head (F-18 FDG)

5

250

2.3 years

(1) UK average background radiation = 2.2 mSv per year: regional averages range from 1.5 to 7.5 mSv per year.

With advice from Wall, B. National Radiological Protection Board.

Source: http://europa.eu.int/comm/environment/radprot/118/rp-118-en.pdf