Monday, January 26, 2009

Four Ways of Reducing Radiation Exposure

The health of radiation professionals needs to be protected from over exposure to radiations. The four most effective ways of reducing exposure to radiation have been elaborated below:

1) Radiation protection shielding

When a sheet of absorbing material is placed between a radiation source and a detector, the radiation arriving at the detector decreases to an extent depending on the energy of the radiation and the nature and the thickness of the shield. For gamma rays (g-rays) lead is generally installed because for a given weight it absorbs more radiations than any other readily available shielding material. Its effectiveness for a particular radiation is usually indicated by the half value layer (HVL). The half value layer (HVL) is the thickness of the lead sheet capable of reducing the radiation to one half (50%) of the original. A second half value layer (HVL) will reduce the remaining half to half (i.e. 25%). A third half value layer (HVL) would
reduce the radiation to half of 25% (i.e. 12.5%) or exactly the 1/8th of the original and a fourth half value layer (HVL) would further reduce the radiation to 1/16th of the original. The thicknesses of HVL for some important radioisotopes used in medical field have been given below:

RadioisotopeHVL in cm Lead

2) Time

The exposure time is directly proportional to the time spent at a place of radiation. The necessary time for any procedure ought to be estimated well in advance, allowing a good safety margin. The equipments used in radiation applications should be simple to minimize the time of operation with a view to reduce the exposure time. But any necessary handling precaution should not be omitted to save time.

3) Distance-Inverse Square Law

The ionization radiation travels like the light and as we go away from a point source, the amount of radiation reaching a given area would decrease. The decrease would be proportional to the square of distance in centimeters. For example: A point at 40 cm from the source would receive 1/4th (25%) of the radiation reaching at a point at 20 cm from the source. As 20x20/40x40 = 1/4.

4) The Gamma Factor for Gamma Emitters

The dose rate at 1 cm from the point source of each gamma emitter radioisotope has been determined and with reference to this the dose rates are worked out in terms of roentgens(r). The roentgen (r) is that amount of radiation which delivers a dose of one rad. A roentgen (r) is unit based on ionization in air and rad is a unit of energy absorption, but both have corresponding amount of energy. For example: the gamma factor of Iodine-131 (131I) is 2.18, of Gold-198 is 2.35 and that of Sodium-24 is 18.4 which means that 1 milliCurie (mCi) of these radioisotopes would have a dose rate of 2.18 , 2.35 and 18.4 roentgens or rad respectively at 1 centimeter from point source. Now, if 200 mCi of Iodine-131 (131I) is placed on a table in the laboratory, its dose rate at 1 centimeter distance would be 200x2.18 entgens and at 40 cm distance it would be 200x2.18/40x40= 0.273 roentgens/hour or 273 milliroentgen/hour (by Distance-inverse square law). A dose rate of 15 milliroentgen/hour is considered safe at 40 cm from the point source. So to attenuate the radiation we need to place a shield of lead around the Iodine-131 (131I) container. The attenuation factor for Iodine-131 (131I) could be calculates as 15/273=1/18 of its value; which means we need a little more than 4 half value layers of lead shield. Four half value layers for Iodine-131 (131I) would be 4x0.3=1.2 centimeter thick lead shield.

Sunday, January 25, 2009

Side Effects of Radiotherapy or Overexposure to Radiation

The x-rays and radiation from the radioisotopes play a great role in diagnostic procedures and therapy, but the exposure exceeding certain limits do have side effects on our body as elaborated below:

  1. Skin Reaction: The maximum permissible dose (MPD) for the skin is 100 rads. When a limited area of the skin receives a dose of a few times of MPD within a few days, that may result in the production of erythema and the reaction may stay for a week. A dose of about 1000 rads may cause tanning of skin and hair fall. A dose in the range of 2000 rads to 3000 rads may cause permanent tanning of skin and hair loss. There is possibility of atrophy of sweat glands and serious ulcers.

  2. Radiation Sickness: Even a single exposure of 50 rads or more to the trunk or entire body may cause malaise, nausea, vomiting and diarrhea, collectively called radiation sickness. Some people may not be affected at this dose, however effect may be severe at a whole body exposure at 200 rads.

  3. Effects on Blood: A single exposure of 25 to 30 rads to the whole body or to the major bones/bone marrow may result in mid to moderate leucopenia (low count of white blood cells in blood), but recovery is there. A dose of 200 to 500 rads to the spinal marrow may cause leukemia in some cases. Keep in mind that the radiations have accumulated effect.

  4. Production of Cataract: Excessive exposure of eyes to radiations with an accumulated dose of 100 to 200 rads could impair our vision and cause cataract.

  5. Production of Sterility: Even a shorter time exposure to the radiations around 500 rads to the genital organs may cause permanent sterility in either sex. A long term exposure to lesser dose of radiations may also lead to secondary infertility in either sex.

  6. Exposure during Pregnancy: Although there are a number of cases receiving radiotherapy for pelvic cancer during pregnancy and delivering normal babies, but a pregnant woman receiving a very large dose of radiation in order of 1000 rads within a few weeks may have a miscarriage or still birth. The possible effect on the fetus may vary with the period of gestation. Congenital abnormalities may affect the babies so pregnancy should be avoided during radiotherapy of pelvic cancer.

  7. Genetic Hazards: Irradiation of ovaries or testicles may lead to mutations in genes and affect reproduction. Greater is the exposure, the greater are the chances of genetic abnormalities.

Wednesday, January 21, 2009

Safe Handling of Radioisotopes

General health precautions while handling the radioisotopes or radiochemicals are must for all professionals associated with radioisotopes or radiochemicals. Regardless of the quantity of radioisotope or radiochemical, strict precautions and personal cleanliness awareness are indispensable. Some important tips in this regard are listed below:

  1. Laboratory coat should be worn to protect the clothing.

  2. Rubber or plastic gloves should be worn.

  3. All handling of radioisotopes or radiochemicals should be done on the surfaces lined by absorbent material.

  4. Dispensing of radiochemicals should be done over stainless steel, aluminium or plastic trays to contain any spillage.

  5. Eating and smoking should be avoided in a hot laboratory (the laboratory where radioisotopes are used is called hot laboratory).

  6. At the end of each procedure, the person responsible should cleanup his/her work space, dispose off any contaminated material in a suitable fashion.

  7. If a spill occurs, that should be dealt with immediately and brought to the notice of Radiation Safety Officer.

  8. The hot laboratory should be monitored periodically for unknown radiation from accidental spillage of radiochemicals.

Personal Monitoring of Radiation Professionals

All persons associated with commercial, diagnostic or therapeutic use of radioisotopes or radiochemicals, x-rays or other sources of radiation could be labeled as radiation professionals or radiation workers. Excessive exposure to radiation beyond the maximum permissible dose (MPD) for a person of a particular age may lead to serious healthy problems. The maximum permissible dose [MPD=(N-18)x5=?Rads; where N stands for the age in years] to the whole body, at any age is equal to 5 times of the number of years beyond 18 years. The MPD in consecutive three months should not exceed 3 rads. The accumulated MPD is also the MPD for the head and the trunk of a person associated with the handling and use of radiochemicals or radioisotopes or operating radiation equipments.

The dose received by any individual is ascertained by various types of monitoring procedures. The most commonly used are film badges and small ionization chambers. The film badges and small ionization chambers are supplied and monitored by many commercial firms.

Film Badges:

The film badge is a small packet of photographic film which is sensitive to radiation; or it may be containing two pieces of photographic films of different sensitivities. A part of these films is covered by one or more metal filters to resist the beta radiation and the other part is open to the beta radiation exposure. For x-rays a different type of film and filter is employed in the film badge. The film badge is worn by the person while handling the radiochemicals or operating radiation equipments. The darkening of the film of the film badge is related to the radiation dose received by a worker. Under the open window it gives the effect of beta radiation and under the filter the effect of gamma radiation or high energy x-rays.

Small Ionization Chambers:

The ionization chambers used for the personal monitoring are small instruments about the size and shape of a fountain pen. These are also called "pocket dosimeters". Before putting to use these are charged with electric pulse from a special charger. The charging transfers the electrical charge to the insulated wire inside the pocket dosimeter. The principle of the functioning of an ionization chamber or pocket dosimeter is that on exposure to the radiation the charge on the insulated wire inside it would decrease.

Brain Tumor and Treatment with Gamma Knife

Gamma rays (g-rays) are like high energy x-rays and can be focused with electromagnetic lenses like electron beam. Gamma knife is a new development in the field of Biomedical Engineering for the surgical interventions. Gamma knife in any way does not look like your kitchen knife, but it refers to a customized beam of gamma rays (g-rays). The conventional surgical procedures for the removal of a brain tumor may need a lot of expertise and experience for a neurosurgeon. The conventional neurosurgical procedures also need utmost precision while chopping of a brain tumor. The gamma knife has made it possible to retard the growth of a brain tumor through non-invasive procedure. The well focused gamma rays (g-rays) at the pre-investigated tumor site in the brain put a full stop on the metabolic activity of tumor cells. Though the technique is called gamma knife radio surgery; the brain tumor is not removed physically but dried and destroyed in situ. The patient needs not to be anaesthetized for gamma knife radio surgery and imaging. There are very few centers in the world equipped with the gamma knife radio surgery facility for the brain tumors.

Monday, January 19, 2009

Diagnostic and Therapeutic uses of Radioisotopes of Cobalt

Three radioisotopes of Cobalt used in medical practice are Cobalt-57 (57Co), Cobalt-58 (58Co), and Cobalt-60 (60Co). Cobalt-60 (60Co) has a half life of 5.27 years and it emits gamma rays (g-rays). Half life of Cobalt-57 (57Co) is 270 days and that of Cobalt-58 (58Co) is 71 days only. It is important to note that the radioisotopes with shorter half life are always good for diagnostic use whereas with longer half life are good for radiotherapy or radiation therapy. Cobalt-60 (60Co) is a preferred source of radiation for radiotherapy at present; though earlier it was in dual use. Cobalt-57 (57Co) and Cobalt-58 (58Co) are preferred for diagnostic applications due to shorter half life.

Use of Cobalt-57 and Cobalt-58 in Diagnosis:

The most common use of Cobalt-57 (57Co) or Cobalt-58 (58Co) is in the diagnosis of pernicious anemia (fetal anemia caused by vitamin B-12 deficiency due to poor absorption of vitamin B-12 by small intestine due to intrinsic factor defect). Cyanocobalamin (vitamin B-12) and hydroxycobalamin labeled with 57Co or 58Co are commercially available for diagnostic use. The dose is 0.5 microCurie (mCi) to 2 microCurie (mCi). The administration is oral after 24 hours of fasting and the patient is not allowed to eat for 2 hours following the administration of vitamin B-12 labeled with radioisotope. The patient does not need hospitalization for this investigation. The excretion of radioactive is measured in the 24 hour urine specimen of the patient after the administration of radioactive labeled vitamin B12.

Use of Cobalt-60 in Therapy:

The gamma rays (g-rays) emitted by Cobalt-60 (60Co) are like high energy x-rays. The radiotherapy or radiation therapy is helpful in some types of cancers. The patients are usually hospitalized for radiotherapy with Cobalt-60 (60Co). The exposure time is worked out by the Radiation Physicist. Smaller radiation sources of Cobalt-60 (60Co) are called seeds and needles, and are used for intra-cavity and intra-interstitial radiation therapy. The seeds or needles of Cobalt-60 (60Co) are removed from the body cavities or interstitial tissue after the optimal time of exposure. The large sources of Cobalt-60 (60Co) are used for 'teletherapy' or external radiation like high energy x-rays.

Therapeutic use of Phosphorus-32

Phosphorus-32 (32P) is a radioactive isotope of phosphorus with mass number as 32. It has a half life of 14.3 days and emits beta-particles. In the chemical form of Sodium hydrogen phosphate, 32P is used in the treatment of leukemia (a type of blood cancer) and polycythemia (abnormal high count of erythrocytes in blood due to a variety of causes). It is also used for relief from pain in the patients affected by metastatic cancer of bone and the patient needs not to be hospitalized for such treatment. The doses range from 1 mCi to 10 mCi. In the colloidal form the radioisotope is used in the body cavities in the same manner as Gold-198 (198Au) and the patient may need hospitalization for such therapy.

Phosphorus-32 (32P) could also used in the differential diagnosis of tumors of the eye. A dose of 500 microCurie (mCi) to 750 microCurie (mCi) needs to be administered intravenously and after an hour, two hours and 4 hours, the radioactive concentration is measured in the diseased eye and the normal eye. If the tumor is malignant, the involved eye would take up more 32P. The investigation may be performed on an out patient basis taking all the aseptic precautions by an eye specialist. It is worth to remember that after 10 half lives (143 day) this radioactive would almost exhaust in its content of radioactivity.

Saturday, January 10, 2009

Role of Gold-198 in Arresting the Growth of Cancer

The radioisotope of gold (Gold-198, 198Au) has a Half Life of 2.7 days and its radiations are beta particles (b-particles) and gamma rays (g-rays). In the form of colloidal suspension 198Au is used as the cancer therapy. The most common use of 198Au is in the treatment of ascites (accumulation of fluid in the peritoneal cavity) and pleural effusion due to metastasis of cancer in the serous surface of cavities. Only the medical practitioners associated with nuclear medicine, administer such therapy. The dose for the treatment of pleural effusion is 25 - 75 mCi and for the treatment of ascites, the dose is 50 - 150 mCi. The patient needs to be hospitalized for the first few days during the course of treatment with the radioisotope.

Saturday, January 3, 2009

What is the use of Iodine-131 in Medical Practice?

There are two main uses of Iodine-131 (131I) in medicine or medical practice.

  1. Diagnostic application in some procedures

  2. Therapy for thyroid disorders or thyroid cancer

The Iodine-131 (131I) has a radioactive half life of 8.1 days and its radiations are beta particles(b- particles) and gamma rays (g-rays). This is most widely used radioisotope in the management of hyperthyroidism and thyroid cancer and thyroid function related diagnostic procedures. There are half a dozen investigations associates with the thyroid function which involve the oral or intravenous administration of a few microCuries (mCi) of 131I. Subsequent study of the patient, either by the direct measurement of 131I deposited in the thyroid gland through measurement of 131I excreted in the urine of the patient or by assessment of radioactivity in the blood samples drawn at different time intervals after the administration of 131I. The part of the 131I retained or excreted depends on the normal, hyperthyroid or hypothyroid conditions. The uptake or excretion of 131I exhibits a diagnostic parameter. After absorption of 131I by the thyroid gland the iodine is elaborated into the thyroid hormone which is discharged in the blood. The hyperactive gland produces too much hormone which would be detected in the blood samples taken at 24 to 96 hours after the administration of radioactive iodine (131I). The measurement of the iodine content is computed from the counts of radioactivity detected in the blood samples. The radioactivity is measured as gamma rays (g-rays) by a Geiger Muller counter or it may be measured as beta particles (b-particles) by a Scintillation counter at a 'Hot Laboratory'.

The therapeutic use of Iodine-131 (131I) could be culminated through optimal doses of this radioisotope with reference to the thyroid disorder and the age and weight of the patient. There are specialized clinics at the authorized medical centers having facilities for the Nuclear Medicine and associated research.

Units of Radiation Dose and Maximum Permissible Doses

The radiation energy from the radioisotope is absorbed by the living tissue of the patient subjected to the radiation therapy or radiotherapy. The traditional unit of radiation dose is called Rad and one Rad stand for 100 egrs of radiation energy absorbed per gram of the absorber tissue Therapeutic doses are usually measured in hundreds or thousands of Rads. In diagnostic procedures where it is desirable to keep the dose very low, the unit is millirad (mr). One millirad is the thousandths part of a Rad and only a few hundred millirads are permissible dose.

The radiation protection is an important precaution to avoid over exposure of a patient as well as the radiation worker. The radiation protection unit is called dose equivalent and its traditional unit is REM (Rad equivalent man).

  • REM = Rad Equivalent Man

  • REM = Rad x Relative Badge Effect

  • REM = Rad x RBE

RBE depends upon the type of the radiation energy. RBE for x-rays, g-rays and b-particles is an energy up to 3 MeV (3 MeV = 1 Rad). RBE for positrons, neutrons and a-particles is 10 rem (REM). It is evident that positrons, neutrons and a-particles exert lesser side effects as compared to x-rays, g-rays and b-particles. The S.I. unit of radiation protection is Seivert(Sv).

  • 1 Sv = 100 Rem

  • 1 Sv = 1000 milliSeivert (mSv) = 100 Rem

  • 10 mSv = 1 Rem

Maximum permissible dose (MPD) or maximum limit of per year exposure to radioactivity could be computed separately for the patients (general public) and radiation workers. As a rule anybody below the age of 18 years is never permitted to become a radiation worker. The permissible dose can be worked out by the following formula:

D = 5 (N-18); Here D=Dose in Rem, N= Age in years.

A radiation worker who joins the duty after acquiring the age of 18 years could have maximum exposure of 5 x (20-18) = 5 x 2 = 10 Rem only up till the completion of 20 years of age or 5 Rem per year exposure. General public or a patient could be exposed to radiation energy at a MPD of one tenth (1/10) of the dose for a radiation worker and that comes out to be 0.5 Rem per year (5/10 Rem per year). Hands and feet could be exposed at a higher dose if desired but not more than 75 Rem/year or 40 Rem/3 months. Thyroid, Bone and skin could be exposed at 40 Rem/year and other organs at 15 Rem/year. The fetus should not be exposed to a radiation dose of more than 0.5 Rem.