Internal Radiation Dosimetry

Radiation dosimetry to both the patient and the Nuclear Medicine Technologist requires special attention, especially if one is to maintain compliance with the ALARA guidelines. The smallest dose possible should be injected, consistent with good image quality. In addition, for pediatric patients, the dose should be scaled down based on the patient's body mass. Of necessity, SPECT imaging may require a larger injected dose than does planar imaging since acquisition time at each orbital stop is so short.

Early observations of radiation effects:Marie Curie's Professor proudly wore on his lapel a vial containing the first sample of radium ever produced; shortly thereafter, the first radiation burn was documented. To verify this association, early researchers taped pieces of the new metal to their skin to observe results and confirmed that the radioactive material was indeed causing the burns.

The now familiar story of an epidemiological study of radium-watch dial painters revealed that these workers, who tipped the brushes with their lips and ultimately swallowed large amounts of radium, had an incidence of bone cancer significantly greater than expected for the general population. An increase in thyroid cancer has likewise been observed in adults who, during the 1940's, underwent neck irradiations during childhood.

Researchers have been unable to document a threshhold level for causing radiation damage for long-term radiation effects, so the assumption is made that the probability of radiation carcinogenesis and radiation-induced genetic abnormalities is never zero, regardless of how low the exposure dose. We know that

-the rates of carcinogenesis and radiation-induced abnormalities are too low to measure by most feasible techniques

-time between exposure and manifestation of symptoms is inversely propor-tional to the dose

- at low exposure rates, biological repair corrects some of the damage

Radiation dosimetry to both the patient and the Nuclear Medicine Technologist requires special attention, especially if one is to maintain compliance with the ALARA guidelines. The ALARA Concept, which is based upon maintaining the radiation dose As Low As Reasonably Achievable, was implemented in the late 1970's and has contributed to a significant overall reduction in dose to radiation workers.The smallest dose possible should be injected, consistent with good image quality. In addition, for pediatric patients, the dose should be scaled down based on the patient's body mass. Of necessity, SPECT imaging may require a larger injected dose than does planar imaging since acquisition time at each orbital stop is so short.

For Radiation workers, the Maximum Permissible Dose (MPD), whole body,1 Rem per year for every year the radiation worker has been alive. For example, if a worker is 30 years old, his MPD is 30 R. While he may exceed that value in a given year (5R max), the average may not exceed the value stated above. The MPD is usually expressed by the following equation.Please note that a radiation worker must be more than 18 years old.

MPD (Cumulative) for Radiation Workers = 1(Age) Rem

For our families and other non-radiation workers, the MPD is one-tenth of our MPD, or 0.1 Rem per year. For radiation workers, the extremities are permitted 15 times the whole body dose, or 75 Rem. This is an acceptable level for the hands since there is essentially no bone marrow to be irradiated. Lifetime cumulative records are maintained and transferred from one institution to another when job changes are made.

Ionizing radiation includes charged particles, photons, and other products of natural and induced radiation. In the body, water is the most likely molecule most likely to be ionized by radiation. The water transfers its energy to another nearby molecule, inducing radiation damage in the second molecule. Depending upon dose rate, the tissue may or may not be able to repair itself. Effects recognized as being caused by ionizing radiation include acute burns, dermatitis and hair loss, and carcinogenesis, as well as genetic effects.

When evaluating individual radioisotopes to determine their ability to deliver radiation dose to the tissue, it is necessary to consider the decay scheme and both "input" and "output" data. The input data are those which may be determined directly from the decay scheme; output data include all possible contributions to the radiation dose. This includes, in addition to the primary radiations listed in the decay scheme, characteristic X-rays, internal conversion electrons, and Auger electrons.

The table shown below was extracted from the MIRD Booklet (Medical Internal Radiation Dose Committee, a sub-committee of the Society of Nuclear Medicine).

TABLE HERE

It is important to consider every radiation not only from the stand-point of self-irradiation of an organ, but also radiation transmitted to every other organ in the body. For example, we know that Tc-99m sulfur colloid localizes in the liver, spleen, and marrow. To calculate liver dose, we must be concerned, therefore, with dose from the liver to the liver, from the spleen to the liver, and from the marrow to the liver. In addition, we must take into account both penetrating (X, gamma) and non-penetrating (betas, electrons) radiations.

The equation shown below accounts for all these factors and indicates that the total dose is a summation of both penetrating and non-penetrating contributions.

equation here

The MIRD Committee has developed a hypothetical "reference man", actually a bisexual construct that permits estimation of the factors required to calculate dose to one organ attributable to a source in another organ. A schematic drawing of this "reference man" is shown below.

RADIATION DOSE FORMULAS (see below for definition of terms in equations)

For beta particles, internal conversion and Auger electrons, which deposit essentially 100% of their energy into the body, the following formula has been derived:

D_beta in Rads = E_beta . f_beta . C_o . 1.443 . t_eff

which, when using appropriate conversion factors, yields

D_beta = 73.8 E_beta . f _beta. C_o . t_eff Rads.

2. For moderately energetic X- and gamma-rays, which deposit only part of their energy in the body, the following formula has been derived:

D_gamma in Rads = E_gamma . f_gamma . C_o . 1.443 . t_eff . 0

which, when using appropriate conversion factors, yields

D_gamma = 73.8 E_gamma . f_gamma. Co . t_eff. . 0 Rads.

When the two equations are combined, the total dose equation, which represents contributions from both penetrating and non-penetrating radiation, is:

TOTAL DOSE = 73.8 C₀ . t_eff [E_beta . f _beta +E_gamma . f_gamma .0] Rads

Definition of terms:

C_o = Initial concentration of µCi/g radioisotope in tissue

t_{eff =} Effective half-life Days

f _beta, f_gamma = fraction of decays in which beta, gamma are emitted

E_beta = Average beta - energy in MeV

E_gamma = gamma ray energy in MeV

0 = fraction of gamma energy absorbed

Radiation Absorbed Doses: Natural Whole Body Background:

1. Principal Sources: cosmic radiation, minerals in soil and building materials; naturally occurring K-40, C-14 in our bodies.

2. Minor Sources: radiocontaminants in food chain; emissions from Nuclear power plants; flying in aircraft.

3. At sea level, annual background whole body dose is 120-150 mR; 1 mile high (Denver), 260-300 mR; in areas where radium and uranium are plentiful in soil, may reach 7,000 mR/yr.

Radiographic Procedures: Dosimetry (Estimated)

Chest film (anterior) .............................................10 mR

Chest film (lateral) ................................................42 mR

Abdominal X-ray ................................................300 mR entry dose, 25 mR whole body dose

Skull X-ray ............................................................250 mR entry dose

Lateral lumbar spine ..........................................600-700 mR entry dose

CT Scan (Continuous slices) .........................4-5 R per exam

Cardiac catheterization ....................................50 R

Fluoroscopy .........................................................2-3 R/min

Lower GI series ................................................5R entry dose