316
8 Ionizing Radiation and Life
thyroid gland, the biological half-life of I-131 is 5.5 days. If part of the thyroid is
not functioning or excised, the biological half-life is longer than 5.5 days.
8.14.4 Brachytherapy
Brachytherapy (brachy is Greek for short distance) refers to the use of localized
capsule containing a radiation source which is placed in or near a tumor, exposing
that tumor to a high radiation dose, while reducing exposure of healthy tissue
compared to beam therapy.
A number of other radioisotopes are used for cancer treatment (and diagnostics),
including Bismuth-213, Holmium-166, and Lutetium-177.
8.15 Radiation Dosimetry
8.15.1 UV Light Limitations
Since ultraviolet light does not penetrate far below the cutaneous layer of skin,
measures of UV radiation effects are often calibrated by the energy deposited per
unit area of the skin rather than energy per unit mass used for Equivalent Dose.
By the way, do not let your dentist convince you that UV light on one sunny day is
equivalent to the X-ray exposure he/she is about to give you. X-rays are penetrating;
UV is stopped by the skin. You are likely to be unaware of the development of a
deep tumor, but a melanoma is visible. The issue for getting an X-ray is whether, for
a given intensity, the chance of a later cancer is more of a concern than the present
treatment you may need. A soldier may decide to cross a field of bullets if his present
danger is high, or he decides whether the risk is worth the hoped-for outcome. X-ray
photons act as bullets.
One ‘Standard Erythemal Dose of UV’ (SED) 15 is a measure of the energy, in
joules, deposited over each square meter of skin. One SED is 100 joules per square
meter.
The ‘UV Index’ is an adopted value intended to indicate to the public the
UV sunburn danger. The raw value is the integrated ground-level solar UV light
intensity in each frequency interval (dI/df ) times a skin sensitivity factor called the
‘erythemal action spectrum’. 16 The raw UV index is then normalized to have a value
of 10 for a clear sky mid-latitude zero-altitude midday solar exposure. Finally, the
15 Erythea is a reddening of the skin due to inflammation. Typically, there are several hours after
Sun exposure before reddening becomes evident.
16 The erythemal action spectrum (EAS) is the ratio of the intensity per unit frequency of UV to
just barely produce erythema, divided by the intensity per unit frequency at wavelength 290 nm.
8 Ionizing Radiation and Life
thyroid gland, the biological half-life of I-131 is 5.5 days. If part of the thyroid is
not functioning or excised, the biological half-life is longer than 5.5 days.
8.14.4 Brachytherapy
Brachytherapy (brachy is Greek for short distance) refers to the use of localized
capsule containing a radiation source which is placed in or near a tumor, exposing
that tumor to a high radiation dose, while reducing exposure of healthy tissue
compared to beam therapy.
A number of other radioisotopes are used for cancer treatment (and diagnostics),
including Bismuth-213, Holmium-166, and Lutetium-177.
8.15 Radiation Dosimetry
8.15.1 UV Light Limitations
Since ultraviolet light does not penetrate far below the cutaneous layer of skin,
measures of UV radiation effects are often calibrated by the energy deposited per
unit area of the skin rather than energy per unit mass used for Equivalent Dose.
By the way, do not let your dentist convince you that UV light on one sunny day is
equivalent to the X-ray exposure he/she is about to give you. X-rays are penetrating;
UV is stopped by the skin. You are likely to be unaware of the development of a
deep tumor, but a melanoma is visible. The issue for getting an X-ray is whether, for
a given intensity, the chance of a later cancer is more of a concern than the present
treatment you may need. A soldier may decide to cross a field of bullets if his present
danger is high, or he decides whether the risk is worth the hoped-for outcome. X-ray
photons act as bullets.
One ‘Standard Erythemal Dose of UV’ (SED) 15 is a measure of the energy, in
joules, deposited over each square meter of skin. One SED is 100 joules per square
meter.
The ‘UV Index’ is an adopted value intended to indicate to the public the
UV sunburn danger. The raw value is the integrated ground-level solar UV light
intensity in each frequency interval (dI/df ) times a skin sensitivity factor called the
‘erythemal action spectrum’. 16 The raw UV index is then normalized to have a value
of 10 for a clear sky mid-latitude zero-altitude midday solar exposure. Finally, the
15 Erythea is a reddening of the skin due to inflammation. Typically, there are several hours after
Sun exposure before reddening becomes evident.
16 The erythemal action spectrum (EAS) is the ratio of the intensity per unit frequency of UV to
just barely produce erythema, divided by the intensity per unit frequency at wavelength 290 nm.