Unit of Radiation dose: Gy [with exercise questions]

Unit of Radiation Dose

The gray (symbol: Gy) is a derived unit of ionizing radiation dose in the International System of Units (SI). It is defined as the absorption of one joule of radiation energy per kilogram of matter.

The measurement of absorbed dose in tissue is of fundamental importance in radiobiology and radiation therapy as it is the measure of the amount of energy the incident radiation deposits in the target tissue. The measurement of absorbed dose is a complex problem due to scattering and absorption, and many specialist dosimeters are available for these measurements, and can cover applications in 1-D, 2-D and 3-D.

In radiation therapy, the amount of radiation applied varies depending on the type and stage of cancer being treated. For curative cases, the typical dose for a solid epithelial tumor ranges from 60 to 80 Gy, while lymphomas are treated with 20 to 40 Gy. Preventive (adjuvant) doses are typically around 45–60 Gy in 1.8–2 Gy fractions (for breast, head, and neck cancers).

The average radiation dose from an abdominal X-ray is 0.7 milligrays (0.0007 Gy), that from an abdominal CT scan is 8 mGy, that from a pelvic CT scan is 6 mGy, and that from a selective CT scan of the abdomen and the pelvis is 14 mGy.

Radiation Damage to DNA

When radiation hits DNA, part of the DNA may break depending on the amount of radiation.

It is said that exposure to 1 mGy of X-rays causes a single-strand break at one location per cell on average. This amount of radiation is equivalent to 1 mSv. Double-strand breaks occur less frequently, at 0.04 locations per cell, which means that if 100 cells are evenly exposed to 1 mGy of X-rays, double-strand breaks occur in four cells.

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