Radiation dose to the patient

Radiation dose to patients
The usage of radiation dose on patients vary in terms of procedure, patient’s medical condition, age and which tissue is being treated. Radiation dose is measured in many ways dependent on different factors that have to be taken into account because of the risk of damage, both acute and long-term, caused to the patient by exposing them to any dose of radiation.

Importance in clinical medicine
Low doses of radiation doses are used for imaging techniques such as CT and PET scans. High doses of radiation is used in radiation therapy of cancer. Treatment always requires careful planning, calculating and finding right doses has its own field of medicine called Dosimetry.

The units.
There are different terms we have to get acquainted with when we speak about radiation doses.

Absorbed dose, DT, has the unit Gray (Gy). This measures how much radiation that is required for 1 Joule to be absorbed by 1 kg of matter. However, dealing with patients this unit is not enough since it’s a physical quantity.

For the impact of ionizing radiation biological tissue we need another unit which is called equivalent dose, HT, measured in Sievert (Sv). Equivalent dose takes the damaging effects of radiation into consideration and is derived from the “absorbed dose”. By multiplying the absorbed dose, DT, with a radiation weighing factor WR, based on the type of radiation used since particles cause different amount of damage when absorbed.

DT x WR = Equivalent dose, HT

However all tissues are not equally radiosensitive. The sensitivity is related to rate of cell-division, i.e. bone marrow and skin are very sensitive to low doses while bones and mature cartilage is not. This is why the dose varies alot depending on which type of tissue/organ is being imaged/treated. By using a tissue weighing factor WT, based on each tissues radiation-sensitivity we derive the effective dose, also measured in Sievert (Sv).

WT x HT = Effective dose, E

Effective dose, is for estimating the general long-term “over-all” effect on the whole body taking both the radiation type and sensitivity of the tissue into account. This is only a calculated unit giving the probability/risk of long term effects patients might suffer, i.e. risk of secondary cancer or genetic damage.

Grey and Sievert are numerically the same. For radiation treatment Gray (Gy) is used since it’s a measured unit and high doses of radiation are absorbed by portions of the body.

Doses.
As a point of reference, an annual dose of background-radiation is about 3 mSv/year.

CT scans vary from 6-8 mGy up to 14 mGy while we in radiation treatment use quantities as 20 -40 Gy (curative), 60-80 Gy (curative, tumors), to 45-60 Gy (preventive).

In today’s radiation therapy doses are often fractionized, split up into smaller portions. 1,8-2 Gy given daily (or even multiple times per day) about 5 days/week. This is called hyperfraction. Commonly used technique is also hypofraction, where the dose is lager and not given with greater intervals.

Fractionation is a good method since cancer cells don’t have time to repair between each dose and the negative effect on healthy tissues is minimized. There is also a good chance to catch the cancer cells at a sensitive phase in mitosis.

Conclusion.
Different radiation schedules and doses are tried to optimize the eradication of cancer cells and recovery of healthy cells, the challenge here is to find a way to minimize the damage of surrounding tissues. While in modern-medical-imaging the most used technique is CT-scan, where the struggle is to get an good image as possible using the least amount of radiation dose possible.