Nuclear Medicine Physics Radiation Dose Jerry Allison, Ph.D. Department of Radiology Medical College of Georgia

Medical College of Georgia And Sameer Tipnis, Ph.D. A note of thanks to Z. J. Cao, Ph.D. Medical College of Georgia And Sameer Tipnis, Ph.D. G. Donald Frey, Ph.D. Medical University of South Carolina for Sharing nuclear medicine presentation content

Definitions Absorbed dose D (Gy): energy deposited in a unit mass of absorber 1 Gy = 1 joule/kg (SI unit) 1 rad = 100 erg/g (traditional unit) 1 Gy = 100 rad

Definitions Equivalent dose HT (Sv): quantity that expresses absorbed dose across an organ or tissue with a weighting factor for type and energy of radiation HT = DT . wR DT: absorbed dose in a tissue wR : weighting factor that denotes relative biologic damage for type of radiation For x, , e- , e+ : wR = 1 For n: depends on energy For p (> 2 MeV): wR = 2, For a, fission fragments, heavy ions: wR = 20

Definitions In diagnostic radiology, equivalent dose (Sv) is equal to absorbed dose (Gy) since: wR = 1 for x, , e- , e+

Definitions Effective dose E (Sv): measure of absorbed dose to whole body, the product of equivalent dose and organ specific weighting factors Whole body dose equivalent to the nonuniform dose delivered

Definitions From: International Commission on Radiological Protection: Recommendations of the International Commission on Radiological Protection. ICRP Publication 103; Ann ICRP. 37 (2-4)2007 © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps

Definitions Dose equivalent H (Sv) An obsolete unit that expressed absorbed dose at a point in an organ with a quality factor that depends on type of radiation Rem (traditional unit) 1 Sv = 100 Rem Presumably effective dose

Example The mean glandular dose to the ACR mammographic accreditation phantom must not exceed 3 mGy (300 mRad) per view at the recommended techniques for imaging an average breast.

Example What is effective dose to the patient if breasts receive an absorbed dose of 3 mGy? wR = 1 wT = 0.12 E = D * wR * wT E = 3 mGy * 1 * 0.12 E = 0.36 mSv So, absorbed dose to the breasts of 3 mGy results in effective dose to the patient of 0.36 mSv (to the whole body)

Definitions Effective dose equivalent HE (Sv) An obsolete unit that expressed absorbed dose to whole body, the product of dose equivalent and organ specific weighting factors Rem (traditional unit) 1 Sv = 100 Rem Presumably effective dose

Definitions Exposure X (C/kg): charge liberated per kg of air by x- or g-rays C/kg (SI unit) 1 R = 2.58×10-4 C/kg (traditional unit) Measures number of ionizations

Definitions Air kerma K (Gy): kinetic energy liberated per kg of air by uncharged radiations (g-rays, neutrons) Kinetic energy of particles liberated through Photoelectric absorption Compton scattering Pair production Gy (SI unit) In nuclear medicine K (Gy) = X (C/kg) * 33.7 K (Gy) = X (R) * 0.00869

Unfortunate Caveat: In radiology: Gy and Sv are often used loosely Rad and Rem are often used loosely 1 Gy = ~100 Rad 1Sv = ~100 Rem 1 Air Kerma Gy = 33.7 C/kg = 0.00869 R

Effective dose of NM procedures

Dose limits Occupational: ALARA 1 & ALARA 2 Embryo/fetus: 5 mSv total © Physics in Nuclear Medicine: Cherry, Sorenson and Phelps