PHY138Y Nuclear and Radiation

Transcription

1 PHY38Y Nuclear and Radiation Professor Tony Key MP40 Announcements MP problems set #4 due Sunday at midnight PS#5 WRITTEN now posted! - do in teams, no Lone Wolves!! NB correction to PS#4, question ; (HVL, TVL) Replace Energy Attenuation Coefficient by Energy Absorption Coefficient Today (and Monday?) SNIV Biological Effects of Radiation Review, then complete... Worked Example - complete Exposure, Dose, Energy Fluence,, W T Background radiation (Attenuation and Absorption Question) Read Notes (& References) photon in HUMAN TISSUE Total Energy Out = Total Energy In ATTENUATION? ABSORPTION? YES NO photon electron Dose, Fluence, Exposure D air = 8.69 x0-3 X (Gy to R) (x) = (0) exp(-µ en x) D m (x) = (µ en /ρ) m (x) (including D air = (µ en /ρ) air ) REMEMBER AND UNDERSTAND THESE THREE RELATIONSHIPS!

2 Dose in air, D air = X air = body Then use absorption of fluence in body (x) = (0) exp(-µ en x) AND D m (x) =S( in - )/M A person is exposed to a beam of X-rays. Which quantities do you think will be the same on both sides of the infinitesimally thin air-skin boundary? A. the exposure B. the dose C. the energy fluence of the beam D. none of the above E. all of the above air = body ASSUMPTIONS, CONSTANTS. Effective X-ray energy is 0/3 = 40 kev. No loss of energy between machine and chest - μ en is VERY small in air 3. Energy fluence at skin = energy fluence entering body 4. Chest has surface area S = 5 x 5 cm = 65 cm 5. Tissue Bone Lung x t = 5cm x b = 5 cm x lung = 5 cm Thus, chest has depth d = x t + x b + x lung = 5 cm

3 ( ) e = e = deposited AVERAGE ABSORBED DOSE ~ 50 μgy Is this a serious health risk? Rolf Sievert ( ) the Sievert was adopted as the unit of equivalent dose 3

4 Photons*, electrons Protons 0 Photons*, electrons Protons 0 * = can be used for the full range of X- ray energies used in this course. * = can be used for the full range of X-ray energies used in this course. RADIATION WEIGHTING FACTORS The Equivalent Dose is the absorbed dose (in Gy or rad) multiplied by. H = x D ΣW T ΣW T Colon, Lung, Breast, Stomach, Bone Marrow Colon, Lung, Breast, Stomach, Bone Marrow Bladder, Oesophagus, Thyroid, Liver Bladder, Oesophagus, Thyroid, Liver Bone, Skin, Salivary Glands, Brain Bone, Skin, Salivary Glands, Brain TISSUE WEIGHTING FACTORS The Effective Dose is the Equivalent Dose multiplied by W T and summed over all organs H eff = Σ W T H T. The Effective Dose (H eff ) to the tissue is that dose which, if it were given to the whole body, would have the same risk to health. (e.g. probability of death from cancer). AVERAGE ABSORBED DOSE ~ 50 μgy Is this a serious health risk? 4

5 Protons Photons*, electrons * = can be used for the full range of X-ray energies used in this course. Colon, Lung, Breast, Stomach, Bone Marrow Σ 0 Suppose a chest X-ray delivers an absorbed dose (D) of 00µGy to a patient s lungs. Photons*, electrons Protons 0 Bladder, Oesophagus, Thyroid, Liver Bone, Skin, Salivary Glands, Brain The equivalent dose (H) is -- A. 00 µsv B. 4 µsv C. µsv D. 6 µsv E. don t know Suppose a chest X-ray delivers an equivalent dose (H) of 00µSv to a patient s lungs. The effective dose (H eff ) is -- Colon, Lung, Breast, Stomach, Bone Marrow Bladder, Oesophagus, Thyroid, Liver Bone, Skin, Salivary Glands, Brain A. 00 µsv B. 4 µsv (good try!) C. µsv D. 6 µsv E. don t know Σ Suppose a chest X-ray delivers an equivalent dose (H) of 00µSv to a patient s lungs. If one lung is now totally shielded during the exposure, the effective dose (H eff ) is -- Colon, Lung, Breast, Stomach, Bone Marrow Bladder, Oesophagus, Thyroid, Liver Bone, Skin, Salivary Glands, Brain Other Organs (grouped) A. 50 µsv B. 4 µsv C. µsv D. 6 µsv E. don t know Σ ~80% of total 5

6 ~0% of total Deterministic Dose Stochastic Dose! Probability of fatal cancer ~ 4 x 0-5 x (No. of msv received). Chest X-ray (~5 msv) increased risk ~ 0.006% Stochastic Dose Probability of fatal cancer ~ 5 x 0-5 x (No. of msv received) ~ 5% per Sv The Canadian Nuclear Safety Commission sets limits of allowable radiation to less than 50 msv per year The Health Physics Society recommends against quantitative estimation of health risks below 50 msv or 00mSv in a lifetime 6

7 Model for Electron Beam Therapy Calculations Isodose Lines Thanks to Professor D.W.O. Rogers 7