Transcript Radiation Protection in Radiotherapy
Introduction to Radiation Units & Biological Effects
Lec 2 of Rad. Prot.
March 13, 2014
•
Absorbed Dose of Photon (D) Gray
dV
•
D
(Gy = 1J/kg) =
dE
/
dM
•
dE
= amount of energy loss only via • ionizations & excitations of atoms • in mass
dM
by 2 nd e ± • 6 MeV Photons • in 50 cm-thick water
Dose, Effective Dose, etc.
• Absorbed energy,
ε
: Joule (J) • Absorbed dose ,
D D
=
ε
/
m
: Gray (Gy), rad (Gy) absorbed energy per mass unit,
m
1 Gy = 1 J/kg, 1 mGy = 1/1000 Gy • Equivalent dose,
H H T
= S
w R ·D T,R
: Sievert (Sv), rem biologically effectiveness ( α,β,γ) • Effective dose,
E H E
= S
w T ·H T
: Sievert (Sv), rem weighted according to stochastic risk
Radiation weighting factors
Neutron weighting factors
A: ICRP 60 histogram B: ICRP 60 function C: ICRP 103
w
R = 2.5 + 18.2 exp[-(ln
E
n ) 2 /6] ……….
= 5.0 + 17.0 exp[-(ln 2
E n
) 2 /6] …… for
E
n
<
= 2.5 + 3.25 exp[-(ln 0.04
E n
) 2 /6 ] … for
E
n 1 MeV for 1 MeV <
> E
n
< 50
50 MeV MeV
Tissue weighting factors
1990 Gonads Bone marrow Colon Lung Stomach Breast Bladder Liver Oesophagus Thyroid Skin Bone surface 0.20 0.12
0.05 0.01 2007 Breast Bone marrow Colon Lung Stomach Remainder Gonads Bladder Liver Oesophagus Thyroid Skin Bone surface Brain Salivary glands Remainder 0.05 0.12
0.08
0.04
0.01
Tissue weighting factors
1990 2007 Remainder (10): 0.05 Remainder (14): 0.12
Adrenals Brain Upper large intestine Small intestine Small intestine Kidney Muscle Pancreas Spleen Thymus Uterus Adrenals Kidney Muscle Pancreas Spleen Thymus Uterus/Cervix Oral mucosa Extrathoracic region Gall bladder Heart Lymphatic nodes Prostate
Absorbed Dose vs. Exposure
• •
D
m (rad (1) ) =
f
m (rad/R) ·X(R)
f
air = 0.876,
f
tissue = 0.94,
f lucite
= 0.78 @ 80 keV
W
/e = mean energy to produce an ion pair in dry air = 34 eV/ion pair = 34 J/C (1 eV = 1.602 × 10 -19 J) • D air [Gy] = X [R, C/kg] ×
W
/e [J/C] = 2.58 × 10 -4 [C/kg] × 34 [J/C] = 0.876 × 10 -2 [J/kg/R] × X [R] D air [rad] = 0.876 (rad/R) × X [R]
Annual Effective Dose Limits
Annually permissible risk factor Radiation risk factor Annual effective dose limit
Occupational
8x10 -4 /yr 4x10 -2 /Sv 20 mSv/yr
Public
5x10 -5 /yr 5x10 -2 /Sv 1 mSv/yr
Effective Dose Limits
• • Old Radiation Dose Limits – Occupational: 50 mSv/yr – Public: 5 mSv/yr New Radiation Dose Limits – Occupational: 20 mSv/yr (5 year average) with a maximum of 50 mSv in any one year, (150 mSv for lens and 500 mSv for skin and extremities) – Public: 1 mSv/yr
6 MV X-ray in aluminum
6 MV X-ray in iron
6 MeV e-beam in water
18 MeV e-beam in aluminum
Radiation & Radicals
Photon, E = hv < nsec
e
Scattered photon E' = hv' Secondary electron
e -
<
μsec
e -
Secondary electron
H 2 O
< msec Free Radicals
H 2 OH H O + e aq , etc.
hours
cell
years
gene
Time Frame for Effects of Ionizing Radiation
Chemical development of a 4 keV e track in water
Intuitive Knowledge from above
• Photons (x-or g -ray) are the main concern • Materials with a high density & high atomic number are effective for shielding • Scatter and 2 nd radiation (Bremsstrahlung x-ray & neutron) should be taken into account • Now, how much (at what level) to be protected?
Epidemiology & Experiment
Breast Cancer vs. Dose
EBCTCG Lancet 2005
Biological Effects
• Deterministic Effects – Result from acute damage too extensive for the body repair – Effects become observable once the radiation dose exceeds a threshold . Increasing the dose above the threshold increases the severity of the damage – Examples include erythema, cataracts, and death from whole body exposure • Stochastic Effects – Result from DNA damage which is mis-repaired but still leaves the cell able to function and divide. This damage then proliferates as the cell divides – There is no threshold dose, “risk” of occurrence increases with dose – Examples include cancer and hereditary defects Severity Threshold Risk Dose Dose
Deterministic Effects
Stochastic Effects
• Cancer risk: malignant neoplasms – 4.0 x 10 -2 /Sv • Hereditary risk: hereditary mutations – 1.0 x 10 -2 /Sv • Risk factors – Occupational: 4.0 x 10 -2 /Sv – Public: 5.0 x 10 -2 /Sv
The Linear-No-Threshold (LNT) Assumption
[1/3]
• What is LNT?
– Most of the information available on stochastic effects is from relatively high dose data (e.g., atomic bomb survivors) – High dose results are then extrapolated linearly down to zero dose Risk Radiation Dose
The Linear-No-Threshold (LNT) Assumption
[2/3]
• Why is LNT controversial?
– Some of the studies at low dose indicate no effect (e.g., Letourneau’s study of Radon in Winnipeg homes) – Others studies even suggest a beneficial effect at low doses (e.g., Cohen’s U.S. Radon study). Such a beneficial effect is known as “ radiation hormesis ” Aspirin has a similar dose response, a few can help you but too many at once can make you sick or even kill you Risk Benefit Linear extrapolation from high dose Dose or # of Aspirin
The Linear-No-Threshold (LNT) Assumption
[3/3]
• The bulk of scientific evidence to date either supports the LNT hypothesis or suggests that it is, at worst, conservative (i.e., it overestimates the risk at low doses) • For these reasons, most radiation protection experts and agencies (including the ICRP, NCRP, NRPB * ) advocate the use of LNT until such time as there is
definitive
scientific evidence that a different dose response relationship would be more appropriate at low doses * ICRP - Internal Commission on Radiological Protection NCRP - National Council on Radiation Protection (US) NRPB - National Radiological Protection Board (UK)
Basic Assumptions of Radiation Protection
• • LNT model for stochastic effects • Protection far below the level that no deterministic effects occur at all
ALARA
, as low as reasonably achievable – applies to all aspects of radiation protection, e.g., the cost of increasing shielding beyond the minimum value often represents only a small increase in cost
UNSCEAR, BEIR etc ICRP 2007 Recommendations IAEA Basic Safety Standards National Legislations
Target
Production of Radiation Types in LINAC
Sources of photo-neutrons*: either (1) Linac head or (2) Rm shielding e.g., 54 Fe(
h
n , n ) 53m Fe* → 53 Fe + g Neutron capture gammas: 1 H(n, g ) 2 H , 27 Al(n, g ) 28 Al → 28 Si + b + g *NCRP Report No. 79
Goals of Radiation Shielding Design
• To limit radiation exposures to members of the public and employees to an acceptable level or • To ensure that all exposure levels to the public and occupational personnel are maintained below the regulatory limits, any special regulatory requirements, and within ALARA limits established at the facility
Shielding in Radiotherapy Facilities
• Equipment to be included in radiation oncology typically consist of a CT simulator & linear accelerators – NCRP 147: CT-simulator shielding – NCRP 151 (which supersedes NCRP 49, 51, 79): Linear accelerator shielding
Quantities and Units
[1/2]
• The quantity for shielding design calculations: dose equivalent ,
H
• The quantity for the limitation of exposure to people: effective dose ,
E
• The quantity air-kerma,
K
a environments (no neutron) in low LET – Exposure (R) is divided by 114 to obtain air-kerma (Gy), or by 104 to obtain absorbed dose (Gy) or dose equivalent (Sv) at a point in tissue
Quantities and Units
[2/2]
Neutron fluence-to-dose equivalent conversion coefficients for photon or e-beams of energy > 10 MeV Typically, dose-equivalent rates < 0.02 mSv/h
Controlled Areas
• Limited-access area in which the occupational exposure of personnel to radiation or radioactive material is under the supervision of an individual in charge of radiation protection – Areas are usually in the immediate areas where radiation is used, such as treatment rooms and control booths, or other areas that require control of access, occupancy, and working conditions for radiation protection purposes – Workers in these areas are those individuals who are specifically trained in the use of ionizing radiation and whose radiation exposure is usually individually monitored
Uncontrolled Areas
• All other areas in the hospital or clinic and the surrounding environs • Trained radiation oncology personnel and other trained workers, as well as members of the public, frequent many areas near controlled areas such as examination rooms or restrooms – Choice of appropriate occupancy factors ensures the protection of both those who are occupationally exposed as well as others who might be exposed in these areas
Shielding Design Goals, P
[1/2]
• Shielding design goals: levels of dose equivalent (
H
) used in the design calculations & evaluation of barriers • It is not practical to base shielding design directly on
E
• The shielding design goals will ensure that the respective annual values for
E
for controlled & uncontrolled areas are not exceeded
Shielding Design Goals, P
[2/2]
• Controlled areas: 5 mSv/y & 0.1 mSv/wk dose equivalent (H) (pregnant workers continued access to their work areas) • Uncontrolled areas: 1 mSv/y & 0.02 mSv/wk dose equivalent (H) • Typical international shielding design goals – 0.12 mSv/week for controlled areas – 0.004 mSv/week for uncontrolled areas • It is the weekly dose equivalent that is used to determine compliance with the shielding design goals
Workload, W
• Time integral (in a week) of the absorbed-dose rate determined at the depth of max. absorbed dose, 1 m from the source, Gy/wk • Product of the average number of patients (or fields) treated in a week and the absorbed dose delivered per patient (or field) • For dual-energy machines, the workload at the higher energy will usually determine the shielding requirement (at least 250 Gy/wk at high MV mode ) • IMRT: workload efficiency factor
NCRP 151 Recommended Workload
[1/2]
• 450 Gy/wk maximum weekly workload cited in NCRP 151 – Kleck (1994) • Maximum 350 Gy/wk for 6 MV • Maximum 250 Gy/wk at high MV for dual energy – Mechalakos (2004) • Maximum 450 Gy/wk for 6 MV single-energy • Maximum 400 Gy/wk for dual energy – NCRP 151 Section 7 examples assume 450 Gy/wk at high MV • 450 Gy/wk is the default weekly workload
NCRP 151 Recommended Workload
[2/2]
• 3 Gy per patient treatment and 30 patients treated per day is default assumption for busy facility – Consistent with 450 Gy/wk with 30 patients treated per day, 450 Gy/wk = 5 treatments/wk/patient x 3 Gy/treatment x 30 patients – Equivalent to 219 cGy treatment fraction (0.73 tissue maximum ratio) • Intentionally conservative (compared to ~200 cGy fraction) since no specific allowance for QA or maintenance workload – Can be based on direct knowledge of accelerator use instead • 450 Gy/wk is consistent with 30 patients & 3 Gy/treatment
Use Factor, U
• Fraction of a primary-beam workload that is directed toward a given primary barrier – Traditionally U = 0.25 for lateral barriers, ceiling, & floor (but see Table 3.1) – U = 0.1 for tapered portions of ceiling barrier – Applies to primary barrier calculations only , usually not secondary – Caution for TBI treatments and facilities that perform a large number of tangential breast treatments
Occupancy Factor, T
• Average fraction of time that the maximally exposed individual is present while the beam is on: not the fraction of time that it is occupied by any persons, but rather it is the fraction of the time it is occupied by the single person who spends the most time there • Occupancy factor of 1/40: the maximally exposed individual would spend an average of 1 h/wk in the area every workweek for a year • Occupancy factor of > 40 h workweek (like in SNUH): ratio of the average time the maximally exposed individual will be present to the total average time that the equipment is used during the week
NCRP 151 Recommended Occupancy
• T = 1: Areas occupied full-time by an individual, e.g., administrative or clerical offices; treatment planning areas, treatment control rooms, nurse stations, receptionist areas, attended waiting rooms, occupied space in nearby building • T = 0.5: Adjacent treatment room, patient examination room adjacent to shielded vault • T = 0.2: Corridors, employee lounges, staff rest rooms • T = 0.125: Treatment vault entrance • T = 0.05: Public toilets, unattended vending rooms, storage areas, outdoor areas with seating, unattended waiting rooms, patient holding areas, attics, closets • T = 0.025: Outdoor areas with only transient pedestrian or vehicular traffic, unattended parking
Protective Barriers
Secondary
Basic Principles
• Increasing the distance between the individual and the sources of the radiation • Limiting the exposure time , and • Interposing protective shielding between the individual & the radiation sources • Administrative controls or additional surveillance
Protected Location
distance in meters
Other Consideration
• Careful planning may result in appreciable savings, particularly in the high-energy range where shielding is very costly • Provision for future requirements may prevent expensive alterations • Periodic inspections during the entire construction period
should
be performed • A corridor can be used to separate offices & support rooms from the tx rooms rather than leaving these rooms adjacent to one another