Lec 2 of Rad. Prot. March 13, 2014

Introduction to Radiation Units & Biological Effects

• 6 MeV Photons• in 50 cm-thick

water

• D (Gy = 1J/kg) = dE/dM

• dE = amount of energy loss only via • ionizations & excitations of atoms • in mass dM by 2nd e±

dV

•Absorbed Dose of Photon (D) Gray

Dose, Effective Dose, etc.• Absorbed energy, ε : Joule (J)• Absorbed dose, D : Gray (Gy), rad

D = ε/m (Gy) absorbed energy per mass unit, m

1 Gy = 1 J/kg, 1 mGy = 1/1000 Gy• Equivalent dose, H : Sievert (Sv), rem

HT = S wR·DT,R biologically effective-

ness (α,β,γ)• Effective dose, E : Sievert (Sv), rem

HE = S wT·HT weighted according to stochastic risk

Radiation weighting factors

wR = 2.5 + 18.2 exp[-(ln En)2/6] ………. for En< 1 MeV

= 5.0 + 17.0 exp[-(ln 2 En)2/6] …… for 1 MeV < En < 50 MeV

= 2.5 + 3.25 exp[-(ln 0.04 En) 2/6] … for En > 50 MeV

A: ICRP 60 histogram

B: ICRP 60 function

C: ICRP 103

Neutron weighting factors

1990 2007 Gonads 0.20 Breast 0.12Bone marrow 0.12 Bone marrowColon ColonLung LungStomach Stomach Remainder Breast 0.05 Gonads 0.08Bladder Bladder 0.04Liver LiverOesophagus OesophagusThyroid ThyroidSkin 0.01 Skin 0.01Bone surface Bone surface

BrainSalivary glands

Remainder 0.05

Tissue weighting factors

1990 2007 Remainder (10): 0.05 Remainder (14): 0.12Adrenals Adrenals Brain -Upper large intestine -Small intestine Small intestineKidney KidneyMuscle MusclePancreas PancreasSpleen SpleenThymus ThymusUterus Uterus/Cervix Oral mucosa Extrathoracic region

Gall bladder Heart

Lymphatic nodes Prostate

Tissue weighting factors

Absorbed Dose vs. Exposure

• Dm (rad(1)) = fm (rad/R)·X(R)

fair = 0.876, ftissue = 0.94, flucite = 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)

• Dair [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]

Dair [rad] = 0.876 (rad/R) × X [R]

Annual Effective Dose Limits

Occupational Public

Annually permissible risk factor 8x10-4/yr 5x10-5/yr

Radiation risk factor 4x10-2/Sv 5x10-2/Sv

Annual effective dose limit 20 mSv/yr 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

e

e- H2O cell gene

Photon, E = hv

Scatteredphoton E' = hv'

Secondary electron

< nsec

< μsec

< msec

hours yearsH2O+

OHH

e-aq, etc.

Free Radicals

e-Secondary electron

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 2nd 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 ex-ceeds a threshold. Increas-ing 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 in-creases with dose

– Examples include cancer and hereditary defects

Severity

Dose

Threshold

Dose

Risk

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 informa-

tion available on stochastic effects is from relatively high dose data (e.g., atomic bomb sur-vivors)

– High dose results are then extrapo-lated linearly down to zero dose

Radiation Dose

Risk

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 sug-gest 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-re-sponse, a few can help you but too many at once can make you sick or even kill you

Dose or # of Aspirin

Risk

Benefit

Linear extrapolation from high dose

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*) ad-vocate the use of LNT until such time as there is defini-tive 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 de-

terministic 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

National Legislations

IAEABasic Safety Standards

ICRP 2007 Recommenda-tions

UNSCEAR, BEIR etc

Production of Radiation Types in LINAC

Target

1H(n,g)2H ,27Al(n,g)28Al →28Si + b- +g

e.g., 54Fe(hn,n)53mFe* →53Fe + g

Sources of photo-neutrons*: either (1) Linac head or (2) Rm shielding

Neutron capture gammas:

*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 be-low the regulatory limits, any special regulatory requirements, and within ALARA limits estab-lished at the facility

Shielding in Radiotherapy Facilities

• Equipment to be included in radiation on-cology 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, Ka in low LET environ-ments (no neutron)– 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 coefficientsfor 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 radia-

tion is used, such as treatment rooms and control booths, or other areas that require control of access, occupancy, and working conditions for radiation pro-tection 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 moni-tored

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 pub-lic, 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 ex-posed 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 di-rectly 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 ac-cess 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 shield-ing requirement (at least 250 Gy/wk at high MV mode)

• IMRT: workload efficiency factor

NCRP 151 Recommended Work-load [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 Work-load [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 di-rected 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 in-dividual 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 indi-vidual would spend an average of 1 h/wk in the area ev-ery workweek for a year

• Occupancy factor of > 40 h workweek (like in SNUH): ra-tio of the average time the maximally exposed individual will be present to the total average time that the equip-ment is used during the week

NCRP 151 Recommended Occu-pancy

• T = 1: Areas occupied full-time by an individual, e.g., administrative or clerical offices; treatment planning ar-eas, 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, stor-

age areas, outdoor areas with seating, unattended wait-ing rooms, patient holding areas, attics, closets

• T = 0.025: Outdoor areas with only transient pedestrian or vehicular traffic, unattended parking

inte

rior

exte

rior

Protective Barriers

Sec-ondary

Basic Principles

• Increasing the distance between the individual and the sources of the radiation

• Limiting the exposure time, and

• Interposing protective shielding between the in-dividual & the radiation sources

• Administrative controls or additional surveillance

Protected Locationdistance in meters

Other Consideration

• Careful planning may result in appreciable savings, par-ticularly 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