McGill Residency Session Notes

Geometrical penumbra is less for collimation near the patient's skin (actually on the patient's skin for orthovoltage) than it is for simple collimation using the main collimator

Also, geometrical penumbra is greater for extended SSD treatments, as can be seen from the figure below.

Orthovoltage collimation is done with lead right on the patient's skin for collimation

this has the advantage of reduced penumbra
and the added advantage of being able to use reduced margins

reduced margins are possible because the patient motion is less (ie motion due to breathing and setup error is less as the field is right on the patient's skin and the collimation will move with the patient (William showed example using piece of paper with a hole in it and patient in high-E and ortho)

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Large Patients

Large patients for which part of the patient is outside of the field-of-view of the planning CT image can have their tumor underdosed

Basically, there is extra tissue that is not shown on the CT but which will cause attenuation of the beam.

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Bolus, Compensator, Build-up Bolus, Beam Spoiler

Bolus

P 259 Kahn
G-32 Dosimetry notes
A layer of tissue-equivalent material placed directly on the skin surface to even out irregular contours of a patient in order to present a flat surface to the beam - ie this accounts for missing tissue and is a form of manual isodose line correction
Bolus works well for orthovoltage radiation but for higher-energy beams it results in the loss of skin-sparing => use compensators for high-energy beams
Example of bolus in clinic???

Compensator

P 259 Kahn
G-32 Dosimetry notes
An irregularly shaped layer of material placed in the beam to account for missing tissue (the irregular shape accounts for the shape of the missing tissue)
Design of the compensator must account for

Beam divergence
The difference in the attenuation coefficients between the compensator material and the soft tissue it is replacing - in order to replace exactly the right amount of tissue.

The compensator basically reduces the amount of radiation into the body where there is missing tissue.
The compensator is placed away from the skin in order to maintain skin-sparing.
The compensator should be at least 20 cm away from the skin to avoid electron contamination.

Build-up Bolus

P 259 Kahn
A layer of tissue-equivalent material placed directly on the skin surface to provide adequate dose buildup over the skin surface.

Ie build-up bolus is used to bring dmax closer to the surface - ie to maximize the dose on the surface
This is similar to a beam-spoiler (I think), except that a beam-spoiler is held at an appropriate distance from the skin.

For example, build-up bolus may be used to treat scar tissue with an MV beam by intentionally maximizing the dose on the skin surface (p280 Kahn)

Beam Spoiler

P 281 Kahn
A beam spoiler is used to bring the dmax for a high energy beam close to the surface, while maintaining the greater penetration advantage of the high-energy beam as desired

Essentially, the secondary electrons produced in the beam spoiler provide electron contamination to increase the build-up dose under the skin of the patient

For example for a large breast patient (in which the tumor extends deep into the breast from the surface of the breast), the beam spoiler can be used to bring the dmax close to the surface to ensure that the tumor close to the surface gets enough dose, while simultaneously ensuring that the deeper tumor gets adequate dose
This is similar to build-up bolus (I think), except that build-up bolus is placed directly on the skin

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Wedges

Advantages of dynamic wedges

Physical wedges have a maximum field size limit, dynamic wedges do not
Physics is easier without wedges - no need for calibration of physical wedges, etc
Faster treatments - less MUs and no need to go into room to change wedge
Clearance issues are simplified

Disadvantages of dynamic wedges

Jaws only do dynamic wedges in one direction, so planning more difficult

Are wedges better for superficial or deep-seated tumors?

Better for superficial tumors
Dose at thin part of wedge will be higher than prescription
Dose at thick part of wedge less than prescription

Wedge pairs

Use when have two obliques - eg when treating superficial tumor at forehead
Hinge angle - need to study

(180 - angle between beams)/2

Typical physical wedge factors

0.4 - 0.7

If forget wedge for one fraction

Investigate - see what the problem was exactly
Possible solution is to use a steeper wedge for the remainder of the treatment so that the part of the field that got more dose than it should have gets less the next time round

MLCs

If MLC doesn't work and patient treated with open beam
Patient will get the more dose for the same number of MUs, based on the different between IMRT and open beams

Orthovoltage Therapy

Interesting Facts re Orthovoltage

Used to treat surface lesions
PTV is on the patient's skin, collimation is on the patient's skin

PTV = CTV since margins tiny
Tiny geometrical penumbra and collimation moves with the patient

50% dose is at what depth? About 7 cm

In orthovoltage, electron contamination is an issue

Especially from high-Z materials in the beam
Applicators often made of plastic (low Z material) and may be closed to reduce electron contamination

Compared to electrons,

Orthovoltage is cheaper (x-ray tube versus linac)
Geometrical penumbra is less

Since collimation on the skin

Shielding is more effective

For example, if treating the eye, electrons would produce a higher dose behind the shield than orthovoltage photons would - why? because of bremsstrahlung? -John Kildea 2/2/10 1:29 PM

Orthovoltage therapy is prescribed to the surface, electrons are prescribed to R90 or R80

Orthovoltage Prescriptions (notes R3)

Prescribed to the surface in Roentgen (exposure) or Gray (dose)
Can be prescribed in 3 ways and it is important to know exactly which way the physician is using:

Exposure in air
Exposure with full backscatter
Dose in tissue (at surface)

Orthovoltage X-ray Machines

Electrostatic x-ray production (as opposed to cyclic, which is a linac or a cyclotron)

Electron Therapy

Advantages

Good for concurrent boosts
No need for additional infrastructure

Disadvantages

Difficult to achieve exact desired dose distribution
Bremsstrahlung contamination
Need for applicators on treatment machine - risk of collision

Definitions
QA

Cerebrospinal Irradiation (CSI)

This is irradiation of the brain and the spinal cord

About 12 cases per year at the MGH
Of which maybe one is an adult

It requires field matching

In all cases the lateral brain fields need to be matched with the spinal cord field
For adults, more than one spinal cord field will likely be required and must also be matched

The patient can be either prone or supine

Prone was the original position used in the 70s, since it was easy to feel the spinal cord
Supine is the preferred position today, since it is easier to setup and is more comfortable for the patient

Tumors treated with CSI include

Meduloblastomas and apendenomas in which the cells metastasize by travelling in the cerebrospinal fluid (CSF)

Today at the MGH, most of these cases are treated using Tomo

Trade off is a dose bath

Conventional treatment includes the following:

Lateral fields for the brain, face blocked off with MLC
Posterior field(s) for the spinal cord, with the patient in the prone position

Don't use laterals for the cord, since the arms and the body get in the way

Problem is the junctions - hot and cold spots (hot beyond junction, cold before junction due to diverging beams)

Solve junction issue using:

Half block technique (one jaw in at field center, other jaw out to the edge of the field) => gives sharp dose drop off at the center, that can be matched without hot and cold spots
Extended SSD (means that a single beam can cover the whole spinal cord)
Feather the junction(s) - move the junction around so that the affect of any mismatching are lessened. Typically, change the junction once per week

For the cerebrospinal junction (CSJ),

Rotate the collimator for the lateral fields to match the edge of the diverging posterior field
kick (ie move slightly) the couch

Field Matching - General Notes

Field-matching at the skin is the easiest

Can use the light fields
However, the result will be overdosing at depth due to beam divergence

Clinically, most field matching is done at depth

Diverging edges must match at the required depth
The 50% isodose levels should add up at the required depth

Simple field matching can be done using a half-block technique and rotating the gantry

Example is McGill breast technique
A block is used to shield one side of the field from the isocenter to the edge

This produces a non-diverging field edge

The first field is irradiated
The gantry is rotated and the first field is blocked while the second field is irradiated

The second field has a non-diverging field edge that matches the non-diverging edge of the first field

Can also kick the couch to obtain to account for beam divergence without having to use a half-block

See examples from junction lab residency session

Image below shows how to calculate the surface gap between two fields based on them matching at depth z

Pelvis Targets

Generally treated with a four-field box

AP/PA and two laterals
The laterals could be higher energy to account for the lateral width of the patient being greater than the anterior-posterior distance

Could also treat with a 3-field arrangement with wedges if want to avoid a structure in the posterior or anterior beam - eg want to avoid the colon

High and Low Energy Beams

When considering energy need to consider the following points

Depth of tumor
Depth of zmax/build-up region
Location and size of tumor (eg a small tumor deep into the lung will be within the build-up region for the air-tissue interface within the lung)
Leakage and neutron dose (high when put a lot of material in the beam, such as in IMRT)

Low energy (6 MV) used for

IMRT
Shallow tumors
Small tumors in the lung (since would be in electronic disequilibrium region for high-energy beams, particularly if beams are not parallel opposed where the exit dose might compensate)

High energy (18 MV) used for

Deep tumors
Patients with large separation

Typical PDDs

Simple numbers to remember

Dmax

6 MV: 1.5 cm
18 MV 3.5 cm

PDD 10 cm

6 MV: 0.666
18 MV: 0.8

Surface dose:

Cobalt: 30%
6 MV: 15%
18 MV 10%

PDD at arbitary depth

Draw point for dmax location and PDD at 10 cm and interpolate between them

Miritza's rules of thumb

5 cm ~80% + E
10 cm ~60% + E
20 cm ~35% + E
30 cm ~15% + E

Radiation Physics

HVL and TVL of Co-60 in Lead

Co-60 => 1.25 MeV => = 5.88 x 10-2
=>
=>
=> - would take a lot to make a lead jacket thick enough to wear in a Co-60 beam.
Of course TVL = 3.32HVL (since and => )

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Range of Electrons

Rules of thumb for photons->electrons->range in H20:

Mean energy of an MV photon spectrum = 1/3rd the MV energy
Examine mean energy position in curtains plot to see what interaction is dominant
Use aveage energy transfer fraction for the dominant process to determine the mean photon energy transferred to the initial kinetic energy of the secondary electrons (p254 Dr. Podgorsak's book for summmary of processes, p246 for curtins plot and p199 for the Compton graph 0.02 0.14 0.44 0.68 0.80)

10-2 10-1 100 101 102

Take 1/2 of the initial energy of the secondary electrons to be their mean kinetic energy (see "Slowing Down Electron Spectra" notes from Radiation Physics class - the square plot)
Use the rule of thumb that the range of electrons in H2O in cm = 1/2 the kinetic energy of the electrons in MeV
Example:

6 MV beam
=> 3 MeV mean photon energy
=> Compton dominant
=> Compton fraction about 0.5 or so
=> 0.5 x (3 MeV) = 1.5 MeV is mean initial electron kinetic energy
=> (1.5 MeV)/2 = 0.75 MeV mean kinetic energy of electron slowing down spectrum
=> 0.75/2 = 0.375 cm is range of electrons in water for 6 MV beam
Actual value from NIST is more like 0.3 cm2/g = 0.3 cm => reasonably close!

Going from H20 to air

Density H2O = 1g/cm3
Denisty air = 1.293 x 10-3 g/cm3 at STP
So, air is 1000 times less dense than water
So, electrons should go about 1000 times further in air than water
So, 0.3 cm in H2O should give 0.3 x 1000 cm in air = 300 cm = 3 m (for the example of 0.75 MeV electrons above)
Actual value from NIST is 3.136E-01 cm2/g => (3.136 x 10-1) / (1.293 x 10-3) cm = 242 cm = 2.42 m (bit less than estimate but not bad)

Range of 6 MeV electrons 3 cm in H2O => 3 x 1000 cm in air = 3000 cm in air = 30 m by estimate

By NIST it is 3.25 g/cm2 = (3.25)/ (1.293 x 10-3) cm = 2513 cm in air = 25.13 m (little less than estimate but not bad)

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Orthovoltage - Dose to Bone

The collision air Kerma in air is related to exposure in air by

Dose to small mass of medium in air is such that :

. For X-ray below 350 kVp, = 1. So where .
The f-factor or Roentgen-to-tad factor converts Exposure to Dose for beam under 3 MeV.

Therefore for low energy beam like in Orthovoltage therapy treatment, absorbed dose to the bone can be up to about 4 times the absorbed dose to tissue.

General Physics and Math

The Inverse-Square Law

Valid:

for point sources only
for primary beam only
really applied to areas on a sphere, not on planes

If going from shorter distance f1 to longer distance f2, expect dose will decrease from larger dose dp1 to lesser dose dp2

=> divide by larger number

Say have orthovoltage beam with 50 mm SSD into curved eye socket for which outer part of socket is closer to beam by 5 mm than inner part - what is the % larger dose to the outer part of the socket and how can a better dose distribution be achieved? Say 5 Gy to central (lower) part of socket

closer by 5 mm => more dose => divide by smaller number
=> % increase is

How can a better dose distribution be achieved?

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The Taylor Expansion & The Inverse-Square Law

Taylor expansion & inverse-square law used together to estimate dose differences at different SSD:

Eg 1: Patient needs to be treated at SSD = 100 cm, but mistreated at SSD = 105 cm. Quick estimation of dose difference:

using inverse-square law (further from source => dose reduced => divide by larger number)

So more or less 10% decrease in dose would be observed.

Eg 2: Patient to be treated at SSD = 100 cm but accidentally at SSD = 98 cm

Using inverse-square law (closer to source => dose increased => divide by the smaller number)

This explains why in Orthovoltage the use of cones with larger SSD (50 cm) are preferred over cones with smaller SSD (20 cm). A small variation in treatment SSD for cone with small SSD has a higher impact on dose delivery as compared to the same variation in treatment SSD for cones with larger SSD.

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Quick Dose Estimates

Units

Exposure, KERMA and Dose

Exposure: The charge of either sign collected for a given mass of air

Unit: Roentgen
Notes: Valid only for air and for photons with energy less than 3 MeV (measurement is impractical above 3 MeV)

KERMA:
Absorbed Dose:
Equivalent Dose:
Effective Dose:
Collective Effective Dose

Relationship between Dose and Exposure

So, for 1 R: - ie 1 R is approximately equivalent to 1 cGy (or 1 R 1 cSv)

Administrative

Ordering a New Machine or Something New for the Department

Things to think about:

Identify the needs for the machine - why is it interesting/useful/needed in the department?
What (if any) other machines in the department can do the same job?
Where will it go?
What shielding/resources are required for it?
Will a license be needed?
How much will it be used?
What sort of maintenance is needed?
Is there enough staff?
What sort of measuring/calibration equipment will be required?
Will construction be required?

How much do things cost?

An ion chamber: ~5 000 CAD
Ion chamber calibration at standard's lab: ~2 000 CAD
Full equipped Linac ~3M CAD
Scanning Water Tank ~100K CAD

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Quality Assurance

Ion Chambers and Beam Calibration Notes

At low energy a graphite chamber with an aluminum central electrode are used. This ensures a flat energy response

At low energies the photons get attenuated in the graphite wall but their loss is compensated for by increased photoelectric electrons from the aluminum central electrode
In fact chambers can be "tuned" in all sorts of ways with all sorts of materials in order to ensure a flat energy response

At high-energies the secondary electrons have a long range

=> they are produced in the phantom and penetrate into the cavity where they ionize the air
So, at high energies the secondary electrons ionize the air in the ion chamber
And it is assumed that the photons do not ionize the air

At low energies the secondary electrons do not have a range long enough to penetrate the chamber wall

So, at low energies the ionization of the air is as a result of the photons themselves

Narrow beam geometry is preferred (even though real work is with broad beams) because all calibrations in the standard's lab and elsewhere are done using narrow beams

It is easier to replicate narrow beams than it is to replicate broad beams => narrow beams are the standard

Air Kerma is only defined for build up

What does this statement mean?
has to do with energy transferred, right?

Parallel plate chambers versus cylindrical chambers

PP use in the build-up region
But need to worry about Compton current - check dosimetry notes (R2 and R6) -John Kildea 1/4/10 10:37 PM

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TG-51 (Clinical Reference Dosimetry)

General Notes

Applies to:

Photons: 60Co to 50 MV
Electrons: 4 MeV to 50 MeV

TG-51 is used to determine absorbed dose to water at the point of measurement of the ion chamber placed under reference conditions

Where is the absorbed dose to water at the point of measurement, under reference conditions
is the fully corrected ion chamber reading
is the quality conversion factor to convert the calibration factor from to the beam quality Q
and is the calibration factor for a beam

Essentially, when carrying out a TG-51 calibration, everything is provided, except for Q and , which must be measured

Q is determined using

for photons
and for electrons

M is the raw ion chamber reading corrected for ion recombination, temperature and pressure, electrometer (if electrometer and chamber calibrated separately as is the case in the US) and polarity

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Health Physics

Probability of Developing/Dying from Cancer

40% of Canadian women and 45% of men will develop cancer during their lifetime.
Approximately 1 out of every 4 Canadians will die from cancer.
Three types of cancer account for the majority of new cases in each sex:

prostate, lung and colorectal in males and
breast, lung and colorectal in females.

Lung cancer remains the leading cause of cancer death for both men and women.
Colorectal cancer is the second leading cause of death from cancer.

Risk of developing cancer & heritable effects as a function of dose (ICRP 103)

Table1 – Detriment-adjusted nominal risk coefficients (10-2 Sv-1) for stochastic effects after exposure to radiation at low dose rate.

The practical system of radiological protection recommended by the Commission is based upon the assumption that, at doses below about 100 mSv, a given increment in dose will produce a directly proportionate increment in the probability of incurring cancer or heritable eﬀects attributable to radiation

The Linear-Non-Threshold (LNT) model

Conclusions on the in-utero risks of tissue injury and malformation at doses below about 100 mGy of low-LET radiation (ICRP Publication 90 - Biological eﬀects after prenatal irradiation (embryo and fetus)):

Embryonic susceptibility to the lethal eﬀects of irradiation in the pre-implantation period of embryonic developments will be very infrequent at doses under 100 mGy.
Risks of malformation after in-utero exposure to doses well below 100 mGy are not expected.

a Limits on effective dose are for the sum of the relevant effective doses from external exposure in the specified time period and the committed effective dose from intakes of radionuclides in the same period.

b Averaged over 1cm3 area of skin regardless of the area exposed.

c With the further provision that the effective dose should not exceed 50 mSv in any single year. Additional restrictions apply for the occupational exposure of pregnant women.

d In special circumstances, a higher value of effective dose could be allowed in a single year, provided that the average over 5 years does not exceed 1 mSv per year.

NCRP Report no. 160: Ionizing radiation exposure of the population of the US

All exposure categories – collective effective dose [%] - 2006

Effective radiation doses from medical examinations

o Radiography: chest 0.1 mSv ; upper GI tract 2 mSv ; lower GI tract 4 mSv

o Mammography: 0.7 mSv.

o CT: head 2 mSv ; chest 8 mSv ; abdomen 10 mSv

Effective radiation doses from image guidance in RT

o Portal vision: 4 – 10 cGy per image pair

o Cone beam CT: pelvis (125 kVp) à 3 – 5 cGy to the skin

head (100 kVp) à 0.5 – 4 cGy to the skin

General Radiation Protection

CNSC Dose Limits

The CNSC dose limits are:

NEW: 50 mSv/yr
NEW: 100 mSv/(5 yr) - so effectively 20 mSv/yr over 5 years
Pregnant NEW: 1 mSv for remainder of pregnancy
Public: 1 mSv/yr

An effective TADR of 20 mSv/hr is in place for instantaneous dose rate

Not sure what the source of this is but it seems to be used in practise...

Peripheral Dose

Could be dose to film left in the room during treatment or dose to a fetus
Comprises

Leakage
Scatter

Collimator scatter
Patient scatter

Neutron dose at high energies
Dose from photo-activated radionuclides at high energies

Important to know the leakage and collimator scatter components

Since these can be reduced by placing shielding over the region of interest
Dose due to scatter within the patient/phantom will still reach the region of interest by travelling within the patient/phantom

TG 36 deals with fetal dose in RT
Experiment at MGH

3 TLDs left on the couch of the Novalis linac during irradiation -John Kildea 4/19/10 12:35 PM

results will come with TLD readings from Health Canada

Physical Data

Linac Shielding Data

Density of concrete: 2.2 g.cm-3
Density of Lead: 11.35 g.cm-3
Density of steel: 7.8 g.cm-3
TVL of concrete for 6MV: 35 cm
TVL of concrete for 18MV: 45 cm
TVL of lead for Co-60: 46mm
TVL of lead for 6MV: 57mm
TVL of lead for 18MV: 57mm
Z(Muscle) = 7.42
Z(Water) = 7.42
Z(Air) = 7.64
Z(Bone) = 13.8

Treatment Site Details

Spine

Prostate

Two targets (and two plans)

Prostate
Nodes

Lung

Use AP-PA
6 MV or 18 MV

Some clinics use one, some use the other
Use 18 MV as the lesion is deep

so less dose outside the lung for more inside (see plot of opposing beams below)
build-up in tumor not a concern since there are two beams, so although build-up deeper for 18 MV, the tumor will get exit dose from the opposing beam

Use 6 MV since low density in lung

so build-up in the tumor
but more dose outside the lungs

Breast

The 2 cm region in the air is known as the "flash"
It's purpose is to account for patient motion/anatomical changes and penumbra
Breast patients are setup lying on incline rather than flat so that breasts don't fall to side

Having the breast to the sides would result in more dose to the lungs, which is avoided by having the patient on an incline

Head and neck

Esophagus

Dose to treat esophagus

Worry about

lungs, and
spinal cord (45 Gy)

Spinal cord has dose constraint of 45 Gy

Heterogeneity Corrections

There is a TG report that deals with hetrogeneity corrections
Hetrogeneity corrections are important for lungs
If turn on hetrogeneity corrections then patients are from then on treated differently than before

A big issue on whether to turn on or not turn on hetrogeneity corrections

Information Technology

DICOM

Digital Imaging and COmmunications in Medicine

DICOM-RT

Includes structures, beam, blocks, mlcs, etc

Radiosurgery

Small-field dosimetry is difficult but not such a big deal in radiosurgery
All the work is in localisation, less worried about dosimetry
Want to blast the tumor with dose, if dosimetry off by 10% due to dosimetry not such a big deal

Literature

Good-To-Know Reports:

ICRU 71 Prescribing, Recording, and Reporting for Electron Beams
ICRU 50 & ICRU 62 Prescribing, Recording, and Reporting for Photon Beam
ICRU 38 Intracavitary Therapy in Gynecology
TG40 Comprehensive QA for Radiation Oncology
TG45 AAPM Code of Practice for Radiotherapy Accelerators
TG 65 Tissue Inhomogeneity Corrections for Megavoltage Photon Beams
TG 66 Quality assurance for computed-tomography simulators and the computed-tomography-simulation process
TG 106 Accelerator beam data commissioning equipment and procedures
TG 53 Quality Assurance for Clinical Radiotherapy Treatment Planning
TRS 430 Commissioning and Quality Assurance of Computerized Planning Systems for Radiation Treatment of Cancer
TG 42 Stereotactic Radiosurgery
TG 29 The Physical Aspects of Total and a Half Body Photon Irradiation
TG 30 Total Skin Electron Therapy: Technique and Dosimetry
TG 63 Dosimetric considerations for patients with HIP prostheses undergoing pelvic irradiation.
TG 65 Tissue Inhomogeneity Corrections for Megavoltage Photon Beams
TG 76 The Management of Respiratory Motion in Radiation Oncology
AAPM Guidance document on delivery, treatment planning, and clinical implementation of IMRT: Report of the IMRT subcommittee of the AAPM radiation therapy committee.
TG51 Protocol for Clinical Dosimetry of High-Energy Photon and Electron Beams
TG61 AAPM protocol for 40–300 kV x-ray beam dosimetry in radiotherapy and radiobiology.
TG43 Dosimetry of Interstitial Brachytherapy Sources
TRS398 Absorbed Dose Determination in External Beam Radiotherapy: An International Code of Practice for Dosimetry based on Standards of Absorbed Dose to Water
TG36 Fetal Dose from Radiotherapy with Photon Beams
ICRU 103 The 2007 Recommendations of the International Commission on Radiological Protection
NCRP160 Ionizing Radiation Exposure of the Population of the United States
NCRP49 Structural Shielding Design and Evaluation for Medical Use of X Rays and Gamma Rays of Energies Up to 10 MeV
NCRP147 Structural Shielding Design for Medical X-Ray Imaging Facilities
NCRP151 Structural Shielding Design and Evaluation for Megavoltage X- and Gamma-Ray Radiotherapy Facilities