Vault Shielding NCRP 151
Terms
Controlled Area: Limited access areas where the occupational exposure of personnel to radiation is under supervision of a radiation protection program. These include treatment rooms, control areas and other working areas for radiation workers where nonmonitored persons are not able to enter.
High Energy Accelerator: Any accelerator delivering a maximum photon energy greater than 10MV.
Low Energy Accelerator: Any accelerator delivering a maximum photon energy of 10MV or less.
Primary Barrier: A wall, ceiling, floor, or other structure that will directly intercept the primary radiation beam.
Secondary Barrier: A wall, ceiling, floor, or other structure that will not intercept the primary beam but will receive radiation scattered by interactions within the patient or other object including accelerator leakage radiation.
Uncontrolled Area: All areas not considered controlled areas are considered uncontrolled areas.
Conservative Assumptions
NCRP151 makes several conservative assumptions designed to produce safe vault designs at reasonable cost. The following are examples of conservative assumptions:
 Neglects patient attenuation (30%)
 Assumes beam takes the shortest path through the barrier. (i.e. That the beam is incident normal to the plane of the barrier)
 Head leakage is assumed to be the maximum allowed by IEC (0.1%)
 Occupancy factors are conservatively high
 Unusual procedures are given a safety multiplication factor (e.g. assume 1.5 times dose of measurement for TBI)
 Twosourcerule
Typical Shielding Thicknesses
Room Type  Typical Shielding 

High Energy Linear Accelerator Primary Barrier 

High Energy Linear Accelerator Secondary Barrier 

High Energy Linear Accelerator Door 

Ir192 HDR Suite 

PET/CT Room 

CT Room 

Radiographic Suite 

Workload (W)
\begin{equation} W = \overline{\frac{\textrm{number of treatments}}{week}} \times \overline{\frac{\textrm{dose (Gy)}}{treatment}} \end{equation}
Definition: Workload is the time integral of the absorbeddose rate, determined at depth of maximum absorbed dose, 1m from the source.
Units: W is typically specified over one week making the units Gy/week.
Determining Workload: A workload should be determined for each accelerator energy. The best method is to find workload data from the clinic in question or from nearby clinics with similar patient populations. If no real life data is available, NCRP suggestions may be used.
Additionally:
 NCRP 49 suggests 1,000Gy/week for low energy accelerators.
 NCRP 51 suggests 500Gy/week for high energy accelerators.
Workload and special procedures
IMRT/SRS/SRT
IMRT, SRS and SBRT deliveries often use many small field sizes to achieve a highly conformal dose distribution. This means that more monitor units (MU) will be required per unit of prescription dose. This can significantly impact the head leakage calculations. Therefore, a leakage workload (W_{L}) is used.
\begin{equation} W_{L} = W_{conv} + W_{IMRT} \end{equation}
Here W_{Conv} is the workload as defined above only taking conventional treatments into account.
W_{IMRT} takes into account the increased MU per Gy for nonconventional treatments through a factor C_{I}.
\begin{equation} W_{IMRT} = W \cdot C_{I} \end{equation}
\begin{equation} C_{I} = \frac{
\frac{MU_{IMRT}}{Gy}}{
\frac{MU_{Conv}}{Gy}} \end{equation}
TBI/Special Procedures
Because workload is defined at isocenter, treatments performed at extended SSD (e.g. TBI) must be accounted for using their dose at isocenter rather than prescription dose.
\begin{equation} \overline{(\frac{\textrm{dose (Gy)}}{patient})}_{\textrm{extended SSD}} = D_{Rx} \times (SSD + d_{Rx})^2
\end{equation}
Quality Assurance
Quality assurance deliveries (i.e. machine and patient specific QA) must also be included in workload. If many patient specific QA deliveries are IMRT, the C_{I} factor must also be used.
Key point: Because workload is defined at isocenter, treatments performed at extended SSD (e.g. TBI) must be accounted for using their dose at isocenter rather than prescription dose. i.e. \( \overline{(\frac{\textrm{dose (Gy)}}{patient})}_{\textrm{extended SSD}} = D_{Rx} \times (SSD + d_{Rx})^2 \).
Use Factor(U)
Definition: Use factor is the fraction of the workload at which the treatment beam is directed at a given primary barrier.
Determining Use Factor: NCRP provides the following table with expected use factors for a high energy linear accelerator.
Key point: Accelerators with a high fraction of special procedures may vary sharply from standard use factors. For example, TBI may be performed only at a single gantry angle and will influence use factors for the impacted wall.
Angle (90 degree interval)  U(%) 

0 degrees (down)  31.0 
90 and 270 degrees  21.3 
180 degrees (up)  26.3 
Occupancy Factor (T)
Definition: Occupancy factor is the average fraction of time that the maximally exposed individual is present in a given location while the beam is on.
Determining Occupancy Factor: Standard occupancy factors are provided in the table at right. Note that these are created assuming a 40 hour equipment use week.
Key point: If the beam on time is greater than 40 hours, the occupancy factor is determined by the ratio of the average time the maximally exposed individual in an area will be present to the total average time the equipment is used. (e.g. A person present 40 hours/week near equipment that is operated 60 hours/week would have a use factor of (40/60)=0.67.)
Location  Occupancy Factor (T) 

Full occupancy areas
 1 
 1/2 
 1/5 
 1/8 
 1/20 
 1/40 
Shielding Design Goals (P)
Definition: Maximum acceptable levels of Dose Equivalent for a given location.
Units: mSv/year, mSv/week
Time Average Dose Rate (TADR)
Because measurements of transmission are typically taken as instantaneous dose rate, they cannot be directly used to determine the shielding adequacy. To resolve this problem, instantaneous dose rate (IDR) measurements are averaged over a week (R_{w}) or an hour (R_{h}).
\begin{equation} R_w = \frac{(IDR)WU}{\dot{D}} \end{equation}
\( \dot{D} \) is the absorbeddose output rate at 1m (Gy/hr)
\begin{equation} R_h = \frac{N_{max} R_w}{\bar{N}_w} \end{equation}
N_{max} is the maximum number of patients per hour.
\( \bar{N}_w \) is the average number of patients per week.
Area  Dose Equivalent 

Controlled Areas  P < 5mSv/year P< 0.1mSv/week 
Uncontrolled Areas  P < 1mSv/year P < 0.02mSv/week R_{h} < 0.02mSv/hour (NRC requirement) 
Transmission Factor (B)
Definition: Transmission factor is the maximum allowable transmission which will allow the barrier to achieve its shielding design goals (P).
Determining Transmission Factors: Transmission factors depend not only upon shielding design goals but also on the intensity, type and energy spectrum of radiation incident upon the barrier.
Photon and Electron Calculations
Primary Barriers (B_{pri})
\begin{equation} \label{eq: Primary Barrier} B_{pri} = \frac{Pd^2_{pri}}{WUT} \end{equation}
Because primary barriers experience fluences significantly higher than the expected fluence from patient scatter or head leakage, these factors are ignored for primary barriers.
Key point: The minimum distance beyond the barrier is taken to be 0.3m as it is not expected that persons will stand directly against the wall.
Secondary Barriers
Secondary barriers must shield both patient scatter photons and head leakage photons. Because the intensity and spectrum of each of these components will vary significantly with treatment type, they are handled separately.
Patient Scatter (B_{ps})
\begin{equation} \label{eq:patient scatter} B_{ps} = \frac{P d^2_{sca} d^2_{sec}}{aWT}\frac{400}{F} \end{equation}
a is fraction of the primary beam absorbed dose that scatters from the patient at a particular angle. This can vary by two orders of magnitude depending on the angle of scatter.
Scatter Fraction (a). Partially reproduced from NCRP 151 Table B.4Angle (degrees)  6MV  10MV  18MV  24MV 

10  1.04x10^{2}  1.66x10^{2}  1.42x10^{2}  1.78x10^{2} 
45  1.39x10^{3}  1.35x10^{3}  8.64x10^{3}  8.30x10^{3} 
90  4.26x10^{4}  3.81x10^{4}  1.89x10^{4}  1.74x10^{4} 
135  3.00x10^{4}  3.02x10^{4}  1.24x10^{4}  1.20x10^{4} 
Head Leakage (B_{L})
\begin{equation} \label{eq:leakage transmission} B_L = \frac{Pd^2_L}{10^{(3)}W_{L}T} \end{equation}
\( W_{L} = W_{conv} + W_{IMRT} \)
10^{3} is taken from the 0.1% maximum allowable head leakage for a clinical radiation therapy machine.
Determining Minimum Barrier Thickness
\begin{equation} \textrm{Number of TVLs (n): } n = log(B) \end{equation}
Primary Barriers
The required number of TVLs (n) is found using B_{Pri} in the above equation.
Secondary Barriers
The required thickness of the secondary barrier is determined by the twosource rule. To apply the two source rule, n should be calculated using the above equation for B_{ps} and B_{L}.
TwoSource Rule treats the patient scatter and leakage components of secondary radiation as distinct sources. If the patient scatter and leakage transmission factors are approximately equal, shielding thickness may be taken as the larger of the two barrier thicknesses plus 1 HVL. If the thickness of each source differs by 1 TVL or more, the larger barrier thickness may be used. This may also be applied to different beam energies.
Thickness of barrier can be found from TVLs as in the below equation where TVL_{1} is the first tenthvaluelayer and TVL_{e} is the equilibrium tenthvaluelayer.
Note: TVL_{1} is not equal to TVL_{e} because of spectral changes in the radiation as a function of depth.
\begin{equation} \tag{barrier thickness (t)} t_{barrier} = TVL_1 +(n1)TVL_e \end{equation}
Total transmission of a barrier (B_{tot})
Barriers greater than 1 TVL
\begin{equation} B_{tot} = 10^{(1+[\frac{(tTVL_1)}{TVL_e}])} \end{equation}
Laminated (multimaterial) Barriers
Total transmission of laminated barriers can be calculated as the product of the total transmission of each component.
e.g. A barrier made of concrete, lead, and steel would have a total transmission of:
\begin{equation} B_{tot} = B_{con}B_{Pb}B_{steel} \end{equation}
Note that the above equation does not account for neutrons or neutron capture gamma rays.
Neutron and Neutron Capture Photon Calculations
For high energy linear accelerators, photoneutron production in the treatment head, fixation equipment, and primary barriers must be considered.
Key point: Neutron and neutron capture gamma equivalent dose may be considered safe for concrete barriers meeting their photon shielding goals. This is because of the high hydrogen content of concrete.
Laminated Primary Barriers
Laminated primary barriers typically include a layer of steel or lead encased within concrete to save space.
Total dose equivalent transmitted through a primary barrier is the sum of the neutron and photon dose equivalents.
\begin{equation} H_{Tot} = H_{n} + H_{phtn} \end{equation}
Neutron Dose Equivalent
The following empirical formula is used to compute neutron dose equivalent for linear accelerators.
\begin{equation} H_n = \frac{D_0 R F_{max}}{\frac{t_m}{2}+ t_2 + 0.3}[10^{\frac{t_1}{TLV_x}}][10^{\frac{t_2}{TVL_n}}] \end{equation}
H_{n} = neutron dose equivalent per week (μSv/week)
D_{0} = Xray absorbed dose per week at isocenter (cGy/week)
R = neutron production coefficient ( in neutron μSv per Xray cGy per beam area in m^{2}) (i.e. \(\frac{μSv}{cGy m^2}\))
F_{max} = maximum field area at isocenter (\(m^2\))
t_{m} = metal slab thickness (m)
t_{1} = first concrete slab thickness (m)
t_{2} = second concrete slab thickness (m)
TVL_{x} = tenthvalue layer in concrete for Xray beam (m)
TVL_{n} = tenthvalue layer in concrete for neutrons (m)
0.3 = distance from outer surface of the barrier to point of occupancy as defined in NCRP 151 (m)
Neutron Capture Gamma Dose Equivalent
For 15 and 18MV photon beams, it has been shown that the following equation gives a conservatively safe estimate of total photon dose equivalent (primary photon plus neutron capture gammas).
\begin{equation} H_{phtn} = 2.7H_{tr} \end{equation}
H_{tr} = Xray dose equivalent. If B_{pri} is known H_{tr} may be a calculated using \( B_{pri} = \frac{H_{tr}d^2_{pri}}{WUT} \).
Neutron Capture Gamma Energy
BPE = 0.48MeV
Hydrogen (concrete) = 2.2MeV
Structural Considerations
Primary Barrier Width
The primary barrier should be extended at least 30cm beyond the maximum field size on either side.
Key Point: Maximum field size will be the diagonal of the maximum collimator setting. The maximum field size of a 40 x 40 cm^{2} field is about 50cm at isocenter.
If the barrier protrudes into the room, the maximum field size should be taken at the plane of the inner portion of the secondary barrier. If the barrier extends out of the room, the barrier is calculated at the outer part of the primary barrier.
Door Design
High energy vault doors must be able to shield for high energy Xrays as well as neutrons and neutron capture gamma rays. Because of weight and volume concerns, doors usually use a laminated construction method.
Typical door construction consists of three layers:
 Inner layer of high z material (typically lead). In addition to attenuating the incident photons, this layer is also able to reduce the energy of fast neutrons making the BPE layer more effective.
 A middle layer of Borated Polyethylene (BPE) attenuates the thermal neutron flux. This layer, however, will produce neutron capture gamma rays.
 BPE Neutron TVL assumed to be 4.5cm
 BPE Neutron Capture Gamma Energy = 0.48MeV
 The attenuation crosssection of Boron is approximately 10,000 times that of hydrogen!
 The outer layer of high z material (typically lead) attenuated the neutron capture gamma rays produced in BPE.
Key Point: Hydrogen is a superior neutron lead or tungsten because it has approximately the same mass as a neutron. Therefore, conservation of energy and momentum allows the hydrogen atom to absorb a maximum of the neutron’s energy.
Mazes
Many high energy vaults utilize a maze to reduce the size, weight, and complexity of the vault door.
Maze calculations require special attention as both the reflected and transmitted dose must be accounted for in the shielding design. Because of their complexity, the reader is encouraged to review the full NCRP151 report.
TenthValueDistance (TVD)
Tenth value distance (TVD) is the maze distance required to reduce thermal neutron fluence by a factor of 10.
\begin{equation} TVD \ (meters) \approx 3 \times \sqrt{height \times width} \end{equation}
For most mazes the TVD is approximately 5m.
Thermal neutron fluence also reduces by a factor of approximately 3 for each leg of the maze.
Skyshine and Groundshine
Skyshine
Skyshine refers to radiation scattered off of the atmosphere back to the ground or surrounding buildings.
Skyshine can become an issue for treatment vaults with lightly shielded ceilings.
Groundshine
Groundshine refers to radiation scattered off of the ground below the vault back to the surface outside the vault.
Groundshine is sometimes a problem with vault designs that use earth as the floor shielding.
Knowledge Test
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