Innovative Vault Innovative Vault Design and gPractical Shielding TiTips

Robert J Barish, Ph.D.,

I t d tiIntroductionExperience matters when theExperience matters when the concept of “value engineering” is applied to the design ofis applied to the design of therapy shielding.Inexperienced people tend toInexperienced people tend to over-shield in order to “err on the side of safety” without a full appreciation of the cost differential, which may be

id blconsiderable.

Varian’s planning department uses a gcomputer program that can produce shielding recommendations that are gillogical. I have seen a physicist add 1/8” Pb to an existing concrete wall because the Varian program produced a calculation to four significant figures that resulted in a thickness that was ¾” greater than an existing concrete barrier. Even at 18 MV, ¾” concrete = 0.14 HVL = dose reduction of 10%. This added $14,000 Pb to the job.

Why adding “safety factors” is nonsensical.

1) Patient attenuation of primary beam: minimally 1 HVL1) Patient attenuation of primary beam: minimally 1 HVL

2) Leakage radiation has a TVL several inches less than primary yet using primary TVL is recommendedprimary, yet using primary TVL is recommended.

3) Is occupancy really “1” in an uncontrolled area dj t t b i M b bl liadjacent to a barrier. Maybe, on an assembly line, a

person can be found in a single location during a full day (minus coffee breaks) but even an office workerday (minus coffee breaks) but even an office worker doesn’t sit at their desk without moving to get files, greet visitors, visit the restroom, etc. g , ,

4) Broad-beam TVLs are used when modern treatments usually employ small fields What percentage of yourusually employ small fields. What percentage of your work uses 40 cm x 40 cm fields?

Every job deserves a fresh l k ith t t ibllook with respect to possible innovations that are site-specific.Even highly-experiencedEven highly experienced people may be locked in to a set of methods that fail toset of methods that fail to explore alternate possibilities.

Architects will usually present you with a layout that comes y ystraight from whatever vendor’s planning manual they have onplanning manual they have on hand.It’ bli ti t l kIt’s your obligation to look beyond the standard configuration and to explore other possibilities.p

Also, actively participate in recommending beam energies for modern facilities. In the age of IMRT does anyone really need >10 MV?

d= 10 cm %DD(18 MV; 10x10 cm) = 78%(18 MV; 10x10 cm)

d= 10 cm %DD(15 MV; 10x10 cm) = 77%

d 10 %DD 75%d= 10 cm %DD(10 MV; 10x10 cm) = 75%

Neutron production = 0.004% (10 MV)

= 0.1% (15 MV)

0 15% (18 MV)= 0.15% (18 MV)

Almost 40 times the neutrons at 18 MV vs. 10 MV

= 3” – 4” greater polyethylene in doors (700 lbs) $4k

Topics of DiscussionSite plansInverted bumpsInverted bumpsIndented ceilingsRamps (not RAMPS)Restricted couch swingg“Backwards linac”TomoTherapy unitsTomoTherapy unitsCyberknivesHigh-density options

2,000 cGy = 1.08 cGy to 20 µGy requires 2.73 TVL

36” t (6 MV)(43)22 = 36” concrete (6 MV).(43)

Beam DivergencePlanning manuals always show

l i l ti ith th

g

a plan view or elevation with the beam at 90 degrees or at 0 degrees. Architects use this 28 degree angle on their g gpreliminary drawings.But the beam can be angledBut the beam can be angled toward the room corners, which obviously increase its size.

I t d BInverted BumpsPlanning manuals always show aPlanning manuals always show a ceiling bumping down into a roomroom. For single-story free-standing f iliti it k tfacilities it makes more sense to bump up. This saves material on all of the walls of the room as well as the maze.

Consider the total footprint of a d ith th llmazed room, with the maze wall

included. For a room size of 40’ x 32’ the saving of concrete by bumping “up” instead of “down” p g pwould be on the order of 60 yards of concrete At an installedyards of concrete. At an installed price of $500 per yard, this would save $30 000 on thewould save $30,000 on the project cost.

Bump thicknesspNote also that bumps in single-

f di f ili i dstory free standing facilities need only account for skyshine to adjacent areasadjacent areas.Even for high-energy linacs this is on the order of 76 cm of concreteon the order of 76 cm of concrete.Control of access must be establishedestablished.A security TV camera with monitor on control console ismonitor on control console is acceptable.

Indented CeilingIndented CeilingIndented ceilings work well when th i li it d di t b tthere is a limited distance between the floor of the radiotherapy area

d th fl band the floor above.This situation often occurs when there is a fully occupied area above the new accelerator vault.the new accelerator vault.It is an ideal solution when there is only about 4 5 m between floorsonly about 4-5 m between floors.

Indented CeilingIndented CeilingThe required 10’ clearance isThe required 10 clearance is maintained over the linac.Th t f th ili iThe rest of the ceiling is designed to the requirements for the secondary barrier or as close as possible.pThe amount of lead saved is on the order of 25 000 poundsthe order of 25,000 pounds

RampsA grade of 1:12 is acceptable and practical for a ramped

Rampsand practical for a ramped entry into a treatment room.A high energy linac room withA high-energy linac room with a maze has a maze length that easily allows a one-foot dropeasily allows a one foot drop with a ramp starting just beyond the door swing.y gThe door may also be designed to swing out, giving 5 g g , g gextra feet (1.5 m) for the ramp.

Compared with the previous, the primary 8’ x 8’ lead has been reduced by 2”.been reduced by 2 .The two side “wings” have also been red ced b 2” andalso been reduced by 2” and are 1’ less wide. The saving of 18,000 lbs of lead represents a similarlead represents a similar dollar cost saving of $25,000.

The total saving of lead gin the previous two t i th d fsteps is on the order of

40,000-45,000 pounds.40,000 45,000 pounds. This is real money!

The extra 2+ feet of clearance produced by a lowered ceiling and alowered ceiling and a ramped maze may allow for the elimination of all high-density materialhigh density material leaving a requirement for

l di tonly ordinary concrete.

Restricted couch swingThink about your current facility

Restricted couch swing

facility.Are there linen baskets, film holders, accessory carts or other “obstructions” in the “sacred semicircle?”Do you treat parallel opposedDo you treat parallel-opposed coronal and perineal fields?

If there is full couch clearanceIf there is full couch clearance in one direction, patients can be positioned on the treatmentbe positioned on the treatment table in two ways – “head first “ or “feet first.” Enough swing remains to allowEnough swing remains to allow for field matching when only 12 degrees of couch rotationdegrees of couch rotation exist.

Five feet (1.5 m) or more of ( )room width can be saved by restricting the full couch swingrestricting the full couch swing to only one side.Thi i h t ll fThis is enough to allow for a high-energy linac to be placed in the space previously used for a low-energy unit.for a low energy unit.

Backwards linacIt is typical for no-maze rooms that, yp ,

when you enter, the gantry is at the far side of the room and the treatment side of the room and the treatment couch is toward you.

Turning the machine 180-degrees from g gthat usual orientation allows for the placement of a small “stub” or mini mazeplacement of a small stub or mini-maze.

This stub will protect the door from direct This stub will protect the door from direct leakage radiation – both neutron and hphoton.

Th hi i li htl ff t t i The machine is slightly offset to insure full screening of the head leakage at any g g ygantry position.

Shielding is designed to an ALARA level of 100 µSv per week The permitted dose is 100 µSv per week. The permitted dose is evenly divided between neutrons and yphotons, i.e., 50 µSv is allowed from neutrons the other 50 µSv from photonsneutrons, the other 50 µSv from photons.

The workload is 500 Gy/week 350 Gy at The workload is 500 Gy/week. 350 Gy at 18 MV. And 150 Gy at 6 MV, an energy below the threshold required for neutron production.

Neutron production at 18 MV = 0.15% pSv/Gy at the isocenter position. Of the 350 Gy workload at 18 MV assume that 50% is Gy workload at 18 MV, assume that 50% is delivered with IMRT.

Compared with conventional treatments, pIMRT techniques are assumed to require an increase in monitor unit settings by a factor increase in monitor unit settings by a factor of five to deliver the same dose to the patient.

Neutron production for “conventional” treatments:treatments:

175 Gy x 0 0015 Sv/Gy = 263 mSv175 Gy x 0.0015 Sv/Gy = 263 mSv

Neutron production for IMRTNeutron production for IMRT treatments:

175 Gy x 0.0015 Sv/Gy x 5 = 1,313 mSv

Total = 1,576 mSv

Door (neutrons) d=6.05 m

1,576 mSv = 43.1 mSv(6.05)2

To reduce this to 50 µSv requires 2 94To reduce this to 50 µSv requires 2.94 TVL

1 TVL = 8.5 cm of borated polyethylene f t d dfor no-maze rooms, so a standard thickness of 25.4 cm of borated polyethylene is specified.

Door (neutron capture gamma rays)Door (neutron capture gamma rays)

Assume 0.1 of neutron equivalent dose is converted to capture gammasdose is converted to capture gammas. Dose equivalent at plastic is 43.1 mSv.

The capture gamma dose is 4.31 mGy.

Door (90-degree scattered photons)( g p )

d1=4.05 m d2=5.15 m U= ¼ α 90°=0.001

500 Gy x ¼ = 7.62 Gy(4 05)2

7 62 G 0 001 7 62 G

(4.05)2

7.62 Gy x 0.001 = 7.62 mGy

5 78 mGy 0 29 mGy5.78 mGy = 0.29 mGy(5.15)2

D (l k h t ) d 6 05Door (leakage photons) d = 6.05 m

500 Gy x 0 001 = 13 7 mGy500 Gy x 0.001 = 13.7 mGy(6.05)2( )

The total photon dose at the door is:

13.7 mGy + 0.29 mGy + 4.31 mGy = 18.3 mGy.

U i TVL f 6 1Using an average TVL of 6.1 cm, as recommended (McGinley 2002) for ( y )both of these components, the lead requirement to reduce 18 3 mGy torequirement to reduce 18.3 mGy to 50 µGy is 2.56 TVL = 15.6 cm Pb.(NCRP 151 says just use leakage and add 1 HVL.)

Using standard material thicknesses, th d ld b ifi d 0 5the door could be specified as: 0.5 cm steel, 25.4 cm of 5% borated polyethylene, 15.6 cm lead and 0.5 cm steel in that order, from the treatment ,room out to the control area.

For better balance, the lead would usually be distributed with 7.8 cm on yeach side of the borated polyethylene. 7 8 cm lead will adequately absorb the7.8 cm lead will adequately absorb the capture-gammas from the plastic.

Door (neutrons) d1=4.10m d2=3.05 m

1,576 mSv = 94 mSv(4.10)2

3.05 m/5.0 m = 0.61 TVD 10-0.61=0.245

94 mSv x 0.245 = 23.1 mSv. To reduce this to 50 Sv requires 2 67 TVLto 50 µSv requires 2.67 TVL.

1 TVL = 5 1 cm of borated polyethylene for1 TVL 5.1 cm of borated polyethylene for this short maze, so a standard thickness of 14 0 cm of borated polyethylene is14.0 cm of borated polyethylene is specified.

Door (neutron capture gamma rays)Door (neutron capture gamma rays)

Assume 0.1 of neutron equivalent dose is converted to capture gammasis converted to capture gammas. Equivalent dose at plastic is 23.1 mSv.

The capture gamma dose is 2.31 mGy.

Door (90-degree scattered photons)( g p )

d3=4.65 m d4=3.75 m U= ¼ α 90°=0.001

500 Gy x ¼ = 5.78 Gy(4 65)2

5 78 G 0 001 5 78 G

(4.65)2

5.78 Gy x 0.001 = 5.78 mGy

5 78 mGy 0 41 mGy5.78 mGy = 0.41 mGy(3.75)2

The total photon dose at the door is: 0.10 mGy + 0.41 mGy + 2.31 mGy = 2 82 mGy (The 0 10 mGy is leakage2.82 mGy. (The 0.10 mGy is leakage through the maze wall)

Using an average TVL of 6.1 cm, as d d (M Gi l 2002) frecommended (McGinley 2002) for

both of these components, the lead prequirement to reduce 2.82 mGy to 50 µGy is 1 75 TVL = 11 cm Pb50 µGy is 1.75 TVL = 11 cm Pb.

Using standard material thicknesses, the door is specified as: 0 5 cm steel 14 0 cm ofdoor is specified as: 0.5 cm steel, 14.0 cm of 5% borated polyethylene, 11 cm lead and 0.5 cm steel in that order from the treatmentcm steel in that order, from the treatment room out to the control area. For balance, put 5 5 cm Pb on either side of the BPE

Th t li d d i h 3 200 k

5.5 cm Pb on either side of the BPE.

The streamlined door weighs 3,200 kg and is 26 cm thick.

3,000 kg and 16 cm are saved.(11.5 cm of BPE and 4.6 cm of lead.)

Shielding Considerations for gTomotherapy

No primary beam; there is a5-inch thick lead “beamstopper.”

A scatter pattern similar to that provided for CT scanner is used toprovided for CT scanner is used to evaluate secondary radiation.

6MV High Dose Source(800MU/ i 1 5 i )

Binar MLC

(800MU/min, 1.5mm point source)

Primary Collimator(0 to 5.0 cm)

Binary MLC(64 leaves, ea @ 0.61cm)

85 cm Gantry Aperture

40 cm VRCT FOV

Approximate 85cm

40 cm VRCT FOV

Approximate 50cm

VRCT Detector System

Courtesy of T. Schultheiss

From Mackie, et.al. AAPM 2003 summer school proceedings

Workload

Assumptions: 800 cGy/min at isocenterBeam on 1/3 timeWorkload=(800 cGy/min)x(2400 min/wk)/3

= 640,000 cGy/wk

Courtesy of T. Schultheiss

East (control area; restricted) d = 3.81 m T = 1 P = 0.10 mGy transmission = 3.8 x 10-5

(6,400 Gy)(3.8 x 10-5) = 24.32 cGy to 0.1 mGy = 3.40 TVL = 46” concrete.

North (mechanical area; unrestricted) d = 2.74 m T = 1/8 P = 0.02 mGy transmission = 2.7 x 10-5

(6,400 Gy)(2.7 x 10-5) = 17.28 cGy to 0.16 mGy = 3.03 TVL = 7” Pb.

ShieldingShielding Considerations forConsiderations for

Gamma KnifeGamma Knife

Gamma knife dose patternsGamma knife dose patterns are provided both for the couch in the treatment position and for the couchposition and for the couch withdrawn, since the 192withdrawn, since the 192 cobalt sources create a radiation field around the unit even when it is “off ”even when it is off.

Treatment times are in the range f 45 i t t 1 5 h ithof 45 minutes to 1.5 hours with

treatment dose patterns often pcreated by having more than a single focal pointsingle focal point.

Workloads are not easilyWorkloads are not easily predicted, and may rely on the p y ybusiness plan of the site with respect to what types ofrespect to what types of treatments will be provided.

Once a workload is established, the total dose is determined bythe total dose is determined by multiplying the values from the p y ggraph for “beam on” conditions b th t d “b ” tiby the expected “beam on” time and then adding the doseand then adding the dose calculated for the “beam off” ti Th TVL f 60C 20 6time. The TVL for 60Co = 20.6 cm concrete or 4 cm Pbconcrete or 4 cm Pb.

Cyberknife ShieldingAll barriers except the ceiling areAll barriers except the ceiling are primary barriers to an angle of 22°from the horizontal plane of thefrom the horizontal plane of the isocenter. Typically 80 -100 beams

d f t t t f tare used for treatment, a use factor of 1/20 is conservative.

A total photon workload no greaterA total photon workload no greater than 32,000 cGy per week at 6 MV at a distance of 100 cm with a photon “IMRT factor” of 14photon IMRT factor of 14 producing 32,000 cGy per week of

i b d 450 G fprimary beam and 450 cGy of leakage radiation at the nominal genergy of 6 MV.

Additionally for SRS treatments ofAdditionally, for SRS treatments of 1,300 cGy, an hourly primary dose

f 1 300 G t di t f 100of 1,300 cGy at a distance of 100 cm and leakage dose of 19 cGy is g yassumed for locations where lower occupancy factors in unrestrictedoccupancy factors in unrestricted areas would limit the dose to 2 mrem in any single hour.

The newest model Cyberknife has aThe newest model Cyberknife has a maximum circular collimator size of 6 cm so using the “broad beam”cm, so using the broad beam attenuation value for 6 MV radiation (1 TVL 33 f t ) i t lTVL = 33 cm of concrete) is extremely conservative.

The newest model Cyberknife actually changes its o n collimators b takingchanges its own collimators by taking them out and replacing them back into a collimator tray!

East (primary; unrestricted; equipment room MB036) d=3.94 m U=1/20 T=1/8 P=0.02 mGy

32,000 x 1/20 = 103 cGy to 160 µGy = 3.81 TVL =(3.94)2 126 cm concrete

East (secondary; unrestricted; equipment room MB036) d=3.94 m U=1 T=1/8 P=0 02 mGyU=1 T=1/8 P=0.02 mGy

450 = 29 cGy to 160 µGy = 3.26 TVL =(3 94)2 108 cm concrete(3.94) 108 cm concrete

The difference is less than 1 TVL, so 1 HVL is added to the calculated primary thickness: 126 cm + 9.9 cm = 136 cmp y

High-density optionsg y p

Concrete is made by mixing Portland Cement with small pieces of material called aggregateof material called aggregate.

The usual aggregate is stoneThe usual aggregate is stone (gravel) which creates concrete with (g )a density of around 145-147 lb/cu-ft

High-density concrete is created b i l h i th tby simply changing the aggregate.

Generally, two densities of high-density concrete are available in prefabricated blocks. The originalprefabricated blocks. The original supplier of these called the material “L dit ” b t th i l d d“Ledite” but there is no lead used.

The high density aggregates areThe high-density aggregates are usually iron ores. These create yconcrete with densities of either 240 lb/ ft 288 lb/ ftlb/cu-ft or 288 lb/cu-ft.

For photon attenuation, the pscaling with density at

lt i imegavoltage energies is practically linear So calculatedpractically linear. So calculated barrier thickness can simply be yscaled down by the ratio of 1/1.6f 240 lb bl k 1/2 f 288 lbfor 240 lb block or 1/2 for 288 lb blockblock.

For neutron attenuation, the TVL in iron at megavoltage

i (35 ) i l t th tenergies (35 cm) is close to that of ordinary concrete (25 cm)of ordinary concrete (25 cm). When calculating barrier gthickness with high-energy li t l blinacs, one must always be aware of the neutron componentaware of the neutron component

For neutron attenuation, the need for a hydrogenous component makes the requiredcomponent makes the required concrete thickness i d d t f d itindependent of density.This can overrule the commonThis can overrule the common conception that “if a barrier is thi k h t b b ththick enough to absorb the photons, it will always be p , yadequate for the neutrons.”