Shielding Calculation Techniques

Transcription

1 Objective Shieling Calculation Techniques Design of shiel with aequate attenuation to achieve the require (or acceptable) ose equivalent (rate) limitation (or ALARA) Calculation Methos Linac Rooms Shieling Issues Barrier Materials Primary, scatter an leakage Barriers Maze Design Neutron Shieling Simulator (x-ray an CT) HDR Brachytherapy Rooms Special Topics Reports Factors o be consiering in shieling esign ALARA Available space Constructions techniques Regulatory limits (or constraints) Shieling materials Source term Trens in regulatory limits with times Calculation Metho Conventional Primary Barrier We calculate the ose rate at a certain istance from the source ue to primary, scattere an leakage raiation an from it erive how many TVL s we nee to bring the raiation levels to the ose constraints (occupational or public). [if the attenuation curves are not exponential, use attenuation curves] bibliography NCRP Report No. 49, for accelerators operating at 0 MV or less No. 5 (44), for higher energy machines Patton H. McGinley: Shieling Techniques for Raiation Oncology Facilities. Meical Physics Publishing, Maison, Wisconsin. WU pri P

2 Patient Scatter Leakage P P W sec l W sec P IMRT Maze an Door Primary Barrier: Source term L W m P What is the Dose Rate at a certain istance from the source? W Workloa average ose per week at one meter from the target (source) U Use Factor (orientation) average fraction of time per week that the primary beam falls on the barrier T Occupancy Factor fraction of the time that a person will be in the area outsie of that barrier (the most expose iniviual) pri istance from source to the point (zone to protect) where person will be staning WUT Dose Rate@ pri H unshiele pri Calculate the transmission of the Barrier (B x or T x ) to bring the Dose Rate to constraint Levels H shiele Primary Barrier shiele ose per week require outsie of the protection barrier Regulatory constraint H H shiele m pri Bx Hunshiele WUT Seconary Barrier Transmission for Seconary Barriers Consier Scatter from Patient, B p, U Scatter from Barrier, B s Leakage thru Barrier, B l, U Do the calculation for each barrier Do the calculation for each primary barrier

3 Scatter from Patient Scatter from Barriers H mseci 400 Bp awt F sec istance from the scattering surface to the point to be protecte i istance from the x-ray source to the patient a a ratio of scattere raiation at one meter from the scattering object to the primary raiation at m from the x-ray target (erive with fiel size 0x0) F F fiel size at patient (cm ) H mseci Bs αawtu sec istance from the scattering surface to the point to be protecte i istance from the x-ray source to the barrier α reflection coefficient for the barrier material, scattering angle, an x-ray beam energy (per m ) A A area of beam at the scattering surface in m Leakage thru Linac Hea Workloa Consierations 000H m Bl WT sec /000 Leakage factor thru the hea of linac sec istance from the isocenter (or target) to the point to be protecte Energy (@90 o ) is about the prouce by E 0 /3 but conservatively assume equal to the primary Choose the maximum weekly expecte average value of the ose at a meter from the target. Survey (Klech et al, 994) For single x-ray beams W350 Gy/week For ual x-ray beams W 50 Gy /week for the highest energy x-ray beam Disregar electron beam workloa IMRT affect all barriers TBI affect only one barrier Workloa Example Average No. of patients/ay 40 Average No. of fiels/patient 5 Average No. of MU/fiel 50 MU cgy No. of ays/week 5 patients fiels W ay patient W 50000cGy 500Gy MU fiel cgy 5 MU ays week Dose Constraint Controlle Hm 50 msv/year (actually 0 msv/year) ( 0/ msv/week) Due to ALARA Hm 0.0 to 0.0 msv/week (NCRP6) Uncontrolle General Public Hm.0 msv/year or 0.0 msv/week Due to ALARA Hm 0.00 to 0.0 msv/week 3

4 Scatter-primary ratio Scattering from barriers Scatter-primary ratio (a) at m from a human-size phantom for a size 400 cm at the phantom, target to phantom istance of m Scattering angle 6 MV 0 MV 30 o o o o o Since scatter raiation has low penetration compare with leakage raiation it can be ignore above 0 MV T ½ to /5 /8 to /40 Occupancy Factor Type of area Full occupancy: : work areas, such as offices, shops, laboratories, chilren s play areas, occupie nearby builings, living quarters, wars, nurse's stations Partial occupancy: : corriors, rest rooms, elevators using operators Occasional occupancy: : waiting rooms, toilets, stairways, unattene elevators, janitor s closet, outsie areas use only for peestrians or vehicular traffic parking lots Barrier Materials Physical Properties of Common Shieling Materials Shieling Material Density (gcm -3 ) Atomic Number Relative Cost/mass Orinary concrete.3.0 Heavy concrete Low-carbon steel Lea Earth, ry -packe.5 - low Barrier Thickness Transmission curves can be use to calculate barrier thickness 6 MV scattere raiation Barrier Thickness Or we can use the tenth value layer (TVL). The number (n) of TVL can be obtaine by n log (/ B ) The thickness S can be obtaine by S T + ( n ) T 0 x e 4

5 Barrier Thickness Tenth value layers (TVL) in concrete, steel, an lea Data base on NCRP 5 Shiel TVL TVL e Energy/MV Material 6 concrete steel lea concrete steel lea Barrier Thickness Tenth value layers (TVL) in concrete, steel, an lea Data base on NCRP 5 Shiel TVL TVL e Energy/MeV Material 5 concrete steel concrete steel concrete steel concrete steel Primary Barrier Photon Tenth-Value Layers (mm) Come from a Variety of Sources Lea Concrete Steel Earth Borate Poly MV TVL TVLe TVL TVLe TVL TVLe TVL TVLe TVL TVLe NCRP 49 NCRP 5 Nelson & LaRiviere McGinley Estimate from Concrete Barrier Thickness Tenth value layers (TVL) for primary an seconary leakage raiation at 90 o. Data from Varian. Megavoltage from BJR 7 X-ray Shieling TVL prim TVL 90lkgg MV Material 6 concrete earth steel lea concrete earth steel lea Barrier Thickness Tenth value layers (TVL) for primary an seconary leakage raiation at 90 o. Data from Varian. Megavoltage from BJR 7 X-ray Shieling TVL prim TVL 90lkgg MV Material 5 concrete earth steel lea concrete earth steel lea Barrier Thickness Tenth value layers (TVL) for primary an seconary leakage raiation at 90 o. Data from Varian. Megavoltage from BJR 7 X-ray Shieling TVL prim TVL 90lkgg MV Material 0 concrete steel lea concrete steel lea

6 First an thir TVL for hea leakage raiation* Angle TVL /TVL 3 6MV 0MV 5MV / / / / / / / / /0.35 * Aapte from Nelson an LaRiviere (984) for orinary concrete Rule of thumb for oblique raiation base on NCRP 49 rules for 60 Co an 37 Cs:. If the barrier is compose of concrete, an attenuation of 000 is require, an q is 50 o, increase the barrier q HVL for low energy an HVL for high energy raiation. The aitional shieling is require to account for t S scattering shown by ray (a) in the fig.. For angles of 60 o an 70 o each of the thickness nee to be increase by an HVL respectively 3. For lea shieling with a require a attenuation of 000, the barrier is increase by HVL at 60 o Rules for 60 Co, 4, 0 an 8 MV x-rays. Biggs (996). For 60 Co an q <45 o, eviations from obliquity are noticeable for barrier transmission factors less than 4x0-3 ; for q 60 o ; they are noticeable below x0 -.. The concrete shieling thickness for x-rays of 0 MV or greater can be base on the slant thickness (t) proviing q is less than 70 o. 3. For steel the slant thickness shoul not be use for an 8 MV beam if the value of q is greater than 60 o an t is greater than 35 cm. 4. For lea the slant thickness shoul not be use for 8 MV beams if the value of q is greater than 60 o an t is greater than 7 cm. With an Length of Primary Barrier The with of a primary barrier is equal to the maximum beam size at the barrier plus foot (0.305 m) on either sie to prevent raiation from leaking through the seconary barrier. The maximum fiel size is normally 40x40 cm an the maximum with is the iagonal of the fiel which will be equivalent to a with of 56.6 cm. The require with W is W X X is the istance of the barrier in meters from source. Primary Barrier With Figure meter margin on each sie of beam rotate 45 egrees Barrier with require assuming 40 cm x 40 cm fiel size w C C ' m Fiel typically not perfectly square (corners are clippe) 35 cm x 35 cm fiel size typically use to account for this Target to Narrow Point Distance ( ) C' * Target Isocenter Target to Narrow Point Distance ( ) C' * Target Isocenter Target to Narrow Point Distance ( ) C' * Target Isocenter 0.3 m 0.3 m 0.3 m 0.3 m C' w C C C' w C C Metal 0.3 m 0.3 m w C 6

7 Maze Calculations for up to 0 MV beams Scattere raiation to the oor from the primary beam (Ss). Primary beam scattere from room surface. Hea leakage photons scattere by room surface 3. Primary scatter from the patient (Sp) If space is at premium use a oor with seconary barrier thickness P L m W where Doα Aα A S s ( ) S s D A o r r r r Dose at oor Workloa of accelerator α Reflection coefficient at first reflection base on a beam energy of ½ the MV of linac A Beam area at first reflection (m ) α Reflection coefficient at secon reflection Base on a beam energy of 0.5 MV Cross section of maze (m ) Distance from target to first reflection Centerline istance along first leg of maze Centerline istance along secon leg of maze Figure 3. Figure 3.3 The following restrictions apply to Ss. The height to with ratio of the maze must be between one an two, an the value of r /(A ) / must be between two an six. Values of the reflection coefficient may be obtaine from Figure 3.3. For photon energies higher than 0 MV, the 0 MV reflection coefficient is use. This is consiere to be a conservative estimation of the ose at the oor. 7

8 Scattere raiation at the oor from hea leakage Figure 3.4 where LoDoαA L ( ) L L A o D α i s o s i Dose at oor ue to hea leakage Ratio of ose ue to hea leakage at m from target to the ose at the isocenter Workloa of accelerator Reflection coefficient for wall reflection Area of wall C that can be seen from maze (m ) Distance from target to maze centerline Centerline istance along the maze McGinley an James (997) observe a factor of between the calculate an measure hea leakage scatter. The hea leakage use was the measure not the stanar 0.%. The energy assigne to the hea leakage was.4 an.5 MV for the 6 an 0 MV linac as suggeste by Nelson an LaRiviere. Scattere ose at the oor from the patient scatter where ado( F / 400) αa S p ( ) S p D o a F α sca sec r sca sec r Dose at oor ue to the patient scatter Workloa of accelerator Reflection coefficient for patient Fiel area at patient (cm ) Reflection coefficient for wall reflection E0.5MV A Area of maze back wall that can be seen from outer maze entrance (m ) Distance from target to patient Distance from patient to maze centerline Centerline istance along maze Transmitte ose at the oor thru leg of maze 8

9 where T B '' L ( D B o o ' ' ) Barrier transmission factor for wall D Distance from target to the oor Total Dose (Dc) when beam is pointing at wall C D f c S p + fs c + L + T Fraction of beam transmitte through patient Values of f for 6 MV (0.3) an 0 MV (0.7) have been reporte by McGingley an James (997) Total Dose (D t ) from all sources of raiation at the oor Dt. 64D c For typical case in which the factors U are ¼ each. This equation can be use for rooms with a similar layout as Figure 3.. The transmission factor require for the oor shieling is calculate by iviing the permissible ose at oor by D t. The thickness of lea for a 6 MV can be calculate from Figure 3.6. For a 0 MV beam the thickness can be base on broa beam ata for 0. MV photons. When the energy of the linac is above 0 MV. This techni- Que still applies. We nee to consier the neutrons. Photoneutrons Relative yiel of photoneutrons in semi-infinite infinite thick target as a function of incient electron energy Target Electron Energy (MeV) Element Al Cu Fe Pb W Figure of linac hea Figure 4.3 9

10 Φ Φ ir + Φ sc + Φ Total flux is ue to irect neutron prouction between a photon an a neutron in the nucleus of the target atom, the scattere neutrons from the concrete surfaces of the room, an a thermal neutron energy group. McCall et al (979) have foun that the irect neutron fluence, which accounts for 5% of the photoneutron. is given by Φir aq / 4π where a is the transmission factor for neutrons that penetrate the hea shieling, is the istance (cm) from target to the point where fluence is evaluate an Q is the neutron source strength in neutrons emitte by unit photon ose. The factor a is.0 for lea an 0.85 for tungsten shieling th The room scatter an thermal neutron fluence are constant in the room given by Φ Φ th.6q / S 5.4aQ / S sc S is the surface area of the treatment room in cm. The total fluence is given by aq Φ 4π 5.4aQ + S +.6Q S c Neutron source strength for meical accelerators Manufacturer Moel State MV Q neutrons/gy Siemens KD 0 0.9x 0 Varian Varian Varian Philips SL-5.37 Philips SL GE Saturne GE Saturne GE Saturne GE Saturne Varian 00C/D an 300 C/D are similar to the 800 series State Rules an Regulations for Hea Leakage. For operations proucing the maximum leakage raiation, the absorbe ose ue to neutrons an photons, at any point in a circular plane (patient plane or area) of m raius centere on an perpenicular to the CAX of the beam at the isocenter an outsie of the maximum beam size, shall not excee 0.% of the absorbe ose ue to the x-rays at the isocenter.. Points outsie the patient area an at m from the path of the electron beam through the accelerator shall receive an absorbe ose ue to photons that is < 0.% of the x-ray ose at the isocenter an < 0.005% ue to neutrons. Activation of Materials Raionuclies prouce in meical accelerators Reaction Moe of ecay Half-lifelife Photon energy 7 Al(n,g) Al b -.3 min.780 MeV 63 Cu(g, g,n) Cu b min 0.5 MeV 56 Mn(n,g) Mn b -.6 hour MeV 63 Cu(n,g) Cu b + /b -.7 hour.346 MeV 65 Cu(g, g,n) Cu b + /b -.7 hour.346 MeV 86 W(n,g) W b hour 0.479/ Ni(g, g,n) Ni b hour.378/.90 Activation of Materials Near the accelerator the ose rate immeiately after treatment is ominate by 8Al an 6Cu, an after one hour the longer-live live isotopes 87W an 57Ni prouce the majority of the ose rate. It was foun by Almen et al (99) that the annual ose receive by the technician from inuce activity was in the range of.0 to.8 mgy for the trunk region of the boy an 0.7 to 3,3 mgy for the hans. A workloa of 40 ays per year an 3500 high-energy ports treate per year was use to estimate the annual ose. McGinley measure the ose rate at the istal en of the collimator for several Varian linacs operating at 8 MV an foun the average level of 0.80 mgy/h, two minutes after the machine ha been running for 30 minutes. The engineer shoul wait for 40 minutes before repair near the target, after a long run. 0

11 Materials for Neutron Shieling Properties of shieling materials Hyrogen TVL TVL TVL content fast slow capture Neutron Materials (atoms/cm3) neutrons gammas activation Concrete x low Polyethylene 8.0x very low 5% Boron very low Steel meium Lea low TVL in cm IN Mazes an Doors for High-Energy Rooms Polyethylene 0. cm Lea.7 cm Steel cm Most meical accelerators operating above 0 MV use a maze with a oor shiele for neutrons an photons at the outer maze entrance. A typical oor consists of a steel case cm thick containing 0. cm of borate polyethylene (5% B by weight) an a.7 cm lea slab. The polyethylene is use to moerate the fast an intermeiate energy neutrons, which react with the boron ant prouce a MV photon. The lea is place after the polyethylene, where it will attenuate the photons prouce in the boron an any capture gamma rays generate in the maze by neutron capture in the concrete walls, ceiling an floor. Metho was evelope by Kersey (979). Kersey Technique H H T T o / 5 o o H is the neutron ose equivalent at the entrance of the maze per unit ose of x-ray at the isocenter. Ho is the neutron ose equivalent at a istance o from the target. T/To is the ratio of the outer maze area to the inner maze entrance nce area. is the istance from the isocenter to the point on the maze center line from which the isocenter is just visible. For a maze with one ben, is the istance from A to B of Figure 5.. TVD is the tenth value istance 5 m for neutrons. 0 Neutron ose equivalent Ho at.4 m from the target per unit ose of x-ray at the isocenter (msv/gy X) Accelerator Moel State Beam Ho* Reference manufacturer (MV) MV Varian Unpublishe U Unpublishe U 0.04 Unpublishe Siemens KD McGinley 988 MD 5 U 0.7 Unpublishe Philips SL McGinley et al SL GE Saturne Fenn an McGinley *Ho in msv/gyx U unknown H H o T T o o 0 0 / 5 3 / 5 3 Distance 3 is equivalent to istance between points B an C of Figure 5.. The neutron ose equivalent a the oor will epen on the collimator opening an gantry angle.

12 Relative neutron an photon ose equivalent at the outer maze entrance as a function of beam irection*. Beam irection base on Figure 5.. State Energy Relative neutron Relative photon (MeV) ose equivalent ose equivalent to 3 3 to up own to 3 3 to up own The average neutron energy at the maze entrance has been reporte to be aroun 00 kev, an the corresponing TVL in polyethylene is 4.5 cm (NCRP 984). Capture Gamma Ray Shieling Most linac room have a leae oor esigne to attenuate the scattere x-rays from the treatment room into the maze. For high-energy x-rays beams, there is a nee to shiel for capture gamma-rays prouce in the maze. The average of the capture gamma-rays in concrete is 3.6 MeV. A metho was evise by McGinley et al (995b) to etermine the ose ue to the capture gamma-ray per unit ose of x-ray at the isocenter. / TVD D KΦtotal0 where K is the ratio of the capture gamma ose to the total neutron fluence at point A in Figure 5. Base on experimental ata, an average value of 0.77x0- cm Gywas foun for K. The TVD is approximately 6. m for 6. to MV x-ray beams. Raiation ose equivalent rate ue to capture gamma rays an neutrons at the outer maze entrance Type of Capture gamma Neutron ose Total maze an ose equivalent equivalent N+g oor rate (nsv/s) rate (nsv/s) rate (nsv/s) Conventional a. Reuce inner opening b. Inner B oor c. Inner poly oor The maze length is 6.5 m an the ose rate at the isocenter is 6.67 cgy/s. The reuce inner opening (a) was one with a 45.7 cm thick wall aroun the inner opening which was.x.3 m. A panel 7 mm thick containing 8.9% boron by weight was use in the technique b. A 5 cm thick polyethylene (5% boron) oor was use for techniquec. c. Total raiation ose equivalent at the outer maze entrance for a workloa of 500 cgyper week an a maze length of 6.5 m. Type of Photon ose Neutron ose Total ose maze an equivalent/w equivalent/w equivalent/w oor (csv) (csv) (csv) Conventional a. No oor b. Inner B oor c. Inner poly oor Figure 5.3 an 5.4 Direct Shiele Doors Maximum ose equivalent rates at the sliing oor, console, an gona levels below the HVAC penetration. Dose equivalent rate Neutron Photon Total Beam irection Location (nsv/s) (nsv/s) (nsv/s) Down oor face oor frame below HVAC < <.9 console < <0.53 Up oor face oor frame below HVAC < <.9 console < <0.53 Down oor face oor frame below HVAC < <.9 console < <0.53

13 Maximum ose equivalent expecte in any one hour of operation, in seven consecutive ays, an in one year. Location Total ose equivalent One hour Seven Days Annual (msv) (msv) (msv) Door face Door frame Below HVAC Console <0.7 <4.0 <00 Laminate Primary Shiel Primary ceiling shiel for 8 MV accelerator facility Accelerator Layer No. Shieling Thickness material Varian 00C concrete lea polyethylene concrete lea concrete 0.4 Patient Neutron Dose Skyshine One linac room with a 4 cm steel slab in one wall Another linac room with a 0 cm lea slab in one wall Both 8 MV Lea increase the total boy ose by 4% Steel increase the total boy ose by 0% For TBI lea increase the total boy ose by a factor of.3 an. for steel. B B D s D XS XS O Ω 6 D( s ) 4.0x0 Skyshine.3 D Ω O Roof shieling transmission ratio Photon ose equivalent rate at groun level (nsv/s) Distance from isocenter to point where ose equivalent rate is D Distance from x-ray target to a point m above the roof X ray ose rate at m from target (cgy/s) Soli angle of raiation beam (steraians 3

14 B B H Φ NS NS 0 Ω.9x0 6 H Φ Ω 0 Roof shieling transmission ratio Neutron ose equivalent rate at groun level (nsv/s) Distance from x-ray targetto to a point m above the roof Neutron fluence rate at m from target (cm - s - ) Soli angle of raiation beam(steraians Measure an calculate x-ray skyshinefor an 8 MeV accelerator with no ceiling shiel Distance from isocenter s Measure photon Calculate photon Ratio (meters) rate (nsv/s) rate (nsv/s) m/c 7.5 (at wall) Measure neutron skyshine for an 8 MeV accelerator with no ceiling shiel Distance from isocenter s Measure neutron ose equivalent rate (meters) (nsv/s) Consierations for Intensity Moulate Raiation Therapy (IMRT) IMRT requires increase monitor units per cgy at isocenter Typical IMRT ratio is 5 MU per cgy, as high as 0 for some systems Percent workloa with IMRT impacts shieling 50% typically assume; 00% if vault is eicate to IMRT Account for IMRT by multiplying x-ray leakage by IMRT factor IMRT Factor % IMRT x IMRT ratio + ( - % IMRT) 3 is typical IMRT factor (50% workloa with IMRT ratio of 5) IMRT factor lower for neutrons if machine is ual energy e.g.,.5 if ual energy linac with 50% of treatments below 0 MV Simulator P KUX WUT P( sec)600i B WT N 3.3log (/ B) S N ( HVL ), HVL 0.8mmlea for5kv x rays L KUX P awt 0 sca sec W is in milliamperes x minutes Raiography 60 Fluoroscopy 300 CT F Patient scattering coefficient (a) for 5 kv x-rayfrom NCRP No. 49 Angle(eg) a

15 CT Isoose (horizontal) CT Isoose (vertical) µgy/slice µgy/slice 0 kv, 30 ma,.5 s 0 kv, 30 ma,.5 s Transmission in Steel for CT Transmission in Concrete for CT B ux B ux mm mm Transmission in Lea for CT Transmission in plaster for CT B ux B ux mm mm 5

16 HDR P B WT W γfat γ 0.48R / Ci / h / m f 0.96cGy / R A activity ( Ci) t time Brachytherapy Rooms LDR Brachytherapy P B 0.80fγA γ gamma factor f 0.96cGy / R A activity( mci) Shieling LAB (homework) en Shieling calculation for a Linac room for regular work an consiering IMRT Shieling calculation for a CT room (simulator) 6