Overview Traditional Shielding Issues ● Shielding Common Isotopes ● Shielding Issues ● Exposure Rates

PET Shielding Issues

• Preparing a Lab for F-18 FDG Imaging Patient Issues

• Recommended Shielding • Portable Shielding • PET/ CT Issues • Meeting regulatory requirements can be expensive • Work with equipment vendor, facility architect, and qualified medical physicist

Traditional Shielding Issues

• Shielding Common Isotopes:Tc-99m, Ga-67, In-111, Xe-133, I-123, Tl-201 • Most isotopes require only 1/8 to 1/4 inch of lead • Shields are not bulky • Distance is a good ALARA practice also • Only other problematic isotope is I-131.

1. Partially because of higher energy (0.365 MeV) 2. Partially because of high activity (10 – 300 mCi)

• Additional shielding was needed in Hot Lab • Shielded syringes are useful • Some institutions added ¼ - ⅛ inch of shielding in walls • We all became fairly comfortable...

PET Shielding Issues ...Changes Occurred

• Prior to ~ 2000, PET had been limited primarily to only institutions with a cyclotron on site

• Technology has improved • PET has become much more common • Shielding changes were needed to accommodate PET scanning

Positron Emission Tomography

• PET uses annihilation radiation, produced when a positron collides with a negatron (electron), for Nuclear Medicine imaging

• This collision always produces two 0.511 MeV photons emitted at a 180o angle to each other

Shielding 0.511 MeV Photons

• Photons with energy this high are difficult to shield • Higher penetrating ability • Increased radiation (Compton) scatter

Radiation sources requiring shielding

• Doses • Patients

Preparing a Lab for F-18 FDG Imaging

• We all know that exposures from PET doses have the potential to be high... How high is high??

Dose Rate from Injection

• Positron-emitting isotopes have high gamma ray dose constants (G) • Hand dose can be very high • Dose rate 5 cm from unshielded syringe with 555 MBq (15 mCi) of F-18 is 33

mSv/hr (3300 mrem/hr)

Reducing Hand Dose

• Tungsten syringe shield can reduce hand dose by 85%. Downside: extra weight (0.75 kg) can make it difficult to inject

• Lead glass window in syringe shield can increase whole body dose

Other options:

• Automatic dispensing systems • Divide responsibilities among staff

• Many good products exist: Good ideas can come from vendors and other PET facilities.

• Radiation Safety and PET: The $20K Solution

Dose Rates from Patient

• American Association of Physicist in Medicine (AAPM) task group published the patient dose rate for F-18:

The expected dose rates from a patient are relatively high

=3.4Rxcm2mCixhr

PET Shielding Considerations: F-18 Injection & Uptake

• Patient Injection: either in Hot Lab or Uptake Room with a typical dose: 370 -740 MBq (10-20 mCi)

• FDG Uptake: Patient waits 60-90 minutes post injection before imaging is initiated • Activity will decay while waiting. Remaining activity 0.83, 0.68 and 0.57 for wait

times of 30, 60 and 90 minutes, respectively

F-18 Scanning

• Patient Void: Patient clears ~15% just before scanning • Patient Scan: Whole-body PET scan for 30-60 minutes

Radiation Safety: Areas of Concern

• Adjacent offices

• Adjacent hallways

• Rooms with radiation sensitive equipment (i.e. cameras, uptake systems)

• Storing Patients:

Waiting rooms will probably need additional shielding

• Dose to employees • Dose to visitors • High background for imaging • Patients wait 30-90 minutes • Transient Patients • Some departments do not have a waiting area near injection room • Radiation from patient may have effect on radiation sensitive equipment (increased

background)

1. Scintillation well counters 2. Thyroid uptake systems 3. Scintillation cameras

How Much Shielding Is Needed For PET? ALARA considerations

● Many facilities have set ALARA limits to be 10% of the applicable limits for radiation workers, 5 mSv/yr (500 mrem/yr)

The More Lead, the Better?? ● Not exactly true- consider the law of diminishing returns ● Yes…it will reduce exposure rates ● Yes…it is $$$$

Factors Affecting Radiation Protection

Each institution must find a happy medium between safety and cost ● Number of patients imaged ● Activity administered per patient ● Length of time patient remains in facility ● Location of department within facility

Physicist performs calculations based on:

● Number of patients imaged ● Activity administered per patient ● Length of time patient remains in facility ● Location of facility relative to controlled and uncontrolled areas

Suggestions for amount of shielding needed to keep doses ALARA

● Imaging room will most likely need additional shielding ● Many facilities add ½ inch lead in walls facing occupied areas (offices, busy

hallways) ● Some could add only ¼ inch if they do not expect a large patient load (realistic??) ● Some facilities only add ¼ inch lead in walls facing semi-occupied areas

• Don’t forget floors and ceilings- If patient load is high enough, shielding is needed

Photograph illustrating sheets of lead installed during construction of imaging room.

Photograph illustrating shielded space for Nuclear Medical Technologists in PET Imaging facility- Leaded Glass Windows and Lead in Walls.

Portable Shielding:

● Pb shields 2.5 cm and 5.0 cm thick are available providing dose reduction factors of 40 and 1900, respectively

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Problems ● Patient may move in relation to gantry ● Shield can limit access to patient

Portable Shield Warning

● Traditional portable shields are not adequate- only have a few mm of lead ● Won’t do much to stop 0.511 MeV photons

PET/CT Shielding

• PET uses high levels of high energy photons • CT uses higher levels of lower energy photons (x-rays) • If shielded for PET – no additional shielding needed • If shielded for CT only – additional shielding will be necessary