Further PPQ Flashcards
Services in the Nuclear Medicine department of your hospital
Trust are expanding to include a new radiopharmaceutical
labelled with Lu-177, a beta emitting radioisotope. You are
planning some external dose rate measurements in order to
assess the retention pattern of the new radiopharmaceutical
and establish radiation protection advice following hospital
discharge of the patient. In terms of potential radiation hazard
(external exposure or contamination) which groups of people
and what dose constrains and dose limits would you consider in
your assessment?
Under IRR - Staff and members of the public. Dose constraints for staff depend on classification. For classified staff the wholebody dose limit is 20mSv and condstraint is 3/10 of this at 4mSv. Site specific dose investigation levels may be in place to ensure that staff are not receiving too high radiation exposures.
For members of the public the limit is 1mSv.
For comforters and carers their exposure limits are the same but if likely to exceed the 1mSv then must be justified under IRMER.
Take dose rate measurements at different times to model clearence for the patients.
Briefly describe the steps required in order to implement
IRR-17 regulations before staring a clinical service for
radionuclide therapies in your hospital Trust.
- Notify the HSE of the use of the radionuclides.
- ensure there are relevent ARSAC licenses
- Ensure appropriate risk assessments are in place.
- Local rules written and boundaries established for designated radiation controlled area.
Briefly describe the requirements related to the EPR-16 regulations with reagrds to establishing Lu-177 therapies.
Alteration to the permits for of unsealed radioactive sources on site. And the radioactive waste being disposed of. New BAT statements would need to be established and using the IRAT tool exposures to the public and the environemtn must be considered.
In the context of radiological optimisation, briefly describe the methodology for receiver operating characteristics (ROC) analysis.
- Define a diagnostic task that is clinically relevent to this imaging, and create a dataset of images, some with and some without the abnormality
- Generate the known ground truth aither by other imaging or by pathollogy with experts.
- Have multiple radiologists review each image and rate with how much confidence they can make a call either way.
- Plot the ROC curve using the observe ratings to calcule true positive rates (sensitivity) and false positive rates (1-specificity) at various thresholds.
- Calculate area under the curve = 1=perfect - =0.5 = random guessing/no diagnostic value.
A new optimised radiological protocol “method B” has been
tested during a pilot ROC study indicating diagnostic
improvements over the default “method A”. As part of the
same study, another protocol “C” performed very close to
‘no predictive value’. Indicate on a graph the anticipated
shape of the ROC curve for each protocol. If protocol B relies
on similar patient radiation exposure as in A, while protocol
C introduces significant radiation dose reduction, briefly
explain your potential recommendation with regard to
introducing these to clinical service.
Cyclotron produced 99mTc will potentially result in an
increased effective dose to the patient Why is this?
(2
Due to impurities in the Tc99m that is produced Tc94, 95 and 96 emit higher energy gamma and some beta. Therefore depositing a higher radiation dose.
State the imaging parameters you would use to acquire data
for a standard renogram examination. Explain your
reasoning.
Radiopharmaceutical - 40MBq Tc99m Mag3. Use weight based for children.
Dynamic acquisition over 20-30minutes. 1-2 seconds per frams for the first minute (vascular phase). 10-15s/frame after for excretion phase.
Have the patient sitting with the camera face pressed up to their back.
Use LEHS to ensure maximal counts during acquisition. Energy window 140keV 10%.
64x64 image matrix size.
In the context of quality control of Nuclear Medicine
equipment, briefly discuss the advantages and disadvantages
of the use of the National Electrical Manufacturers
Association (NEMA) standard.
Advantages: Standardised repeatable tests. They show the best possible performance allowing for comparison acros make/model of camera.
Dissadvantages: Not representative of clinical imaging, difficult to perform the imaging / fill the phantom. Specialist phantoms and analysis software may be required.
Briefly discuss the use of NEMA as part of routine testing of
a gamma camera. What would be a pragmatic approach to
routine testing? Give two examples of a performance test
where this pragmatic approach would apply
Nema tests provide a basline for performance during acceptance testing and periodic QA.
E.G. Intrinsic uniformity:
Nema: uses a point source in air and calculates integral and differential uniformity.
Pragmatic version: Uses a co-57 flood source to analyse uniformity. Performed daily or weekly to monitor detector consistency.
- Spatial resolution: Nema- uses capillary sources and percise setups.
Pragmatic version: Use a bar phantom or line source, visually inspect for blurring or distortion.
How should a gamma camera room be designated under
IRR’17? Discuss your reasoning.
The room should be designated as a controlled area, due to the risk of receiving a radiation dose while in the room. Radiopharmaceuticals will be used in the room.
An RPA should be consulted as to whether or not the room will require shielding to be installed. This will depend on neighboring rooms and their occupancy and will then influence them as potentially being designated as supervised areas.
A radiopharmaceutical is to be transported to another
hospital. Describe the steps that would need to be taken in
order to correctly label the container.
The container must have an appropriate label containing:
- The transport index, which is based on the surface dose rate, and therefore the transport category. The activity at time of disbatch.
- Indication that the contents are radioactive trefoil with correct colouring and wording, the radionuclide present and activity at a reference time of disbatch.
Describe the potential benefits of Time-of-Flight in PET
imaging.
Caused by better timing resolution of the detectors.
- improved spatial resolution which leads to improved SNR
~Faster convergence of iterative recon algorithm.
Describe the advantages of iterative reconstruction over
filtered back projection in SPECT imaging.
- Helps to reduce noise
- ## Improves contrast and therefore lesion detectability.
Describe three potential uses of CT data in the
reconstruction of PET or SPECT images.
Attenuation correction
Anatomical Localisation
Motion Correction
Describe a potential imaging protocol for carrying out
tumour dosimetry of a patient treated with Lu-177 PSMA.
Gamma photon energies are 113keV (6%) and 208keV (11%).
**
SPECT/CT should be used to acquire the images for post therapy dosimitry.
Duel energy window settings with scatter windows adjacent to each peak for scatter correction.
4-6 hours post injection, 24 72 and 120 hours.
image matrix 128 x 128 with 360 degree spect with 120 projections.
MEGP collimators are best to reduce septal penetration of the 208kEv gamma photons.
Use low dose CT acquisition, OSEM iterative reconstruction. Do a reconstruction of both photopeaks (duel-energy) which may improve quantification.
Generate ROI and time activity curves and perform voxel based dosimitry.
What are the advanatesg and disadvantages of the proposed lu-177 imaging protocol
Advantages - quantatative accuracy from multiple time points imaging allows lu-177 kinetics to be modelled, improving dose estimates.
- Anatomical correlation, SPECT/CT helps provide precise tumour localisation and delineation.
- Using both photopeaks improves image quality and quantification, especially when using scatter correction.
Dissadvantages:
-Multiple scans is a large patient burden.
- Lu-177 has low abundance of gamma photons leading to a low count rate and potentially noisey images.
- Additional radiation dose from CT
- Heavy resource intensive using multiple imaging slots.
A patient receives a therapeutic administration of 90Y
microspheres.
State a gamma-camera technique for obtaining a
quantitative image of the activity distribution post-therapy
administration. Discuss the limitations of the technique.
Technique - Bremsstrahlung imaging SPECT/CT
Brem radiation is produced as the beta particle interacts with tissue, allowing for indirect imaging using a wide energy window. 5-250keV.
poor resolution, low quantitative accuracy, high noise and artefacts, need for complex corrections.
A patient receives a therapeutic administration of 90Y
Describe two alternative techniques for generating an image
of the absorbed dose (dose-map) from such a scan. Include
any assumptions associated with each technique. Discuss the
relative merits of each approach.
Voxel-based dosimetry
Use quantitative brem SPECT/CT images to 3d map activity and apply dose kernals to compute 3D dose distribution.
Assumptions: beta particles deposit energy at the site of origin, activity quantificiation is accurate.
Merits: patient specific dose distribution, higher spatial resolution than alternative, allows assessment of dose heterogeneity (liver tumours)
Limitations: Depends heavily on the quality of images and corrections & complex processing.
MIRD schema (organ-based dosimetry) - Uses average activity within organ volumes from imaging and applies standard s-values to compute absorbed dose.
Assumptions: uniform activity distribution & standard anatomical models are valid.
Merits: Simple, fast widely accepted. Suitable for whole-organ dosimitry when high resolution detail is not critical.
Limitations: ignores dose heterogeneity, less personalised.
A patient receives a therapeutic administration of 90Y
microspheres.
The patient has previously received external beam
radiotherapy to the liver. Which radiobiological parameter
could be calculated to allow comparison of the doses
delivered to the normal liver from the respective
treatments? In order to carry out the calculation, which
parameters would need to be known or estimated?
Biologically Effective dose from the radiotherapy.
Dose rate and effective half life of radionuclide therapyy. Relative biological effectiveness of therepy radionuclide.
Explain the steps required to determine a detriment (cancer
risk) to populations exposed to ionising radiation.
- Determine lifetime cancer incidence risk estimates
Use epidemiological data (e.g., A-bomb survivors, radiation workers) to estimate cancer risk over a lifetime.
- Apply Dose and Dose Rate Effectiveness Factor (DDREF)
Adjust high-dose data to account for lower risk at the lower doses and dose rates typically encountered in most exposures.
- Transfer risk estimates across populations
Adapt data from the studied population (e.g., Japanese A-bomb survivors) to other populations, accounting for differences in baseline cancer rates.
- Adjust for lethality
Factor in the probability of fatal vs. non-fatal cancers when estimating detriment.
- Adjust for quality of life
Consider the impact of non-fatal cancers and severe hereditary effects on the quality of life.
- Adjust for years of life lost
Take into account the reduction in lifespan due to cancer or hereditary effects.
- Population Averaging
Average risks across both sexes and different age groups, using weighted averages of Excess Relative Risk (ERR) and Excess Absolute Risk (EAR).
- Weight different cancer types appropriately
Different methods are used for different organs/tissues, e.g., EAR model for breast and bone marrow, ERR model for thyroid and skin.
- Summarise into a single detriment figure
For example, 5.5% per sievert for detriment-adjusted cancer risk, 5% per sievert for fatal cancer risk.
D = L (Rf + Rnf)
Provide a qualitative explanation for how health detriment
data is used when calculating Effective Dose for a given
exposure.
Tissue weighting factors (wT) are based on population-averaged health detriment.
Reflects the combined risk of fatal, non-fatal cancer and hereditary effects.
Provides a single value for stochastic risk.
Allows comparison of different exposures.
What are the problems with using Effective Dose to calculate
the risk to an individual patient?
Averaged across population, not individual-specific.
Based on data with high uncertainty at low doses.
Assumes LNT model, which may not be accurate.
Needs adjustment for real individual patient risk.
Give two conditions of compliance that are included in an
Environmental Permit for Radioactive Substances.
- Keep adequate records of sources accumulation and waste. Ensure maximal levels are not breached.
- Notify the EA if the permit is breached.
- Ensure there are records of locations of each source and unique ID numbers.
- Records of radioactive waste and excretions.
- Must have BAT statements to minimise activity and volume of all disposals of waste & to minimise their impact on the wider environment.
Give one other condition that still applies for substances that
are exempt from the need for an Environmental Permit.
Adequate records of Radioactive sources and their activities and locations they are kept.
- Follow conditions for disposal e.g. VLLW.
- Remove radioactive waste labels before disposal.
How can exemption categories be used for a site with a permit
Lower activity sources which are exempt can be managed seperetley to the permit. For example a low activity sealed source that is exempt can be purchased without alterations to the permit. So long as they comply with adequate record keeping and storage.
