RANZCR-Like PYS Qs Flashcards
Four systems that can be used to avoid and detect dose delivery errors are:
• Record and Verify Systems;
• Interlocks;
• Select and Confirm; and
• Imaging
a. Describe in detail each of the 4 systems above.
Include a specific example in your description of each system.
(2018, Q5A)
4.5 marks
1) Record and Verify System
Ensures that the planned treatment is delivered in a similar manner every fraction
○ Includes daily measurements of (pick 1 e.g):
■ MU
■ beam energy
■ beam mode (photons/electrons)
■ jaw positions
■ collimator, gantry and couch angles
■ wedging
■ SSD
Compared with/linked to LA control system, treatment planning system. If any parameter outside of tolerance treatment cannot proceed until issue resolved.
2) Interlocks
Emergency shutoff mechanisms that are triggered by certain events
○ eg (pick 1)
- last man out button
- emergency stop button
- positioning interlocks: prevents beam on unless collimator, gantry and couch are in correct positions
- beam interlocks: prevents beam on unless jaws and MLCs are in the correct positions.
3) Select and Confirm:
Ensures correct treatment parameters. When a setting is selected, mechanical changes are checked to have occurred before treatment continues.
○ System also checks that the field correlates with the mechanical positions of the field, collimation and energy
○ Discrepancies are highlighted
4) Imaging: Used to identify and correct problems arising from inter- and intrafractional variations in patient setup, anatomy and organ and tumour movement.
e.g CBCT, KV portal imaging
Most modern radiotherapy techniques rely on computer-controlled treatment delivery. This has reduced many of the random human-based errors but has introduced the potential for less common, but more severe, treatment delivery errors.
b. List 5 potential errors that can arise specifically at the time of radiation treatment.
(2018, Q5B)
2 marks
1) Patient movement
2) Changes in anatomy e.g bladder volume at treatment versus plan
3) Mechanical error. E.g sagging of treatment head changing isocenter
4) Radiation error:
Sievert is the standard international unit of radiation absorption. Define in words and explain the rationale for its use in radiation protection.
(2018, Q5C)
2 marks
The seivert (J/Kg) was devised to allow radiation absorption from different sources to be described by a single unit. An EQUIVALENT DOSE is achieved by weighting absorbed energy from different sources by a WEIGHTING FACTOR that reflects differences in the BIOLOGICAL EFFECT (during whole body radiation) of different sources. Similarly, a TISSUE weighting factor can then be applied to reflect different radio sensitivities of the absorbing tissue to arrive at an EFFECTIVE DOSE.
Give millisievert (mSv) estimates for the following radiation exposure examples:
i. Average population background radiation exposure per year
ii. Chest X-ray
iii. Diagnostic CT Chest
iv. PET/CT
v. Maximum allowed occupation exposure of radiation workers per year (averaged over 5 years)
(2018, Q5D)
2 marks
i. Average population background radiation exposure per year = 3 mSv/year per person
ii. Chest X-ray= 0.1 mSv
iii. Diagnostic CT Chest = 7 (Abdo = 8, head =2)
iv. PET/CT = 25!!
v. Maximum allowed occupation exposure of radiation workers per year (averaged over 5 years) 100 mSv in 5 years
Brachytherapy is a treatment modality that can potentially expose staff to radiation.
i. Explain the three main principles used to minimise radiation exposure to staff.
(2018, Q5E i)
3 marks
3 main principles used to minimise radiation exposure to staff:
1) Time: Reduce the amount of radiation absorbed by using safe procedures that minimise the time of exposure and accurately record exposure time.
2) Distance: Inverse square law implies significant reduction in exposure by increasing distance from sources where possible.
3) Shielding: Attenuate exposure by use of shielding either as boundaries, encasing sources, or worn by staff.
A patient is receiving HDR brachytherapy for a locally advanced cervix carcinoma. Following completion of treatment, the radioactive source is unable to be remotely removed from the tandem and remains in the patient.
ii. Give three practical examples of how each of the above three principles could be implemented in this brachytherapy emergency situation.
(2018, Q5E ii)
(1.5 marks)
1) Time: Well-practiced procedure for extracting source from tandem. Can be preformed quickly.
2) Distance: Technique for removal can be done at distance. e.g forceps
3) Shielding: Patient in shielded room.
On the same set of axes, draw two (2) percentage depth dose curves comparing 6MV and 10MV photons for a 10 x 10 cm field in a water phantom. Include surface dose, depth of maximum dose (Dmax) and depth of 50% dose (D50).
(2019, Q11A)
3 marks
X-axis - depth (cm), 5cm intervals
Y-axis %Dose
Table:
6Mv: Surf = 20%, Dmax=1.5cm (D90 = 4.5cm), D50=15.5
10Mv: Surf 12.5%, Dmax=2.5 (D90 = 5cm), D50 18
Also know 15Mv…
i. Using a table, compare a fixed source surface distance (SSD) radiation therapy technique to an isocentric radiation therapy technique with regards to the following characteristics for a multifield treatment: • Source surface distance for each beam • Speed of setup • Set up accuracy • Beam delivery time
(2019, Q11B i)
2 marks
For multi field:
Fixed SSD:
SSD is FIXED for each beam (i.e.. may need to repos patient). Therefore SLOW to setup for multi beam. Less accurate (relies on skin markers). Constant beam delivery
Isocentric: SSD changes but isocenter the same. Faster to setup (pt does not need to move). More accurate (does not rely on skin, rather machine accuracy). Beam delivery time variable
Describe two (2) advantages of prescribing to a fixed SSD technique rather than an isocentric technique for single field treatment.
(2019, Q11B ii)
1 mark
1) Advantage when target is at surface and fall off at dose (PDD) is key variable - simple dose calculation
2) Does not require expensive aids
3) Faster setup
Parallel opposed beams are the simplest technique when using a combination of fields.
i. Draw an isodose chart using equally weighted parallel opposed fields 6MV photons, 100cm SSD, 10 x 10 cm field, 30 cm separation prescribed to midplane. Include 50%, 90%, 100% and Dmax% isodose lines.
(2019, Q11 C i)
3 marks
3 marks down the toilet
ii. How would your isodose chart differ if 10MV photons are used?
(2019, Q11 C ii)
1 marks
1) Dose becomes more evenly distributed with higher energies.
2) less dose at skin/deeper
In practice, multifield radiation therapy can be delivered as a conformal or modulated techniques dependent on clinical requirements.
i. Describe the physical basis and the role of multi-leaf collimators (MLCs) on how dose is delivered for an IMRT beam compared to a traditional 3D conformal radiation beam.
(2019, Q11 D i)
2 marks
In 3D conformal radiotherapy a conformal dose is achieved by shaping the treatment field at each planned beam angle using motorised computer controlled MLCs. After they have moved into position dose is then delivered.
In IMRT MLCs are not static during dose delivery. Rather by moving across the field the are used to modulated fluence. Allowing for nonuniform dose delivery across the field.
For IMRT plans, describe the process of inverse planning following patient data set acquisition to plan acceptance.
(2019, Q11 D ii)
3 marks
1) 3D image acquisition and processing
2) Images loaded into planning system
3) Target volume delineation and organs at risk contoured.
4) Constraints and weights given to inverse planning/optimisation algorithm.
5) Iterative algorithm determines optimal plan given constraints and weights.
6) Planner reviews output, may modify constraints/weights if targets not met and re-iterate.
7) Optimised plan accepted after review process.
Describe how an electron beam for clinical use is generated in a linear accelerator. (An annotated diagram may be used but is not required).
(2018, Q2, a)
3 marks
1) Electron Gun produces modulated/timed pulse of electrons.
2) Klystron/magnetron produces pulsed microwaves (timed with gun via modulator)
3) Electrons enter accelerator waveguide with typical energy around 50KeV, timed with microwaves. Electrons accelerated by EMF of microwaves
4) High energy electrons in pencil beam enter bending magnet (90 or 270 deg), beam is focused.
5) Thin scatter foil of high Z material broadens beam, makes fluence more uniform. 2nd thin filter of low z material may be added.
6) Ion chamber - Intensity and beam uniformity
7) Jaws - opened wider than applicator opening to prevent scatter
8) Applicator - define treatment field
i. For an electron beam, briefly explain how photon contamination occurs and include the magnitude of its effect on patient dose.
(2018, Q2, b i)
1.5 marks
Electron beam interactions with any part of the treatments system (from source to applicator) can produce Bremmstahlung photons.
This contribution can be assed in the asymptotic part of the PDD curve.
Generally for 6-12Mev contributes 1%, 12-15MeV 2%
