Clinical/Physics in treatment: Brachy, QA ect

Question 1

3 broad types of brachytherapy:

Answer 1

Interstitial, cast/surface mould, intracavity

Question 2

Most common technique for exposure to sources in brachytherapy:
Its general features

Answer 2

After-loading technique:
Sources loaded into previously implanted tubes
“Dummy” sources for dosimetry
see Manchester/Paris/Stockholm

Question 3

General outline of a cervix applicator (e.g. Manchester)

Answer 3

Tandem = central tube, surrounded by ovoids separated by spacers (Manchester system), 1-3 radium tubes.
Sources moved into and out of tandem.I.e at least 2 applications

Question 4

Name 3 systems of dose specification in cervix brachytherapy. Which is the best

Answer 4

No system is able to meet all criteria for dose specification. Reference volume is the most informative.

1) Milligram hours (historical): Simply source strength and duration of implant with these numbers matched to extensive data for a specific applicator, loading technique and stage of disease.
2) Manchester System: Attempts to capture more spatial info using 4 standardised points (A, B, Bladder, Rectum)
3) Reference volume: Relates dose distribution to target volume. Is the isodose surface that just encompasses the target volume, with the value of this surface set at 60Gy.

Question 5

How does the Manchester system describe dose specification for cervix brachy?

Answer 5

Captures spatial vacation at 4 points

1) Point A = 2cm above cervical os, 2cm lateral to cervical canal. It is the approximate point where uterine vessels cross the ureters - the tolerance of these structures being the critical limit to dose.
2) Point B 2cm lateral to point A
3) Bladder
4) Rectum

Question 6

ICRU recommend which parameters be recorded and reported in image guided brachy?

Answer 6

Think source strength, dose absorbed, volume, duration, technique:

1) Description of technique: applicator system, source, loading, radiographs
2) Source strength: Reference Air Kerma Rate
3) Description of reference volume: i.e 60Gy isosurface encompassing target volume.
4) Absorbed dose at reference points: Bladder, rectum, lymphatic trapezoid, pelvic wall
5) Time-dose pattern

Question 7

Define the ICRU recommended quantity for specify source strength in brachytherapy? Name the quantity the US use?

Answer 7

Reference Air Kerma Rate: Kerma rate to air (dry) at a reference distance of 1 meter, corrected for air attenuation and scattering (i.e. in vacuo).
Americans use Air Kerma strength (see Khan pg 340)

Question 8

What is the purpose of “bolus” or “build-up material” in radiation therapy?

Answer 8

There are 2 uses for bolus/build-up material in RT
1) Make surface anatomy/skin more uniform by flattening or compensating for missing tissue (remove air cavities) within the treatment volume to allow coverage to conform better with the target volume.

2) Modifying the dose at depth and at the skin surface. For example by applying a bolus with thickness equal to the depth of the build-up region, the skin-sparing effect of megavoltage X-Rays can be removed.

Question 9

Define and explain the concept of the following contours that may be drawn as part of the RT planning process –

• Gross Tumour Volume (GTV)
• Clinical Target Volume (CTV)
• Internal Target Volume (ITV)
• Planning Target Volume (PTV)
• Organs at Risk (OAR)
• PRV *****

Answer 9

GTV := The detectable (by palpation, direct visualisation or imaging) extent of malignant tissue
CTV := The expected extension of undetectable disease beyond the GTV
PTV := A volume encompassing the CTV that anticipates movement of the CTV due to movement by the patient, the treatment system, or the tissues containing or adjacent to the CTV.
ITV := An internal margin added to the CTV to account for its physiological movement or variation in its size or shape
OAR := Critical structures with normal tissues whose radiation sensitivity may influence radiotherapy planning
PRV := Analogous to PTVs relationship with CTV, accounts for movement/geometric variations of an OAR

Question 10

Benefits of CT planning?

Answer 10

○ Excellent spatial localization of patient anatomy including contours and inhomogeneities
○ Good differentiation between bone, soft tissue, air and fat
○ Isodose distributions based on attenuation data from CT
○ 3D
○ rapid acquisition
○ Virtual simulation

Question 11

Name the key pieces of localisation equipment for PT simulation:

Answer 11

● Lasers are used in simulation to assist patient positioning and ensure accurate treatment setup. Usually 3- roof (sagittal) and 2 laterals.
● Skin tattoos are used as reference points but care should be taken as skin is mobile relative to deep structures
● Setup SSD should be recorded
● Portal imaging (usually orthogonal images) are taken and referred to during treatment setup.

Question 12

What are the basic requirements of immobilisation equipment?

Answer 12

Immobilization equipment

● Should be comfortable and reproducible.

Question 13

List 7 most common pieces of immobilisation. Note construction materials, and what it

Answer 13

● Kneefix- shaped foam block. Provides comfort and helps maintain position.
● Vacuum bag- filled with polystyrene beads. When air is removed it becomes fixed in position. Comfortable, inexpensive immobilization device.
● Thermoplastic mask- low melting point plastic mould. Lightweight, perforations allow patient to see and breathe. Limits head movements.
● Breast board- head and buttock rest. armrests with supports allow patients to raise arms above their heads for treatment. Arm positions can be set in place for reproducibility.
● Butterfly board- for thorax treatments. Headrest with armrests overhead, settings similar to breast board.

Question 14

What is the PRV?

Answer 14

Margin around the OAR to account for internal movement and treatment machine inaccuracies.

Question 15

What is the ICRU reference point?

Answer 15

ICRU reference point:
For 3D CRT, ICRU recommends reporting the dose at a single point within the PTV.
1) Point should be clinically relevant,
2) Easily defined
3) within a region of uniform dose.
4) It should be at the center of the PTV and at the intersection of the radiation beam axes if possible.

Question 16

In conventional simulation, individual treatment field geometries are determined:

Answer 16

In conventional simulation, individual treatment field geometries are determined at the time of simulation.

All field data are recorded, and simulator images are taken for each field as a record of the intended treatment and for use in designing field shapes. Target and other data are often drawn or annotated on each film or digital image.

Question 17

The main downside of 2D planning:

Answer 17

The planning process is faster and often possible to perform by hand.

The main downside is less accuracy as planning is done on one slice and structures above and below the slice are assumed to be similar.

Question 18

Steps of 2D planning:

Answer 18

1) Pt placed in Tx position +/- immobilisation devices
2) Pt moved to align target region or reference anatomy with isocentre (SSD/or SAD technique chosen)
3) Simulator mimics gantry, couch movements and uses Dx XR to simulate beam.
4) Simulator radiographs are taken for comparison with treatment port films and planning
5) At least one transverse film is taken for planning, requires transfers contour (might require a marker)
6) Marks placed on pt and/or immobilisation device (remember this step when talking about use of vac bags ect).
7) All information (field sz, gantry and couch positions, ect recorded).
8) 2D computer planning system used for dose distributions.

Question 19

Steps of 3D planning:

Answer 19

1) Pt placed in Tx position +/- immobilisation devices
2) CT data set of the region to be treated. This takes into account dose distributions caused by scattered from multiple planes.
3) Markers on Pt and/or immobilisation device.
4) All information recored.
5) The tumour, target volumes and organs at risk are drawn on CT slices (using computer planning systems). +/- MRI &/or PET fusion to help delineate soft tissues/tumour eg. brain.
6) Beam geometries, field limits and shielding are determined and loaded onto the treatment machine for treatment delivery.

Question 20

How does IMRT work?

Answer 20

Involves subdividing the incident beams into smaller segments (beamlets) and modulating each by the use of MLCs to achieve a non-uniform fluence within the field.

Question 21

Contrast the use of MLCs in 3D CRT versus IMRT:

Answer 21

3D CRT uses MLCs to make the beam shape conform to the tumour shape but the field fluence remains uniform.

In IMRT MLCs control non-uniform fluence. Delivered each both the multiple beam angles/segments using non-uniform fluence combines to allow a much more overall optimised dose distribution.

Question 22

What are the modes of IMRT dose delivery?

Answer 22

1) Step and Shoot: After each segment, beam is turned of and MLCs/or gantry move to next position
2) Sliding window IMRT: While beam is on MLC move across the field. The time dependent position of the MLCs determine the field fluence.
3) IMAT: Leaf pattern changes continuously as the gantry rotates.
4) Tomotherapy: Spiral CT like - fan beam collimated by binary collimator:
- Serial tomotherapy rotates a fan beam once around the patient with the couch fixed.
- Helical tomotherapy the fan-beam rotates while the couch moves towards the gantry.

Question 23

What type of planning is used in IMRT?
What is the process?

What is needed after?

Answer 23

Inverse planning = planning starts with outlining the end result and working backwards to find the best way to achieve it.

In IMRT:
1) Target volumes and organs at risk are contoured,

2) The plan is assigned certain descriptors eg. maximum dose to OAR and minimum dose to PTV, and each descriptor ranked according to relative importance.
3) Optimisation: These factors are incorporated into a mathematical objective function, which guides the optimization process. An algorithm then finds the beam arrangement and fluence that best fits the desired plan.

After planning: Treatment verification.. Also QA ect.

Question 24

IMRT requires additional commissioning and QA checks to maintain patient safety, including:

Answer 24

IMRT requires:
● Testing of dynamic MLC to ensure stable leaf speed, accuracy of leaf position readings and dose profile between leaves.
● Testing of dosimetry including interleaf transmission and increased head scatter
● Treatment verification to ensure that the computer designed plan accurately represents the dose distribution within the patient
● Regular QA including verification of dose prior to start of treatment, daily checks of dose to a test point in each field, weekly checks of dose distribution based on gantry and collimator positions, and annual checks of all machine features

Question 25

Important anatomical patient data to collect for treatment planning includes:

Answer 25

Important anatomical patient data to collect for treatment planning includes:
● Patient surface contour
● Internal structures including target volumes, organs at risk and inhomogeneities.

Question 26

Compare (pros and cons) CT to MRI in terms of planning:

Answer 26

CT:
Very accurate axial anatomical information and precise localization of internal structures.
As reconstructed based on attenuation data, and is useful for heterogeneity corrections on dose calculations.

MRI:
Soft tissue contrast superior to CT but suffers from less spatial accuracy.
Practical problems:
1) The physical dimensions of the MRI scanner also limits use of immobilization devices and compromise Tx positions.
2) Long scaring times - movement artefacts.

Question 27

How does PET work?

Main drawback for planning?

Answer 27

PET localizes areas of increased glucose metabolism and can indicate areas involved with malignancy which may not be apparent on CT and MRI.

Its main drawback is poor resolution and requires fusion with CT to determine the exact position of internal structures.

Question 28

For SXR, How does increasing beam energy effect:
Skin dose
DMax
PDD

Answer 28

Dmax at skin

PDD

Question 29

100 KeV = HVL?

Answer 29

100 keV beam = HVL 2.07 mm Al

Question 30

150 KeV = HVL?

Answer 30

150 KeV = 1.5mm Cu HVL

250 KeV = 3mm Cu HVL

Question 31

250 KeV = HVL?

Answer 31

250 KeV = 3mm Cu HVL

Question 32

Examples of bolus:

Answer 32

1) Flab/superflab
2) Wax (e.g dental wax)
3) Wet gauze or combine
Wet gauze is the least accurate of the bolus types, but is capable of conforming to unusual shapes such as the ear or nose with relative ease. Care must be taken to ensure that air gaps within the wet gauze are minimise

Question 33

The PTV aims to encompass:

Answer 33

1) Random error

2) Systemic errors - though these should be identified an eliminated prior

Question 34

ALARA Principle

Answer 34

As low as reasonably achievable
Takes into account dangers of radiation but also the economics and practicality of radiation protection.

1) Maximise distance (inv sqr law) Staff and public
2) Minimise time: e.g. through practice with dummy
3) Shielding: Should be cost effective and should reduce the dose rate to non-significant levels.

Question 35

The fucking ICRP put out a super boring document in 2007 out lining the principles of radiation protection.

What are they

Answer 35

3 principles, 2 relate to source and apply to all situation, 1 principle relates to the individual and applies to planned exposures.

1) Justification - any DECISION that ALTERS the RADIATION EXPOSURE SITUATION should do more good than harm.
2) Optimisation of Protection: Limit probability of exposure, Limit magnitude of potential exposures, limit number of people exposed
3) Application of dose limits: 1mSv/year public, 20mSv/year averaged over 5 years (and not more than 50 in any 1 year)