Plan Eval and QA

Question 1

What is the Homogeneity index?

Answer 1

Homogeneity Index (HI) is an objective tool to analyse
the uniformity of dose distribution in the target
volume.

HI = (D2-D98)/Dp
is a homogeneity index proposed
in ICRU-83, where D2 and D98 represent the doses
received by 2% and 98% volumes of PTV,
respectively.

Question 2

What is the Conformity Index?

Answer 2

CI is defined as ratio of volume of the body
receiving the prescribed dose (Vp) to the volume of
the PTV receiving the same dose (PTVp),
CI = Vri/TV
e.g. V95%/PTV

Question 3

What are the advantages and disadvantages of the CI?

Answer 3

adv: simplicity and integration of multiple parameters
disadv: Cannot provide practical information, also require visual analysis

Question 4

What is QA?

Answer 4

procedures the ensure consistency and safe fulfilment of the medical prescription
ensure dose to the target volume, minimal dose to normal tissue, minimal exposure of personnel and adequate patient monitoring

Question 5

Why is QA good?

Answer 5

reduces uncertainties and errors in dosimetry, treatment planning, equipment performance,
treatment delivery…, thereby improving dosimetric
and geometric accuracy and the precision of dose delivery

This improves RT results raising tumour control rates as well as reducing complication and recurrence rates

allows reliable inter-comparision of results among RT centres

Question 6

Sources of Error?

Answer 6

• Equipment/ software (tolerances and error logs documented)
• Natural variability (internal and set-up errors included in margins, daily imaging helps further reduce)
• Human error (random, transcription/ omission/ oversight)
• Data transfer

Question 7

What are some DICOM issues?

Answer 7

coordinates, movements and scales

Question 8

What is our responsibility as RT’s?

Answer 8

responsible for accuracy of information recorded in medical charts

• we calculate and document the relevant treatment parameters
• adhere to safe practice procedures

Question 9

What are the pre-planning checks?

Answer 9

CT sim
Primary and secondary datasets are imported into TPS
RT checks prior to treatment planning

Question 10

What are the checks after planning?

Answer 10

treatment plan done
Treatment plan checked by second RT
Treatment plan checked by physicist

Question 11

What are the additional checks for QA?

Answer 11

Day 1 checks
weekly checks
Boost/ Phase 2 checks
Final treatment checks

Question 12

What is the QA checklist?

Answer 12

Treatment patient
correct site
plan matches prescription
total dose, fractionation
daily dose
treatment machine correct
beam type and energy
OAR dose
isocentre moves
MLC shielding
appropriate inhomogeneity correction applied
correct bolus
target volume and field size correlate
DRR generated to the correct isocentre

Question 13

What are the QA tools?

Answer 13

manual/ hand calculation
departmental programs-spreadsheets (excel)
Commercial software (IMSure QA software, MU check)

Question 14

What are the Physics checks for IMRT/VMAT?

Answer 14

• Validate IMRT parameters
• More complex and time consuming
• Dose delivery in IMRT dependent on accurate position of MLC leaves
• Important to verify leaf position
• 2D,3D and 4D verification

Question 15

What is patient specific QA?

Answer 15

• depicts the information of variation in planned and measured dose in the PTV
• Point dose measurements (Checks dose at isocentre)
• Verification of dose delivery of separate beams using 2D, 3D or 4D detectors.
• Gravitational errors affecting MLC present

Question 16

What is 2D verification?

Answer 16

• Check Fluence maps (MLC position)
• Analyse planes of view
• Limitation of 2D array (mapcheck) is inability to capture rotational dose information

Question 17

What is the 3D portal imager?

Answer 17

• Evaluate the agreement between predicted and measured images similar to 2D dosimetry software
• Improved workflow efficiency
• PER fraction 3D, Adaptivo dosimetry, Dolphin QA (measures real time patient output)

Question 18

What is the VMAT QA?

Answer 18

Arc check: displays BEV dose distribution throughout entire arc.

Question 19

What is in vivo dosimetry?

Answer 19

• In Vivo dosimetry refers to measuring the dose received by the patient during treatment. This is different to in vitro dosimetry, which refers to most other physics measurements in phantoms.
• Used in TSET and TBI

Question 20

What are the advantages of In vivo dosimetry?

Answer 20

It allows comparison of prescribed and delivered doses and thus provides a level of radiotherapy quality assurance that supplement port films and computational double checks

Question 21

What are TLD’s?

Answer 21

• Can be placed on to record patient doses

- Place on patient, record results, perform calculations, inform physicist if it exceeds

Question 22

What are silicone diodes?

Answer 22

• small and instantaneous readout

- Limitation: need to be connected to electrometer

Question 23

What is MOSFETS?

Answer 23

• metal oxide silicon field-effect transistors
• small and quicker readout than TLDs, do not need power source
• Limitations: Expensive

Question 24

RVR

Answer 24

Remaining Volume at Risk

Difference between the volume enclosed by the external contour of the patient and that of the CTVs and OAR on the slices that have been imaged

(Similar to NTT principle)

Question 25

Give (3) reasons why reviewing RVR is important for evaluating IMRT/VMAT plans

Answer 25

1) Could be unsuspected regions of high absorbed dose within the patient that would otherwise be undetected (AKA dose dumping)
2) Useful for estimating risk of late effects
3) Important for younger patients who can expect a long life span

Question 26

Stage 1 Optimisation

Stage 2 Optmisation

Answer 26

Generate fluence map

Segments

Question 27

When would in-vivo dosimetry be useful?

Answer 27

• pacemaker

- pregnant patients