Topic 19: dose measurement + QA in radiotherapy

Question 1

Why is dose distribution by X-ray within tissue important?

Answer 1

• Used to determine dose at specific patient volume

Question 2

How is dose distribution measured?

Answer 2

• Using tissue equivalent phantom:
  > Has same atomic number as tissue
  > Same e- density as tissue
• Same density as tissue

Question 3

Why is water used inside the phantom?

Answer 3

• Must be possible to move ionization chambers within phantom

Question 4

Describe isodose charts

Answer 4

• Dose distribution plotted in form of isodose chart
• Isodose lines = line of constant dose
• Expressed as % of max dose

Question 5

Describe effect of depth to dose

Answer 5

• Dose will fall with depth = radiation weaker > deeper beam goes = some energy absorbed + spread via inverse square law

Question 6

Describe the effect of low/energy beams

Answer 6

• Low energy = edges diffuse = blurry due to compton scattering = hard to target tumors
• High energy = edges better defined = good focus for tumors

Question 7

Explain the skin-sparing effect at higher energy

Answer 7

• High-energy beams don’t give max dose at skin surface = they go deeper = skin gets less damage = better for the patient
• At higher energy = 100% dose line below surface of phantom = spot where the full treatment dose hits = inside the body = protects the skin + shallow tissues

Question 8

What is the relationship between depth of peak + photon energy?

Answer 8

• Depth at which the dose is highest = peak = is about ¼ of the energy in megavolts

Question 9

What is the main advantage of high energy therapy?

Answer 9

• Surface dose only 20-50%
• Protects skin + targets tumors deeper

Question 10

Describe treatment plan

Answer 10

• Graphically describes dose distribution when 1+ beams converge on treatment volume
• Treatment volume = volume that has to be treated
• Treated volume = volume actually treated
  > Planning target volume
  > Clinical tumor volume
  > Gross tumor volume

Question 11

Give the criteria for good dose distribution

Answer 11

1) Dose throughout treatment volume uniform ±5%
2) Treated volume should be as close to PTV
3) Dose to treated volume = exceed dose by at least 20%
4) Dose to sensitive sites e.g. eyes/spine = be below tolerance dose

Question 12

How can the criteria be satisfied?

Answer 12

• By using several beams
• Small as possible
• Enter patient as close to treatment volume as possible

Question 13

Describe the steps for LINAC planning

Answer 13

• CT for image guided radiotherapy taken
• Targets defined
• Treatment planned by medical physicist
• Physician verify plan + approve for use

Question 14

Give the actions planned by medical physicist

Answer 14

• Choose radiation beam direction
• Choose beam type + energy
• Conform radiation for each beam
• Execute treatment plan simulation
• Verify plan

Question 15

Describe dose distribution of skin

Answer 15

• Skin = curved = beam enter skin at not 90◦ = alter dose distribution
• Solutions:
  1) Curvature corrected
  2) Distribution corrected
• At low energy = no skin-sparing effect = skin can be built up = using tissue-equivalent bolus = give surface at right angles
• At high energy = bolus not used = mathematical corrections applied

Question 16

Explain the use for tissue compensator

Answer 16

• Placed in beam remote from skin
• Suitably shaped attenuator makes up for missing attenuation of tissue
• If large volume at surface needs to be treated = distribution altered via wedge-shaped compensator
• Wedge = attenuate beam at thick end = tilt isodose curve

Question 17

Why are beam intensities optimized?

Answer 17

• Achieve best coverage of tumor + save healthy tissue

Question 18

What is the use of collimators?

Answer 18

• Spare sensitive organs

Question 19

Where are isodose curves plotted?

Answer 19

• On the anatomy = MRI/CT images

Question 20

Describe IMRT

Answer 20

• Targets + sensitive organs defined = computer finds best plan
• IMRT = delivers radiation beams in multiple arcs
• Uses = sophisticated inverse planning software + multileaf collimators = shape radiation beam + modulate beam intensity
• Delivers optimum dose

Question 21

Describe the simulation

Answer 21

• Diagnostic X-ray set = same mounting + couch movements as treatment accelerator
• Radiographs taken = provide info on treatment area for planning = check for accuracy
• Can be used for localization of sealed sources = brachytherapy

Question 22

Describe positioning

Answer 22

• No point in making accurate plan if patient positioning cannot be guaranteed
• Patient = placed in correct position in relation to beam = remain stationary during treatment
• Patient shell may be used to position = used in 30% patients = head + neck

Question 23

Describe the aspects of QA

Answer 23

• Commissioning of new equipment
• Interface between various equipment
• QA = daily/weekly/monthly/yearly
  DAILY:
• Constancy
• Ionization chamber
  WEEKLY:
• Profiler
  MONTHLY:
• Profiler
• Ionization chamber
  YEARLY:
• Phantom
• Water