Lecture 5 Dose Calculation Algorithms And Helical Tomography

Question 1

What are the features of helical tomotherapy

Answer 1

• fast binary MLC
• continuous rotation
• continuous couch movement (can treat up to 135 length)

Question 2

What are the components of a tomo machine?

Answer 2

• linac gun
• jaws (one set, 23 cm tungsten)
• MLC (64 in total, 32 on each side)
• detector
• beam stop (12.7cm Tungsten = x2 10VL)

Question 3

What is the SAD and bore size?

Answer 3

Both 85cm

Question 4

How many beam angles are optimised in tomo?

Answer 4

51 beam angles

Question 5

The couch velocity is __________.

Answer 5

Constant

Question 6

What are the dimensions of tomorrow MLC?

Answer 6

10cm thick
2mm wide on linac side
3mm wide on patient side

Question 7

What is the time required for tomo MLC to switch between open/closed states?

Answer 7

10 msec

Question 8

What are the jaw options?

Answer 8

1.0 / 2.5 / 5.0 = 3 different settings

Smaller jar size, longer treatment time
Larger jaw size, less conformality

2.5 is the most common

Question 9

What are the planning parameters can be manipulated in the tomo planning system?

Answer 9

• jaw width
• tightness of helix (ie. Pitch)
• modulation factor
• DVH goals

Question 10

What is the dose of MVCT image acquisition?

Answer 10

1-3 cGy

Question 11

What is the benefit of radixact couch over traditional tomo couch?

Answer 11

Radixact couch have independent axes which can eliminate cobra motion and reduce couch sagging
(5mm vs 2mm)

Question 12

With reference to TG148, what are the QA parameters?

Answer 12

• 85 SSD
• 1.5cm depth
• FS = 5cm x 40cm
• 10s warmup added on delivery side
• configured to 1000MU/min

Question 13

What is the equation for pitch in tomo?

Answer 13

Couch travel distance per rotation divided by field width

Question 14

What do pitch <1 means:

Answer 14

Overlap from one rotation to the next
(More tightly wound helix)
Usually must smaller than one

Question 15

How many beamlets do a single gantry rotation have?

Answer 15

51 (projections) x 64 (beamlets per projection)
= 3264 beamlets

Question 16

What is a leaf sinogram?

Answer 16

A leaf sinogram s a 2D representation of the leaf pattern necessary to deliver dose to the tumour

Question 17

What are the setting user input?

Answer 17

• patient image set
• IVDT
• display and use options for structures
• field width
• pitch
• blocking
• modulation factor
• dose calculation grid

Question 18

What do the minimum planning scan distance beyond the target account for?

Answer 18

• irradiation region
• divergence
• penumbra

Question 19

What is IVDT?

Answer 19

Image value to density table
= the IVDT converts CTT numbers to mass density

Question 20

How can minimise thread effect?

Answer 20

1. Set up patient with target closer to the central axis
2. Use smaller numbers for the pitch (0.86/n)

Question 21

What is modulation factor (MF)?

Answer 21

= the modulation factor I a number that the user enters to influence the balance between efficiency and conformality

= upper limit on the range of open times for ‘used’ beamlets

Conformality: unequal weighting of beamlets for treating the target and avoid sensitive structures

Efficiency: the most efficient way to deliver all the dose is to weight all the beamlets equally

Question 22

What is the formula for modulation factor

Answer 22

Max open time divided by average open time

Question 23

What are typical MF value

Answer 23

Prostate = 1.8
HN = 2.4

Question 24

What are the effects when adjusting the MMF

Answer 24

Increasing the MF increases delivery time, decrease efficiency

Decrease the MF may degrade dose conformity

Question 25

How to reduce delivery times?

Answer 25

• use the largest field width that produce an acceptable plan
• use the smallest modulation factor that produces an acceptable plan

Question 26

Will adjusting the pitch affecting the treatment time?

Answer 26

Th pitch does not affect the treatment time as long as 12 sec <gantry period<60 sec / rotation (gantry speed)

Question 27

What should we do before machine quality asssurance

Answer 27

• turn the machine on and wait until the machine water temperature reaches 40 degree
• run a 5 min warm up
• run an air scan