Lecture 8 Adaptive Radiotherapy

Question 1

Why we need adaptive therapy?

Answer 1

-Tumour shrinkage
-Tumour change shape
-Change in relative position between OAR and tumour
-Change in patient geometry

Question 2

Describe the process of offline adaptive.

Answer 2

Start of fraction —> IGRT —> rigid registration —> if not acceptable —> ReCT + replan / re-optimization —> plan QA —> next fraction

Question 3

Describe the process of online adaptive.

Answer 3

Start of fraction —> IGRT —> deformable image registration —> re-optimization —> QA checks —> treatment delivery

Question 4

Describe the process of automatic image registration in terms of similarity index.

Answer 4

The similarity index is calculated for stationary image and moving image. The optimization algorithm tries to maximize the similarity index by changing deformation vector field (DVF). This process is done iteratively until the similarity index reaches its target.

Question 5

Sum of square differences is sensitive to ______________ and works well with noiseless image from ________________.

Answer 5

Image intensity ; same modality

Question 6

Mutual information is a metric for _______________.

Answer 6

Multi-modality image registration

Question 7

What is the assumption and theory of mutual information.

Answer 7

It assumes regions of similar tissues in one image corresponding to regions in other image. Regions are defined in a ‘feature space’. When the images are correctly registered, the join histogram of the 2 images be maximized.

Question 8

What is the theory of demons algorithm for DIR?

Answer 8

It assumes that pixel representing the same anatomical points on each image have the same intensity.
A regular grid of forces deforms the iso-intensity contours derived from the diffusion equation. The diffusion equation is solved iteratively

Question 9

How do MRI systems and LINAC system affects each other?

Answer 9

MRI affects LINAC:
- beam generation
- beam penetration
- impact of Lorentz force on secondary electrons

LINAC affects MRI
- B field inhomogeneity
- radio frequency interference

Question 10

What has been ammended to combine MRI and LINAC

Answer 10

• active magnetic shielding such that at the accelerator gun and tube is B=0
• superconducting coils are put aside and gradient coil is split to allow beam passage
• additional magnetic sources added to create combined magnetic effect independent of LINAC position
• correct schemas for gradient non-linearity and B0 inhomogeneity to improve spatial integrity

Question 11

What is the problem of electron return effect?

Answer 11

Increase dose in tissue-air interface