5. Treatment planning, radiation protection

Treatment planning
- Imaging is used to form a virtual model of the patient, including the tumour
  - Native CT is almost always used, because the physical interactions between the radiation and the tissue is the same in native CT as in radiotherapy
    - This means that a native CT contains the dose-absorbing properties of the tissues of the patient
  - By using image registration and image fusion, multiple imaging modelities may be combined, if needed
    - Image registration refers to “matching” multiple imaging modalities by mapping the coordinates of anatomical structures on the different modalities, so that they “match” on top of each other
    - Image fusion refers to displaying multiple modalities on top of each other after image registration
    - MRI provides good differentiation between different soft tissues
    - PET provides good information of functionality and metastases
    - Ultrasound cannot be used for treatment planning
- Computer systems allow for simulation and calculation of how different radiotherapy approaches would deliver radiation to the tumour and the surrounding tissues
- Modern techniques allow for even more precise radiation planning
  - Intensity-modulated radiation therapy (IMRT)
  - 3D conformal radiation therapy (3DCRT)
  - Intensity-modulated arc therapy (IMAT)
  - Image-guided radiation therapy (IGRT)
- Volumes in radiotheapy planning
  - Gross tumour volume (GTV) = the volume of the macroscopic tumour
  - Clinical target volume (CTV) = the GTV + microscopic, un-imageable tumour spread
  - Planning target volume (PTV) = the CTV + uncertainties in planning or delivery
Fractionation of radiation therapy
- Most cancers respond based on the total (cumulative) amount of radiation, not the size of the individual doses
  - However, side effects are mostly related to the sizes of the individual doses
- As such, the sizes of the doses can be increased or decreased
- Hypofractionated radiation therapy
  - When the total dose of radiation is divided into larger but fewer doses
  - Used for cancers which are sensitive to large individual radiation doses
  - Treatment course is completed quicker
  - Often used in breast cancer and prostate cancer
- Hyperfractionated radiation therapy
  - When the total dose of radiation is divided into smaller but more doses, often given more than once a day
  - May produce fewer side effects
  - Used for cancers with high turnover, like SCLC