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
- 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
- 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
- Imaging is used to form a virtual model of the patient, including the tumour
- 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
- Most cancers respond based on the total (cumulative) amount of radiation, not the size of the individual doses