5. Treatment planning, radiation protection

From greek.doctor
  • 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