Radiation oncology

From greek.doctor

Radiation physics

  • Types of radiation used
    • Natural radiation (due to radioactive decay)
      • Alpha radiation (two protons and two neutrons)
      • Beta radiation (high-energy electrons)
        • Electrons (beta radiation) has low penetration power, so beta radiation is only suitable for treating tumours near the skin surface
    • Gamma radiation (high-energy photons)
      • Photon radiation (gamma and X-ray radiation) has high penetration power, so it is suitable for treating deep tumours
    • Artificial radiation
      • X-rays (gamma radiation which is artificially made)
      • Heavy ion radiation
        • Heavy ions, like carbon ions, are accelerated to high speeds
        • Proton radiation (alpha radiation) and heavy-ion radiation don’t penetrate tissues deeper than a certain point
          • This makes these types of radiation suitable for treating tumours where radiation-sensitive tissue is very close, for example inside the skull
          • They’re also suitable for treating tumours close to the skin
          • This effect is due to the so-called “Bragg peak” on the percentage depth dose distribution
  • Particle therapy
    • Accelerated protons or heavy ions are sent into the tumour
    • Very expensive
  • Compton-scattering is the most important radiation effect on tissues in radiotherapy
    • The photoelectric effect, pair production and coherent scattering are less important
  • The inverse square law
    • The law states that the intensity of radiation decreases in an inverse square ratio with the distance from the source of the radiation
    • For example
      • If the distance from the source is doubled, the radiation intensity is reduced to one fourth
      • If the distance from the source is increased three-fold, the radiation intensity is reduced to one ninth
    • The inverse square law is important in treatment planning in brachytherapy
  • Percentage depth dose (PDD)
    • The PDD is the percentage of the maximum dose which is deposited in tissue (or something, it’s confusing)
    • PDD is important in treatment planning in teletherapy
  • Squamous cell cancers are generally very radiosensitive
    • Adenocarcinomas on the other hand, are not
  • Radiation biology
    • Radiation induces double-stranded breaks in DNA
    • The cell is most sensitive to radiation in the M and G2 phases of the cell cycle
    • The oxygen effect
      • According to the oxygen effect, normoxic tissues and cells are more radiosenstive than hypoxic ones
      • The oxygen effect occurs because oxygen “stabilizes” or “makes permanent” the DNA damage produced by reactive oxygen species
      • In hypoxic tissues, the DNA damage is not made permanent and can therefore be repaired by the cell

Radiation therapy

  • Types of radiotherapy
    • External beam radiation therapy (EBRT)/teletherapy
      • The radiation source is outside the patient
      • The source can be rotated around the patient, allowing the radiation beam to target the tumour from a variety of directions
      • Types
        • Conventional external beam therapy
        • Three-dimensional conformal radiotherapy (3DCRT)
        • Intensity modulated radiation therapy (IMRT)
        • Stereotactic radiosurgery/radiotherapy
    • A multileaf collimator (MLC) is used to shape the radiation beam
      • Especially used in IMRT and 3DCRT
    • Internal radiation therapy/brachytherapy
      • The radiation source is inside or very close to the patient, as close to the tumour as possible, or even inside the tumour
      • Can be temporary or permanent
        • Temporary brachytherapy – the radiation source is placed and then removed after some time
          • Most common
        • Permanent brachytherapy – a small radiation source is permanently placed into the patient
          • Also called “seed implantation”
          • The radioactive “seed” loses its radioactivity after some months, but won’t be removed
      • Can be high dose-rate (HDR), low dose-rate (LDR) or pulsed dose-rate (PDR)
        • HDR reduces the treatment time, and is most commonly used
        • HDR = dose rate of more than 12 Gy per hour
          • Treatment typically lasts a few minutes
        • LDR = dose rate of less than 2 Gy per hour
          • Treatment typically lasts 24 hours
        • PDR = short pulses of radiation are given
          • Treatment typically lasts 24 hours
      • Types
        • Intracavital brachytherapy
          • Into the cervix, bronchi, etc.
        • Interstitial brachytherapy
          • Into the breast, prostate, etc.
    • Systematic radiation therapy
      • An isotope is injected into the patient, by itself or attached to a specific molecule
      • The isotope will travel to the tumour and irradiate it from inside
      • The isotope often gives off alpha or beta-waves, as these waves don’t travel far in the body
        • Alpha waves only travel 100 µm
      • Examples
        • Radioactive iodine given for thyroid cancer
        • Radium-223 given for bone metastases
  • Equipment used in brachytherapy
    • Afterloading
      • The technique where a machine (an afterloader) is used to deliver the radiation source into the patient during brachytherapy
      • This eliminates the need for a person to deliver the radiation source, eliminating radiation exposure for that person
        • Manual delivery of brachytherapy is seldom used for this reason
      • Often used with HDR, sometimes called HDR Afterloading
      • 192Iridium is often used as radiation source
  • Equipment used in teletherapy
    • Cobalt unit
      • Older type of teletherapy
      • The external beam is generated using 60Cobalt
    • Linear accelerator (LINAC)
      • More modern than cobalt unit
      • Most commonly used device for external beam radiation therapy
      • The external beam is generated using linear acceleration
      • A multileaf collimator allows precise modification of the radiation field
      • Used for stereotactic surgery, intensity modulated radiotherapy, particle therapy etc.
    • Gamma knife
      • Used for stereotactic radiosurgery in the brain
    • Tomotherapy
    • CyberKnife

Treatment planning

  • 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