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
- 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