14.2 - Evaluation Of Radiation Protection

Dose Minimisation

Area survey

An area survey is carried out after installation of a linear accelerator and is part of the acceptance testing. The medical physicist must examine all areas outside of the bunker for radiation levels when the linear accelerator is operating. For linacs unable to produce photons over 10 MeV energy, the area survey is carried out with a Geiger-Muller tube (to detect the presence of leakage radiation) and an ionisation chamber (to quantify the amount of radiation if it is present). High energy linear accelerators must also be tested for neutron leakage using a neutron survey meter.

Personal radiation monitors

Personal radiation monitors may be TLD badges, film badges or electronic devices.

Badge dosimeters

The most commonly used type of dosimeter is the TLD badge. This is a small thermoluminescent dosimeter carried by each member of staff when they are working in the department. As part of radiation safety practices, the TLD in every badge is replaced at 3 monthly intervals and checked for absorbed dose.
The purpose of TLD badges is not to protect staff from acute radiation exposure. Instead, regular checks of TLDs identify areas where work practices are leading to unacceptable doses to staff members. These practices may then be reviewed and changed to reduce staff exposure.
The other use of TLD badges it to identify when staff are approaching recommended tolerance doses of radiation. If staff members reach or exceed these guidelines, they are reassigned to work in areas with no risk of radiation exposure.

Electronic dosimeters

In some situations it is necessary to measure exposure during a procedure, particularly during brachytherapy emergencies or spills of unsealed sources. In this situation a small electronic dosimeter is used. This device usually contains a silicon diode dosimeter and gives a real time dosage readout.

Control of radiation areas and sources

Control of radiation areas and sources serves two functions:

  • Prevention of inadvertant exposure to the public, staff and patients
  • Prevention of theft of radiation sources
  • Prevention of inappropriate use of radiotherapy equipment

Control of radiation areas

There are several methods to prevent people from accidentally wandering into 'hot' areas.

  • Areas where radiation may be present should be appropriately signed
  • Computer software should activate warning lights when radiation is present
  • Interlocks at entry points into radiotherapy areas should be present, including:
    • Infrared light beams that trigger if the beam is interrupted
    • Doors which shut when radiation is active
    • Emergency 'off' if a door is opened during radiotherapy delivery
    • 'Last man out' buttons which prevent activation of radiotherapy devices until triggered
    • Emergency 'stop' buttons located at numerous locations throughout the bunker and control room in case the machine needs to be stopped immediately.

Prevention of theft

It is the responsabilty of staff to adhere to government and international regulations when handling radioactive sources, in order to prevent their theft and use for non clinical applications.
Sources are most likely to be stolen:

  • During transport
  • During periods of inactivity of the facility
  • After a facility is closed
  • Following decomissioning of a source

Permanent implanted sources are also kept within a patient.
Similarly to drugs of addiction, a count of sources should be performed when one is added or removed from the safe. The safe should be secure and alarmed. The sources present in a facility should be recorded by their type and serial number.

Prevention of unauthorised use

Devices which may deliver radiation should only be used by accredited staff. To prevent unauthorised use, keys, passwords or other locking systems should be in place to restrict access.

Design of rooms / bunkers

The design of a bunker for a radiotherapy source or machine is dependent on the energies and type of radiation used.

Linear Accelerator / Teletherapy Bunkers

Linacs are capable of producing photons energies from 4 MeV to 18 MeV depending on the model. For lower megavoltage energies, the major concerns are:

  • Sufficient shielding of the primary radiation beam
  • Sufficient shielding of scattered radiation
  • Sufficient shielding of leaked radiation from the linac head

The bunker wall is important in controlling the both primary, scattered and leakage radiation. When determining the thickness of the walls, the highest energy produced by the linear accelerator should be considered. Walls are typically made of normal concrete, but may be replaced at great expense by denser material to conserve space (eg high density concrete is 30 times more expensive than normal density concrete).
The primary barrier of the wall intercepts radiation produced by the linear accelerator that does not interact with matter, and is typically the thickest part. For linacs capable of producing photon energies over 10 MeV, it should be 240 cm thick. 60Co teletherapy units need a primary barrier thickness of about 130 cm.
The secondary barrier of the wall attenuates scattered and leaked radiation. Scattered radiation is typically in the kilovoltage range and is less of a concern, whereas leakage radiation may be of higher energy (but accounts for only 0.1% of the radiation prodcued in the linac). The barrier thickness for leakage radiation is usually applied, unless the HVL for scattered radiation approaches the HVL for leakage radiation in which case an extra HVL is added to the thickness. High energy linacs need a concrete wall at least 120 cm thick for adequate shielding, whereas 60Co units only need 65 cm of concrete.
The maze of the bunker connects the bunker to the rest of the facility. The maze prevents scattered or leakage radiation from travelling directly out of the bunker, as it must undergo attenuation or further scattering to reach the exit.
When photon energies climb above 10 MeV, it is possible for photodisintegration to take place. This adds extra complexity to bunker design, as neutrons may scatter more readily than photons (making a maze by itself inadequate) and also create radioactive substances which can accumulate in the room. These problems are overcome by:

  • A neutron door blocks the exit of the maze, and contains about 12 cm of borated polyethylene (a good neutron absorber). The distant side of the door contains 2.5 cm of lead shielding to attenuate photons generated in the polyethylene.
  • The bunker must be adequately ventilated, cycling air in the room at least 6 times per hour, to prevent buildup of radioactive gases from photodisintegration and neutron interactions. Radioactive byproducts in the linear accelerator and couch are short lived and pose no significant health problems.

Radiation monitoring instruments must be present in linear accelerator bunkers. A normal survey meter is capable of picking up scattered photons, whereas a specialised neutron survey meter must be used in bunkers containing a high energy linac.
Bunkers must be adequately signed with radiation warnings to prevent inadvertant exposure of members of the public and staff.

Superficial X-Ray Bunker

Superficial X-Ray beams have much lower HVL/TVL values than megavoltage accelerators. The are also more likely to undergo photoelectric attenuation. Bunker walls can be much thinner than for linac bunkers (in the order of 10 - 20 cm of concrete only), and lead glass can be used to allow the therapist to visualise the patient without the need for video cameras. Neutron production does not occur.

Brachytherapy Bunker

Low dose rate brachytherapy sources, if kept within a safe and handled behind localised shielding, do not require any special bunker designs.
High dose rate brachytherapy sources must be used within an adequate bunker. The entire bunker must have sufficient 'primary shielding' as brachtherapy sources may be oriented in any direction and typically give out radiation circumferentially. Bunker design must take into account the maximum photon energy released by the source; in the case of 192Ir this is 1.06 MeV and shielding should reduce the dose to areas outside the room by 3 TVL. Wall thickness of about 70 cm is adequate.
A maze should be used to reduce scatter outside of the treatment room. Alternatively, a lead shielded door can be used where space is at a premium. Rooms must be adequately ventilated.

Unsealed Source Bunker

Unsealed sources are typically electron emitters or kilovoltage energy emitters. They have similar bunker requirements to superficial x-ray bunkers (thin walls). Importantly, they should be well ventilated to prevent buildup of radioactive chemicals in the air and cleaned after every patient.
Special mention should be made of material used in the bunker. It should be non-reactive and easily cleanable in the event of a spill.

Dose limits to foetus

The foetus should be considered a member of the public, and therefore dose guidelines for pregnant workers should reflect this.
Recommended dose limits to the foetus are:

  • 5 mSv maximum equivalent dose
  • 0.5 mSv monthly equivalent dose

Standard guidelines place pregnant workers on duties that avoid most sources of radiation. For example, a pregnant radiation therapist might be assigned to planning, but not simulation or treatment. This would mean the dose received by the foetus would be negligible over the background dose rate.
The latest ICRP summary (Report 103) presents evidence that other foetal effects (lethal damage prior to implantation, congenital malformation during organogenesis and mental retardation from exposures 8 - 15 weeks) have threshold doses of at least 0.1 Gy. If the foetus is exposed to doses higher than these levels, consultation with the pregnant woman to discuss therapeutic termination of pregnancy should be performed.

Avoiding foetal dose in pregnant patient

In some situations radiotherapy must be delivered to the pregnant woman. In this scenarios it is important to take foetal dose into account and minimise it where possible.
Strategies (from Stovall et all) include:

  • Use of customised shielding where appropriate (5 HVL usually sufficient)
  • Simulation of treatment with a custom phantom to estimate foetal dose and judge efficacy of shielding
  • Modification of treatment / shielding as pregnancy progresses

Whatever measures are taken, the total dose to the foetus should be documented for the entirety of treatment.

Documentation of Radiation Incidents

There are two processes by which radiation incidents are reported.
Firstly, the radiation safety officer should be informed. This is often one of the medical physicists. They will make an assessment of the incident, the likelihood for harm and submit a report detailing this to the radiation safety commission (the actual name varies depending on the state). A pre-made form is available for this purpose, listing issues such as whether a patient was involved, the estimated dose received and so on.
At my hospital, we also submit an electronic risk management form. This gives feedback to the radiation safety officer, the state/territory radiation safety commission, and the hospital board.