Quality assurance is, for radiotherapy, "all those procedures that ensure consistency of the medical prescription and the safe fulfilment of that prescription regards dose to the target volume, together with minimal dose to normal tissue, minimal exposure of personnel and adequate patient monitoring aimed at determining the end result of treatment". This is the WHO definition. In less words, it refers to the processes which ensure patients receive the intended treatment, and staff are not exposed unnecessarily to radiation. QA is most frequently referred to in terms of linear accelerator testing, but also refers to other parts of the patient journey.
Components of a QA Program
The process of quality assurance should be managed by a multidisciplinary team including medical physicists, radiation oncologists, radiation therapists, nursing staff and administration. This team should meet regularly to discuss current QA processes, any new data that may have come to light, and implementation of QA on new technologies.
Specific QA Areas
Dosimeters need quality assurance! The typical process is to send a department dosimeter to a central labroratory at ARPANSA, which then calibrates it with a centralised ionisation chamber. This dosimeter is then used in the department to calibrate other dosimeters, including those of other types. The main dosimeter should be checked regularly with ARPANSA to ensure it remains stable.
Linear accelerators are the cornerstone of modern radiotherapy departments and most departments have several machines. QUality assurance is divided into:
- Acceptance testing: When the machine is installed, a number of tests to ensure it meets the department's specifications are performed. These include:
- Mechanical tests (does the machine work)
- Energy tests (does the machine do what it should - eg. multiple electron energies)
- Safety tests (is the machine and bunker properly shielded)
- Commissioning: Detailed analysis of the radiotherapy beam characteristics for each energy are performed. This includes:
- Central axis depth dose curves
- Beam flatness and symmetry
- Beam quality
- For MLCs: Interleaf transmission, penumbra around leaf edges and ends, movement of leaves and effects on isodose distribution.
- For jaws: Effect of jaws on output factors
- For dynamic wedges: Output factors, isodose distribution
- Regular quality assurance: This is divided into daily, fortnightly and annual tests to ensure the machine is performing safely and as specified:
- Daily tests: Check of laser position, output and mechanical function of machines is carried out daily
- Fortnightly tests: Measurement of beam energy and field sizes is checked at this frequency
- Yearly tests: Full measurement of beam characteristics, similar to commissioning tests, is carried out annually to ensure the machine continues to perform as expected
Brachytherapy quality assurance is of particular importance, particularly of afterloading devices to ensure that sources do not get stuck within the patient.
Simulation, either with CT or with a simulator, involves both the scanner and any other treatment accessories (such as immobilisation, wires or bolus).
The simulator should be evaluated in a similar way to the linear accelerator, although the number of tests to perform are significantly less:
- Acceptance: Confirmation that the machine is functional (can generate CT images) and safe (minimal radiation leakage from machine and virtually none outside the room)
- Commissioning: Commissioning should determine that image acquisition is accurate and that images can be transferred to the treatment planning system. This involves testing slice thickness, pixel size, measurement of Hounsfeld Units and so on.
- Regular quality assurance: Scanners should be assessed at least monthly to ensure they are capturing patient anatomy appropriately and a full examination of all systems is required yearly
Treatment Planning Systems
Treatment planning software is highly complex and many of the functions are hidden from the end-user (therapist or oncologist). For this reason, errors in treatment planning software can potentially be dangerous, particularly when more complex plans are used (multiple field 3D CRT or IMRT). Quality assurance of treatment planning systems is essential to safe patient treatment delivery. This can be divided into four parts:
- Pre-delivery acceptance testing: It is advised that software developers test their software 'in house' using pre-defined data sets from the IAEA.
- Post-delivery acceptance testing: Prior to accepting the software, it should be tested in house to ensure it can import CT images, perform image fusion, display usable images, allow contouring and beam placement, and calculate plans.
- Commissioning: The commissioning process is complex and involves checking the calculated dose distributions with actual measurements taken in phantoms. This includes similar specifications to the linear accelerator testing. The final stage is testing generated plans on a phantom and comparing the planned versus the actual dose delivery.
- Regular quality assurance: Reliability of treatment planning software is paramount and testing should be carried out monthly.
IMRT adds additional complexity to treatment planning and the users rely heavily on the computer generated plans. It is difficult to use knowledge regarding typical beam isodose distributions when beams are so heavily modulated by the computer system. To ensure safe delivery of dose, most centres perform a phantom test prior to delivering the dose, measuring dose distribution, point doses and fluence maps of individual beams. This reassures staff that the treatment planning system is accurate. During treatment, most centres perform in vivo dosimetry as a further check.
Essential to stereotactic radiotherapy is quality assurance of immobilisation techniques. This is often achieved using a 'hidden target' technique, where a small radio-opaque target is placed within a phantom that is then immobilised using the frame or frameless apparatus for stereotaxy. This process is done repeatedly (maybe up to 50 times) to determine systematic or random errors. Error under 0.5 mm is acceptable for immobilisation as the remaining error (up to 1-2 mm) is due to other factors that are less easily controlled (e.g. dosimetry, patient movement etc).
There are less centralised guidelines regarding quality assurance of newer technologies. These technologies often require more frequent and specialised quality assurance due to potential for serious error.