A very high level of accuracy is needed.
Within acceptable ranges of good practice and equipment performance, the prescribed absorbed dose at the prescribed beam quality should be delivered to the planning target volume (PTV) while doses to other tissues and organs are minimized. In practice, this means that a very high level of accuracy should be striven for at all steps of the treatment chain in external beam radiotherapy.
When considering the percentage change in response for tumour control or normal tissue damage for a 1% change in dose (normalized dose response gradient [Brahme]), the figures are uncertain and depend on tumour site or normal tissue reaction being monitored. A range of 0.5% to 5.0% decrease in probability for tumour control with a 1% decrease in dose has been indicated, with corresponding values for normal tissue complications of around 2-3% [Mijnheer].
A small deviation in absorbed dose of only a few per cent from the intended one might thus mean a reduced probability for tumour control or an increased probability for normal tissue complications [Dische]. Since there are many steps in the treatment chain, each step should be kept very accurate.
By identifying, quantifying, reducing and taking into account all geometric variations in the treatment chain.
When a tissue volume is judged to be likely to contain tumour cells, and a decision has been taken to attempt to cure the patient using radiotherapy, the targeted volume should be optimized. The optimization process is crucial to the potential success of the treatment, and should ensure that this targeted tissue volume has a high probability of being included in the volume receiving a high absorbed dose (treated volume) at all times, while surrounding healthy organs are excluded from significantly high dose (irradiated volume) as much as possible.
The process of optimization of external beam radiotherapy should take into account geometric variations in the location of the target and organs at risk throughout the treatment chain by creating a planning target volume (PTV) and one or several planning organ at risk volumes (PRVs). The geometric variations in the whole radiation treatment chain (such as organ motion and patient set-up variations) all contribute to the size of the margin around the tissue volume where a high dose is required. The identification and quantification of these variations is necessary for the creation of a precise and safe margin.
Modern conformal radiation therapy should not only aim to conform the treated volume to the PTV, i.e. increase the isodose sculpting ability in treatment planning and execution, but also to conform the PTV to the clinical target volume, i.e. reduce geometric variations in the treatment chain, and to conform the clinical target volume to the gross tumour volume, i.e. improve clinical visualization methods of tumour cells. Once the variation components are known and minimized, they have to be combined to an overall margin to ensure the inclusion of tumour cells in the treated volume.
By studying and following the relevant local, national and international guidelines on QC.
According to the International Organization for Standardization (ISO) [ISO9000], QC is the regulatory process through which the actual quality performance is measured, compared with existing standards and finally the actions necessary to keep or regain conformance with the standard. It should be emphasized that in addition to controlling the performance of radiotherapy equipment, it is vital to also control the performance of radiotherapy processes.
The actual parameters to control, and how and when this should be done might be nationally regulated. There might also be recommendations by professional organizations, international bodies or local hospital rules. You need to find out what directly applies in your hospital and country.
Thorough acceptance tests and commissioning procedures must be carried out before new equipment is taken into clinical use. QC is then applied to ensure that relevant parameters of the equipment maintain conformation with what has been found at these acceptance tests and commissioning measurements. The IAEA is developing guidelines and recommendations for the setting up of national programmes in radiotherapy dosimetry [IAEA TECDOC 1040] and for the physical and technical aspects of QC in radiotherapy [IAEA TRS 430]. Overall, QC should be involved in all steps of the radiotherapy process in order to maintain confidence in the continued quality of the patients’ treatment. It is also important that results from all tests are adequately documented.
You ensure this by measuring absorbed dose according to an established protocol within a national/international framework.
To ensure that the absorbed dose is equivalent in different clinics, and thus that experience and evidence in dose versus effect can be shared between clinics, it is necessary for experts in radiation oncology physics to determine the absorbed dose according to established protocols. Some countries have national protocols, while others follow international protocols such as the IAEA code of practice [IAEA TRS 398].
Equipment for this dose determination (e.g. ion chambers, electrometers) should be regularly calibrated in such a way that the calibration can be traced to a primary standard dosimetry laboratory (PSDL) through secondary standard dosimetry laboratories (SSDL), e.g. the IAEA/WHO network of SSDLs. Since these laboratories compare results, 1 gray (Gy) at one site will be very close to 1 Gy at another site. Regulatory authorities should establish the calibration frequency, where applicable.
Dose quality audits and follow-up programmes are powerful tools to ensure that there is a continued consistency in dose determination. The IAEA/WHO provide an independent quality audit of radiation dose delivery in radiotherapy hospitals.
A good start is to learn from past accidents.
Lessons learned from many accidents point to some general conclusions. Accidental exposures occur when there is an insufficiency in:
Counteracting these conditions will lead you a long way towards minimizing the risk of accidents in your clinic. Help in finding the solutions can be found in some key documents [IAEA SRS 17, ICRP86].
Yes, and resources are freely available.
A good way to become informed of why and how accidents might happen is to read the freely downloadable IAEA reports on investigations from accidental exposures [Costa Rica, Panama, Poland]. Even if, in these cases, the equipment, the staffing and procedural set-up is different in your clinic, there can be valuable lessons to learn. This is especially the case, when one is prepared to keep an open mind and is ready to see beyond the details of the specifically presented accident.
In addition to these reports, sometimes accounts of accidents are published in scientific journals or disseminated through other means. By sharing this information with others, clinics help to make radiotherapy safer.
Lack of symptoms is, unfortunately, no guarantee that all patients are treated as intended.
Unfortunately acute symptoms of overexposure might only be showing when it is too late to do something to correct the treatment. It should also be kept in mind that situations of underexposure might only be seen in terms of lower-than-expected cure rates of patient groups after many years. In this type of situation, a vast number of patients might be affected by an accidental underexposure before it is discovered, as seen in one actual case [Ash and Bates] in the figure.
Incidents in radiotherapy should be monitored and approached as important events.
When considering the risks associated with radiotherapy, it should always be taken into account that the patient also has potential benefit from the radiotherapy. The combination of the probability of harm occurring and the consequence of that harm constitute the risk the patient is subjected to from a particular hazard in dose administration.
While the consequence of an incident might be small for the individual patient, the probability of incidents occurring is much greater than the probability of major accidental exposures. Therefore, incidents can still pose a substantial risk in the radiotherapy clinic. One should keep in mind the high level of accuracy for the individual patient required for good practice in radiotherapy.
Another reason for the importance of learning from incidents is that many incidents can have a variable magnitude (e.g. for Patient 1, the mistake causes a dose deviation of 5%, while for Patient 2, the same type of mistake causes a dose deviation of 50%). One should also learn from the potential incidents, or 'near misses'. These are usually even more numerous than the actual incidents, and might correspondingly enable the clinic to find safety critical steps in the treatment chain, and strengthen the defence where it is needed.
Not only should one therefore search for, correct, monitor and learn from incidents in the own clinic, but when an opportunity is given one should learn from incidents in other clinics too. This can be done through a voluntary anonymous incident reporting system shared with other clinics (e.g. Radiation Oncology Safety Information System: ROSIS).
You should follow the procedure that you have set up before the event happened.
All clinics using cobalt treatment units should have a procedure for this situation. This procedure should be written, clearly displayed, available to and known by all relevant staff, and regular practice sessions should be held. While it is the local clinical responsibility to devise a procedure that is relevant for the local circumstances, there are some general elements to consider when action is taken in the event of a cobalt source not returning to its safe position at the end of treatment.
It is important to note that the points below might not be in the order carried out in your clinic, and they might not cover all aspects according to your clinic's local rules.
When considering the great importance of getting the patient out of the primary beam, it should be remembered that the dose received by staff is relatively low under normal circumstances when they are avoiding the primary beam. Make sure that all emergency equipment is available at the unit; that the periodic checks are performed (e.g. every morning); and that the unit functions well before taking it back into clinical use after source stick.