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Second cancer risk, concomitant exposures and IRMER(2000)

In this issue we publish a Commentary by Dr Roger Harrison [1] on second cancer risk following radiotherapy. Harrison looks at the evidence from various sources, including an NRPB summary, and the complexity of comparing risk in the radiotherapy patient population with that in the normal population. He also addresses the particular problem of "concomitant dose" in radiotherapy planning and verification that has been highlighted by other authors in articles in BJR, particularly Waddington and Mackenzie [2] and Munro [3].

The Ionising Radiation (Medical Exposures) Regulations (IRMER(2000)) [4] require justification of all medical exposures [Reg:6(1)]. Under the optimization regulation [Reg:7], the Practitioner is also required to "ensure that exposures of target volumes are individually planned, taking into account that doses to non-target volumes and tissues shall be as low as reasonably practicable (ALARP) and consistent with the intended radiotherapeutic purposes of the exposure". Neither of these regulations distinguishes between the radiotherapeutic dose and the "planning and verification doses" required by the total process. Also, ALARP has such a different meaning for radiotherapy exposures from that for diagnostic exposures that it is almost inappropriate as a concept. (We may need to develop our own language specifically for protection in radiotherapy, but more of that later.)

A specific interpretation is put on these regulations in the Medical and Dental Guidance Notes [5]. In situations where the clinician is required to consider the extra dose from planning and verification exposures independently, "The IRMER Practitioner responsible for the treatment exposure (the Clinical Oncologist) can justify the concomitant exposures at the outset or during the radiotherapy course, but in doing so must be aware of the likely exposures and the resulting dose so that the benefit and detriment can be assessed. This can be achieved by including likely concomitant (extra-target) exposures within site-specific protocols with a total effective dose agreed".

Waddington and McKenzie [2] chose to follow the Guidance Notes to demonstrate the possibility of expressing concomitant exposures using "effective dose", and have presented an excellent paper analysing that part of the "dose" from verification images; and at the same time justifying the need to record these doses by looking at the potential for cancer induction. Harrison [1] has taken this a stage further to analyse in more depth the radiation induced cancer issue for radiotherapy patients. One of his conclusions is that to specify and document individual concomitant exposures separately would seem to be unnecessary.

This Editorial aims to stimulate further debate on these subjects, which are partly about the need for radiotherapy departments to comply with IRMER(2000) [4], but also about the need for all staff to gain a better understanding on cost-benefits of radiotherapy.

It is extremely important that the radiotherapy community addresses this subject and arrives at a consensus that will benefit the patient. In the summary of the current position, which follows, absorbed dose (shortened to "dose") has been used throughout. The use of "effective dose" is questionable in radiotherapy for several reasons: the radiotherapy patient population is a highly selected subgroup of the general population with different longevity; also tissue weighting factors will be different because dose gradients and doses are much greater in radiotherapy than in diagnostic radiography. Therefore it is avoided here.

What do we know about doses in radiotherapy?

1. Dose to various critical structures from CT scanning of radiotherapy patients for planning varies typically between 1 mGy and 40 mGy, depending on scanned volume and CT parameters.
   
2. Dose from a portal image (to sites outside the coned region around the target – part of the "extra-target" dose) will be about 10–20 mGy (for a modern Electronic Portal Imaging Detector) from each image exposure. So 10 images will give 100–200 mGy to these regions.
   
3. Dose (part of the "extra-target dose" from the actual radiotherapy) due to leakage and scatter (for 60 Gy to the target) will give at least 60 mGy to every part of the body; and between 600 mGy and 6000 mGy at between 10 cm and 1 cm from the edge of the irradiated volume (in 30 fractions; fractionation may be important-see below), which probably includes most of the "portal volume" (including the "open" field). For example, the dose to the contralateral breast has been measured to be 900–3400 mGy for 50 Gy to the target [6].
   
4. Dose from leakage and scatter in intensity-modulated radiotherapy may be at least a factor of 2 higher than figures given above [7]. Generally the more complex the delivery plan the higher the dose outside the target volume.
   
5. The future: planning and verification of treatments will bring more dose from CT and portal imaging both within and outwith the treatment room as three-dimensional (3D) imaging is used for verification both on CTSim and using a CT system attached to the linear accelerator.
   

What do we not know?

1. Are low doses, say less than 100 mGy, relevant at all for the radiotherapy patient (when considering potential for second cancer)? We always answer "yes" to this question in radiation protection for staff, the general public and diagnostic patients, but what is the relevance of these small(ish) doses when we take prognosis for the cancer patient into account (assuming it takes 15–40 years for a radiation induced cancer – excluding leukaemia – to be expressed)?
   
2. At what dose do we need to concern ourselves with secondary cancer induction (a chance effect) as well as our concerns over reducing morbidity (a definite effect)?
   
3. Is there a level of dose at which we can assume that no cancer will be induced, because most cells become sterilized? (There is some evidence from recent studies of Hodgkin's patients that tumours have been found in regions of breast tissue irradiated to quite high doses.) What is the risk to an organ where part receives a high dose and part a low dose? Does "mean dose" have any meaning in this case?
   
4. What is the relevance of fractionation of the various doses given to the patient? Classical radiation protection does not include any concept of splitting dose over a period of time, but in radiotherapy it is a very important radiobiological concept that is used to govern biological effect, particularly when considering late morbidity to critical structures.
   
5. What risk factors should we use for cancers at different sites? For example, the National Health Service Breast Screening Programme (NHSBSP) [8] uses very different values from those used by the National Radiological Protection Board (Radiation protection standards). For women from 15 years to 60 years old NHSBSP uses a Lifetime risk factor (for radiation induced breast cancer) in the range 43 to 10 per million per mGy. NRPB use a composite factor for both sexes and all ages for the induction of fatal breast cancer of only 2.5 per million per mGy.
   

Cost-benefit: probability and uncertainty

UNSCEAR [9] has collated the evidence that radiation causes cancer at a certain rate within various organs; over the years we have used these progressively refined published probabilities to justify or not the use of ionizing radiation in medicine. The stochastic effect has dominated our thinking within the diagnostic field of work, but diagnostic exposures are not the subject of this Editorial. We are discussing the tricky problem of the additional doses given to the individual patient by the exposures used to plan and verify the individual treatment that regulation requires us to address. This is not just a scientific matter; it is a medical, ethical and legislative matter; and it is not easy to arrive at a solution.

None of the authors mentioned in the introduction to this Editorial has identified the crux of the matter (although Munro [3] comes close to it). The clinical oncologist requires the team of physicists, radiographers and MTOs to plan and deliver the treatment he/she has prescribed to the individual patient as accurately as possible. There is plenty of evidence to show the advantages of using CT for planning; there is equally evidence to demonstrate that portal images are essential to sustain accurate coverage of the target and avoid critical structures. So these extra-target doses are an essential part of the whole process of accurate delivery of the prescribed dose. However, this is not to say that any amount of extra-target dose can be given. The question then arises as to how to establish constraints for this extra dose to decide when it can be accepted within the prescription and when it can not. This is not straightforward. We also need to consider years of life remaining and the susceptibility of cancer induction. The 17-year-old female having "involved field irradiation" for Hodgkin's lymphoma is not in the same category as the 70-year-old male treated for prostate cancer.

Conclusions

There are still many questions unanswered about radiation-induced cancers in radiotherapy patients. The most important are summarized below:

1. Scatter and leakage form a large part of the extra-target dose; and this contribution to the dose burden to the patient will continue to increase as techniques become more complicated. Do these doses need to be recorded? (Legislation does not require this and yet these doses are just as important or more important than simulator or CT doses). What is the purpose of legislation in this context?
   
2. The clinician has a responsibility when planning the patient's treatment to justify and optimize all the exposures of radiation. This is not a simple matter (very much more complicated than the same regulatory matter for radiologists). The target volume must be optimally treated; the planning of this must include as much information as possible about the target and the critical structures and the delivery of this dose must be checked regularly with portal imaging (or the new linac–CT systems). The more complicated the treatment (to ensure longevity) the more extra-target dose will be given. There will be a balance to be drawn between the deterministic damage to critical structures and the risk of radiation induced cancer. One of these is definite, with a well documented link between dose level and damage; the other is probabilistic, and not well understood in radiotherapy patients.
   
3. A system is needed for recording all significant doses for children and young adults (as recommended by the Royal College of Radiologists in 1996 [10]). This system needs to be developed to include reporting of extra-target doses from both radiotherapy and planning and verification exposures. Such a system should be recognized internationally.
   
4. It is evident that the language and terminology widely used in radiation protection is not always appropriate in radiotherapy. It should be reviewed and standardized.
   
5. Do parts of the IRMER legislation need re-interpreting specifically for radiotherapy? Was sufficient consideration given to the complexities of radiotherapy, and the various sources of dose to the patient, when IRMER(2000) was introduced?
   

References

1. Harrison R. Second cancers following radiotherapy: a suggested common dosimetry framework for therapeutic and concomitant exposures. Br J Radiol 2004;77:986–90.[Free Full Text]
2. Waddington SP, McKenzie AL. Assessment of effective dose from concomitant exposure required in verification of the target volume in radiotherapy. Br J Radiol 2004;77:557–61.[Abstract/Free Full Text]
3. Munro AJ. Motes and beams: some observations on an IR(ME)R inspection in radiotherapy. Br J Radiol 2004;77:273–5.[Free Full Text]
4. Ionising Radiation (Medical Exposure) Regulations 2000 (Statutory Instrument 2000 No 1059). London, UK: HMSO, 2000.
5. Institute of Physics and Engineering in Medicine. Medical and Dental Guidance Notes: a good practice guide to implementing ionising radiation legislation in the clinical environment. York, UK: IPEM, 2002.
6. Epstein RJ, Kelly SA, Cook M, Bateman A, Paddick I, Kam KC, et al. Active minimisation of radiation scatter during breast radiotherapy: management implications for young patients with good-prognosis primary neoplasms. Radiother Oncol 1996;40:69–74.[Medline]
7. Hall EJ, Wuu CS. Radiation-induced second cancers: the impact of 3D-CRT and IMRT. Int J Radiat Oncol Biol Phys 2003;56:83–8.[CrossRef][Medline]
8. NHSBSP Publication No 54 (Feb 2003) Review of Radiation Risk in Breast Screening. Report by a joint working party of the NHSBSP National Coordinating Group for Physics Quality Assurance and the National Radiological Protection Board. Sheffield: NHS Cancer Screening Programme, 2003.
9. UNSCEAR. United Nations Scientific Committee on the Effects of Atomic Radiation. Sources and effects of ionising radiation. Report to the General Assembly, with scientific annexes. (Volume II: Effects) Vienna and New York: United Nations Publications, 2000.
10. Royal College of Radiologists RCR. Guidance on the retention and destruction of NHS medical records concerned with chemotherapy and radiotherapy. BFCO(96)3. London, UK: RCR, 1996.

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