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British Journal of Radiology (2004) 77, 557-561
© 2004 British Institute of Radiology
doi: 10.1259/bjr/67235257
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Full Paper

Assessment of effective dose from concomitant exposures required in verification of the target volume in radiotherapy

S P Waddington, MSc 1 and A L McKenzie, DSc 2

1 Faculty of Health and Social Care, University of West of England, Bristol BS16 1DD and 2 Department of Medical Physics and Bioengineering, Bristol Haematology and Oncology Centre, Horfield Road, Bristol BS2 8ED, UK


   Abstract
Top
 Abstract
Introduction
 Method
 Results
 Discussion
 Conclusion
 References
 
The requirement of the Ionising Radiation (Medical Exposure) Regulation 2000 [IR(ME)R] of justifying all exposures to ionizing radiation includes those from radiotherapy double exposure portal images resulting in exposure to normal tissues outside the treatment volume. Typical effective doses were calculated for a range of common sites using CT data to outline those parts of specific organs subject to concomitant radiation and generate dose–volume histograms. The product of the mean dose and the relative probability of inducing a fatal cancer in specific organs was used to determine a representative total effective dose in mSv per monitor unit for each site. A table of representative effective doses, ranging from 0.32 mSv to 2.56 mSv per monitor unit, was produced, which may be used to monitor cumulative effective doses of individual patients from double exposure portal images, in addition to those received from localization procedures.


   Introduction
Top
 Abstract
 Introduction
Method
 Results
 Discussion
 Conclusion
 References
 
One of the requirements of the Ionising Radiation (Medical Exposure) Regulation 2000 (IR(ME)R) [1] is that we know the cost as well as the benefits of radiation to our radiotherapy patients. The process of justification needs explicit knowledge of the detriment by induction of cancer from ionizing radiation not aimed at the target volume. It is a perceived wisdom in radiotherapy that the potential of localization and verification exposures to induce cancer does not add to the patient's burden because the doses from such exposures are very small compared with the intended therapy dose.

This is true when considering a cell within the target volume, where that cell will receive a tumouricidal dose from a course of radical radiotherapy, whether or not a malignant change has been induced. But a cell just outside the target volume will receive a less than tumouricidal dose, partly from scatter and partly from "concomitant dose", defined in the "Guidance Notes" [2] as the contribution from radiotherapy localization and verification.

In radiation protection, absorbed dose is used to mean the dose averaged over a tissue or organ [3]. The probability of stochastic effects, primarily the induction of malignancy, also depends on the type and energy of ionizing radiation, taken into account by the application of a radiation weighting factor, wR, to absorbed dose to give equivalent dose in tissue T, HT. The vast majority of radiotherapy applications utilize high energy X-ray photons or electrons of similar linear energy transfer (LET) and are assigned a radiation weighting factor of 1.

The relationship between the probability of stochastic effects and equivalent dose also depends on the tissue or organ irradiated. Tissue weighting factors (wT) defined by the International Commission on Radiological Protection (ICRP) [3] give the relative contribution of an organ or tissue to the total detriment from irradiation of the whole body. The weighted equivalent dose is termed the effective dose, E, where E={Sigma}wT.HT. As the sum of the tissue weighting factors wT is normalized to unity, the contribution to effective dose resulting from the exposure of an individual organ can be calculated, albeit as an approximation, because of assumptions and simplifications made as detailed in ICRP publication 60 [3]. Table 1Go indicates the relative probabilities of inducing a fatal cancer in column 2 and simplified tissue weighting factors in column 3.


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  		Table 1. Relative contribution of organs to the total detriment (adapted from Tables 2 and B-20, ICRP [3])

 
It is important to recognise that the risk of cancer induction is additive. In other words, whatever the risk of cancer induction from the leakage and scatter from the treatment beams, the concomitant radiation from localization and verification will add to that risk.

The risk from leakage and scatter from the treatment beams cannot be reduced once the prescription has been fixed. For a radical course of radiotherapy, the effective dose from such leakage and scatter may well be in the order of a few hundred mSv, with a corresponding risk of cancer induction in the order of 1%. However, there is effectively no control on such radiation-induced cancers —they are an unavoidable consequence of the decision to treat with radiation.

On the other hand, there is a degree of choice in the level of concomitant radiation exposure, particularly with regard to the verification process, depending upon the number of times patients are imaged, the size of fields used and the characteristics of the imaging system. Underlying IR(ME)R is the principle that, where there is a choice in the level of radiation exposure, the benefits must outweigh the risks.

In this work, only the contribution of the concomitant exposure to the effective dose was evaluated. This was achieved by using only portal beams and not treatment beams in the treatment planning system, so that the contribution of the treatment beam leakage and scatter was effectively removed.

The effective dose from diagnostic X-ray energy exposures, including simulator fluoroscopy and radiography localization procedures and CT localization may be calculated using readily-available computer programs [4], but there appear to be no similar programs for the calculation of effective dose from megavoltage X-ray photon energies.

Under IR(ME)R [1] the employer is required to establish diagnostic reference levels (DRLs) for routine examinations in diagnostic radiology and nuclear medicine. This is to ensure that the dose is "commensurate with the clinical purpose" by reducing radiation exposure that does not contribute to the medical imaging task [5]. DRLs act as investigation levels to identify unusually high levels, which require review if consistently exceeded.

Several authors have examined the use of portal imaging in radiotherapy with both film–screen combinations and electronic portal imaging devices (EPIDs), and mention the desire to minimize dose outside the treated volume [6–8]. However, current evidence also leads to recommendations that portal imaging should be performed frequently or even daily during a course of radiotherapy, especially for conformal techniques with small margins for set-up error [9–12].

Unfortunately, megavoltage portal images have poor image contrast, leading to difficulties in visualizing bony landmarks to which the treatment volume may be related. Clinically useful portal images often require a double-exposure technique whereby the treatment field is first exposed, followed by a larger field exposure to demonstrate relative anatomy. Many linear accelerators have EPIDs but others do not, requiring the use of film–screen systems with associated higher exposures to give sufficient optical densities [8, 13]. The use of double-exposure portal imaging, by EPID and film–screen system, will lead to an increase in the probability of inducing a fatal cancer in sensitive organs outside the treatment volume. The probability that 1 mSv will induce a fatal cancer in an average person is 0.005% [3], or, using the linear model of dose dependence, an effective dose of 200 mSv has 1% chance of inducing a fatal cancer.

It would be correct, of course, to query the process of calculating the risk of inducing a fatal cancer in patients who already have cancer. While the probability of inducing a cancer in a patient will not generally depend upon whether they already have cancer, the induced cancer may not have the chance of being fatal because the patient may first succumb to the original cancer. Nevertheless, patients being treated radically may have a high expectation of cure, and in those patients, the effective dose from portal imaging will indicate the chance that they may die from a radiation induced cancer.

There is no doubt that portal imaging can reduce systematic and random set-up errors [11]. But does the benefit due to verification exposures outside the treatment volume outweigh the risk, as required by IR(ME)R [1]? This paper attempts to quantify the detriment resulting from portal imaging.


   Method
Top
 Abstract
 Introduction
 Method
Results
 Discussion
 Conclusion
 References
 
The aim of this study was to indicate the effective doses per monitor unit (MU) for portal exposures of a range of sites. The derivation of these effective doses may be illustrated by considering a typical radiotherapy procedure: a conformal prostate cancer technique, with a small target volume to enable dose escalation, requiring verification using portal imaging to ensure a high level of set-up accuracy. Within these criteria, an average-sized male patient was retrospectively selected of approximately 170 cm in height and weighing 70 kg, similar to Reference Man [14] and more recent reference values for the adult male [15]. An average patient was chosen to illustrate a real-life situation and to calculate the order of effective dose to specific organs rather than using an anthropomorphic phantom of geometric shape.

CT localization scans for this patient were transferred to a 3D treatment planning system (Helax TMS v 5.1A, MDS Nordion, Sweden) and a planning target volume was grown around the prostate. The bladder, colon and sites of red bone marrow were outlined on each axial CT image. Under magnification, the walls of the bladder and colon were identified and outlined. Red bone marrow was assumed to be present in the medullary cavity of the os coxae, sacrum and upper half of the femora and outlined as such.

The treatment planning system was used to model portal beams for a 6 MV Elekta SL22 linear accelerator (Elekta Oncology Systems Ltd, Crawley, UK) representing local protocol for double-exposure portal imaging: the planned field size plus 4 cm in each dimension for an anteroposterior (AP) and lateral view. Full 3D planning was employed with normalization to the depth of maximum dose, dmax.

For each tissue/organ in Table 1Go, a volume of interest (VoI) was generated by outlining the organ, or, at least, that part appearing within the portal field. Any part of the tissue/organ exposed to the treatment beam was excluded from the VoI, because it would not contribute to the effective dose. To see why this was necessary, consider a uniform dose deposited in an organ from a portal field. If half of that organ were also in the treatment beam any cancer cells induced in that half would be sterilized. Then the contribution from the portal field to the effective dose in the organ would be only half of that found by multiplying the dose by wT for the organ (essentially, because the probability of inducing surviving cancer cells would be halved in this case).

The mean dose to the VoI was then calculated using the treatment planning computer to generate a dose–volume histogram (DVH) of the VoI. The contribution of the tissue/organ to the effective dose was determined by multiplying the mean dose to the VoI by the ratio of the VoI to the total volume (Vtotal) of the tissue/organ in the body (giving the mean dose to the complete organ) and also by the relative probability of fatal cancer for the complete organ (see Table 2Go).


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  		Table 2. Effective doses due to anteroposterior extended-field portal image for verification of typical prostate technique for an exposure of 1 monitor unit (MU). The precision of these figures is quoted only to illustrate the derivation of the effective dose. The accuracy is discussed in the text

 
For tissues/organs classified as "remainder" (Table 1Go), the weight of each was calculated as a fraction of the 70 kg total. In the case of skin, the field area at the beam entrance and exit was calculated and then expressed as a fraction of the total body skin area. The area of irradiated bone surface was estimated by assuming that the ratio of irradiated area to total skeletal area was equal to the equivalent ratio for the skin. This supposed correlation is crude as will be seen immediately by thinking of specific examples. However, the coarseness of the approximation may be made acceptable by the relatively low contribution of bone surface to the effective dose, so that the degree of error is small.

A separate 3D treatment plan was produced for each portal beam with the isocentre in the tumour, but with the dose distribution normalized to dmax, since the effective doses are calculated for each portal beam separately.

The same method was employed to calculate effective doses for large-field portal images of other sites.


   Results
Top
 Abstract
 Introduction
 Method
 Results
Discussion
 Conclusion
 References
 
The typical prostate isocentric technique required an AP portal image extended field of 18 cm x 15.6 cm, surface-to-skin distance (SSD)=88 cm. Table 2Go indicates the data obtained from the DVHs for the VoI irradiated by this field.

Typically, the number of MU required for the larger field for a double exposure portal image is 4 when using an EPID, and 12 using a film–screen system. This would result in effective doses of 1.4 mSv and 4.1 mSv, respectively, from the AP image.

Approximately 25% of the total bladder volume is outside the treated volume but within the extended portal image field for a typical radical prostate technique. This bladder volume contributes 44% to the total effective dose. If the bladder were fully within the treated volume, as it would be for a radical bladder technique, there would be no contribution from the bladder to effective dose. This would give a total effective dose of 0.19 mSv per MU.


   Discussion
Top
 Abstract
 Introduction
 Method
 Results
 Discussion
Conclusion
 References
 
The results indicate effective doses calculated using "standard" patients, not accounting for individual sizes and shapes, but making conservative overestimates wherever necessary, to give effective doses in the correct order of magnitude. These results are tabulated to give a chart of representative effective doses per MU for a range of typical portal images (Table 3Go), where a double-exposure technique is used to irradiate tissues outside the treatment volume. In our centre, for a 10 cm x 10 cm field, a setting of 100 MU on the linear accelerator will produce a dose of 1.0 Gy at a depth dmax below a water surface 100 cm from the source. Such a chart may be used to record and monitor cumulative effective doses for individual patients in addition to doses received from simulator radiography/fluoroscopy exposures and CT localization scans.


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  		Table 3. Representative effective doses from extended field portal images. The precision of these figures is quoted only to illustrate the derivation of the effective dose. The accuracy is discussed in the text

 
It is important to appreciate that the effective doses quoted in Table 3Go are meant to be taken as representative for an average portal exposure. Individual patients and organs vary in size and shape and there will be differences in the size of the target volume that must be subtracted from the irradiated volume in order to calculate the effective dose. Hence, Table 3Go is intended to be used to provide an approximate or relative indication of effective dose for a particular patient, without attempting to quantify all of the contributing factors for each exposure.

Verification protocols differ between centres and may depend on availability of linear accelerators with EPIDs. Verification frequency may range from once at the start of treatment to every treatment session. Taking the multicentre RT01 prostate cancer trial [16] as an example, treatment verification of all treatment fields during phase 1 is required daily during week 1, using an EPID, and then weekly. Even for the low dose arm of the trial (64 Gy), this would result in at least 30 verification images giving a total effective dose of about 40 mSv if a double exposure EPID technique is used, based on the representative effective doses given in Table 3Go. This would be in addition to an effective dose of the order of 10 mSv from CT localization [17] giving a total effective dose approaching 50 mSv.

Given a 1% chance of inducing a fatal cancer with an effective dose of 200 mSv, 50 mSv to each of the 800 patients recruited into the RT01 trial could be translated into an additional two fatal cancers within that population. IR(ME)R 2000 [1] requires all concerned to reduce unnecessary exposure of patients to radiation. This implies reviewing the use of film–screen systems alongside the use of EPIDs, and particularly amorphous silicon (aSi) detectors.

Studies involving the implementation into clinical use of active matrix flat panel imagers (AMFPI), using aSi photodiode arrays indicate increased image quality alongside the potential to reduce effective dose [18, 19].

However, EPIDs are not always utilized routinely due to their perceived poor image quality when compared with film. Nevertheless, Kruse et al [20] compared a scanning liquid ion chamber (SLIC) with the more recent aSi AMFPI and ready-pack verification film. Results showed no statistical differences between film and SLIC in terms of clarity of anatomical landmarks, and increased clarity for aSi. Set-up errors were identified more accurately with both EPIDs compared with the film. This might not have been the case if a different film had been used, with its increased image quality [18] but this must be balanced against increased effective dose where extended fields are employed.

In addition, the visual inspection and measurement of anatomical landmarks from film has been shown to be subjective and unreliable [21], a further indication for the routine use of EPIDs and image matching software.


   Conclusion
Top
 Abstract
 Introduction
 Method
 Results
 Discussion
 Conclusion
References
 
Based on standard radiotherapy techniques and an average patient size, a table of representative effective doses from extended field verification images has been produced (Table 3Go). In individual patients, depending upon size, shape and details of the tumour volume and portal field dimensions, the range of effective doses will be wide and Table 3Go should only be used to obtain representative, or relative values.

The cumulative effective dose from concomitant exposures may then be recorded and monitored for individual patients, allowing an informed justification of such exposures to be made, in accordance with IR(ME)R [1]. An "action level" of, say, 100 mSv could be set, where the chance of inducing a fatal cancer would be in the order of 0.5% in addition to the natural lifetime risk of approximately 25% [5].


   Acknowledgments
 
We are grateful to Roger Parry (Radiotherapy Physics Unit, Bristol Haematology and Oncology Centre) for advice on the treatment planning system and for producing the dose–volume histograms.

Received for publication March 27, 2003. Revision received October 28, 2003. Accepted for publication December 3, 2003.


   References
Top
 Abstract
 Introduction
 Method
 Results
 Discussion
 Conclusion
 References
 

  1. The Ionising Radiation (Medical Exposure) Regulations 2000, Statutory Instrument No 1059. London: HMSO, 2000.
  2. Medical and Dental Guidance Notes published by Institute of Physics and Engineering in Medicine, York, 2002.
  3. International Commission on Radiological Protection. Recommendations of the International Commission on Radiological Protection. ICRP Publication 60. Annals ICRP 21. Oxford, UK: Pergamon Press, 1991.
  4. Ranniko S, Ermakov I, Lampinen JS, Toivonen M, Karila KTK, Cherjakov A. Computing patient doses of X-ray examinations using a patient size- and sex-adjustable phantom. Br J Radiol 1997;70:708–18.[Abstract]
  5. International Commission on Radiological Protection. Radiation and your patient: a guide for medical practitioners. ICRP Supporting Guidance 2. Oxford, UK: Pergamon Press, 2001.
  6. Mayles WPM, Heisig S, Mayles HMO. Treatment verification and in vivo dosimetry. In: Williams JR, Thwaites DI, editors. Radiotherapy physics in practice. Oxford: Oxford University Press, 1993:227–51.
  7. Hatherly KE, Smylie JC, Rodger A, Dally MJ, Davis SR, Millar JL. A double exposed portal image comparison between electronic portal imaging hard copies and port films in radiation therapy treatment setup confirmation to determine its clinical application in a radiotherapy center. Int J Radiat Oncol Biol Phys 2001;49:191–8.[Medline]
  8. Langmack KA, Goss V. Characterisation of new portal film systems for radiotherapy verification. Br J Radiol 1999;72:479–84.[Abstract]
  9. Antolak JA, Rosen I, Childress CH, Zagars GK, Pollack A. Prostate target volume variations during a course of radiotherapy. Int J Radiat Oncol Biol Phys 1998;42:661–72.[Medline]
  10. Stroom JC, Olofsen-van Acht MJJ, Quint S, Seven M, de Hoog M, Creutzberg CL, et al. On-line set-up corrections during radiotherapy of patients with gynecologic tumors. Int J Radiat Oncol Biol Phys 2000;46:499–506.[CrossRef][Medline]
  11. Hurkmans CW, Remeijer P, Lebesque JV, Mijnheer B. Set-up verification using portal imaging: review of current clinical practice. Radiother Oncol 2001;58:105–20.[CrossRef][Medline]
  12. Alasti H, Petric MP, Catton CN, Warde PR. Portal imaging for evaluation of daily on-line setup errors and off-line organ motion during conformal irradiation of carcinoma of the prostate. Int J Radiat Oncol Biol Phys 2001;49:869–84.[CrossRef][Medline]
  13. Roberts R. Portal imaging with film-cassette combinations: what film should we use? Br J Radiol 1996;69:70–1.[Abstract/Free Full Text]
  14. International Commission on Radiological Protection. Reference Man: anatomic, physiological and metabolic characteristics. ICRP publication 23. Oxford, UK: Pergamon Press, 1975.
  15. International Commission on Radiological Protection. Basic anatomical and physiological data for use in radiological protection: reference values. ICRP publication 89, Oxford, UK: Pergamon Press, 2002.
  16. Medical Research Council Radiotherapy Working Party. RT01: A randomised trial of high dose therapy in localised cancer of the prostate using conformal radiotherapy techniques. London, UK: Medical Research Council, 1997.
  17. Wall BF, Hart D. Revised radiation doses for typical X-ray examinations. Br J Radiol 1997;70:437–9.[Medline]
  18. Langmack KA. Portal imaging. Br J Radiol 2001;74:789–804.[Abstract/Free Full Text]
  19. Antonuk LE. Electronic portal imaging devices: a review and historical perspective of contemporary technologies and research. Phys Med Biol 2002;21:R31–65.
  20. Kruse JJ, Herman MG, Hagness CR, Davis BJ, Garces YI, Haddock MG, et al. Electronic and film portal images: a comparison of landmark visibility and review accuracy. Int J Radiat Oncol Biol Phys 2002;54:584–91.[Medline]
  21. Perera T, Moseley J, Munro P. Subjectivity in interpretation of portal films. Int J Radiat Oncol Biol Phys 1999;45:529–34.[CrossRef][Medline]



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