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Second cancer risk

The two informative articles by Aird [1] and Harrison [2] on second cancer risks following radiotherapy are very thought-provoking. We would like to draw your attention to several additional points, namely:

1. The discussion on doses from diagnostic X-rays, CT scans and scattered radiation from megavoltage therapy assumes a radiation weighting factor (w_r) of 1.0, the standard recommended value. However there is mounting evidence of higher relative biological effect (RBE) values (around 4.0) for certain endpoints in the case of low energy photons [3–5]. While the published data are related to photon energies that are lower than those considered by Aird and Harrison, the RBE values in the complete diagnostic energy range are worthy of scientific investigation, and higher w_r values may eventually be required.
   
2. Repeated imaging studies associated with clinical trials are necessary for data acquisition but are sometimes unnecessary in terms of clinical management of the individual patient. However these protocols can be inadvertently continued following cessation of trials and in such cases represent unnecessary radiation exposure. Whereas there are comprehensive guidelines on optimum imaging strategies in radiotherapy treatment planning [6], similar guidance is not available for patient follow-up.
   
3. One clear-cut answer to some of the questions posed by Aird [1] is that a greater use of proton beams in radical treatments should reduce the incidence of second malignancy, due to the substantially reduced integral dose. The expected yield of radiation-induced secondary cancers is estimated to be reduced by a factor of between 2 and 15 when compared with either conventional megavoltage or intensity-modulated X-ray treatments [7, 8]. We gather that modern radiation-protection regulations are adding weight to the already strong clinical case for proton therapy expansion within the EU. The most important strategic decisions confronting the UK radiotherapy community are whether and when investment in a National Proton Therapy Centre or Centres should be made [9].
   

Yours etc.,

S Green and B Jones

University Hospital Birmingham NHS Trust, Edgbaston, Birmingham, B15 2TH, UK

Received for publication January 7, 2005. Accepted for publication January 18, 2005.

References

1. Aird EGA. Second cancer risk, concomitant exposures and IRMER(2000). Br J Radiol 2004;77:983–5.[Free Full Text]
2. Harrison RM. Second cancers following radiotherapy: a suggested common dosimetry framework for therapeutic and concomitant exposures. Br J Radiol 2004;77:986–90.[Free Full Text]
3. Heyes GJ, Mill AJ. The neoplastic transformation potential of mammography x ray and atomic bomb spectrum radiation. Radiat Res 2004;162:120–7.[CrossRef][Medline]
4. Frankenburg D, Kelnhofer K, Bar K, Frankenberg-Schwager M. Enhanced neoplastic transformation by mammography X rays relative to 200 kVp X rays: indication for a strong dependence on photon energy of the RBE(M) for various end points. Radiat Res 2002;157:99–105. Erratum in: Radiat Res 2002;158:126.
5. Goggelmann W, Jacobsen C, Panzer W, Walsh L, Roos H, Schmid E. Re-evaluation of the RBE of 29 kV x-rays (mammography x-rays) relative to 220 kV x-rays using neoplastic transformation of human CGL1-hybrid cells. Radiat Environ Biophys 2003;42:175–82.[CrossRef][Medline]
6. Imaging for Oncology: Collaboration between Clinical Radiologists and Clinical Oncologists in Diagnosis, Staging and Radiotherapy Planning; Royal College of Radiologists, London 2004.
7. Miralbell R, Lomax A, Cella L, Schneider U. Potential reduction of the incidence of radiation-induced second cancers by using proton beams in the treatment of pediatric tumors. Int J Radiat Oncol Biol Phys 2002;54:824–9.[CrossRef][Medline]
8. Schneider U, Lomax A, Lombriser N. Comparative treatment planning using secondary cancer mortality calculations. Phys Med 2001;17 Suppl. 1:97–9.
9. Jones B, Rosenberg I. Particle Therapy Co-operative Oncology Group (PTCOG 40) Meeting, Institute Curie 2004. Br J Radiol 2005;78:94–7.

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