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It is encouraging to read the detailed analysis of dose distributions to the mediastinum and heart during radical radiotherapy for oesophageal cancer in the UK [1]. This is a desirable consequence of better radiotherapy treatment planning using three-dimensional (3D) techniques and advanced software systems. Although the authors quote the Japanese experience of increased toxicity following chemoradiotherapy [2], there is important pioneering work from Japan using protons to reduce normal tissue doses while delivering a high tumour target dose using only two fields, with a respiratory gating technique linked to a pulsed output of protons.
At The University of Tzukuba Proton Medical Research Centre, the potential indications for proton therapy have been extended by conducting a detailed clinical study of 46 oesophageal cancer patients using limited proton beam availability between 1985 and 1998. These mostly mixed protons and X-rays studies show that high total doses can be given with long term tumour control at least equivalent to X-ray therapy [3]. Toxicity reduced with experience, dose adjustment, increasing the component of protons and the use of respiratory gating. No late symptomatic complications were observed in the tracheobronchial tree, heart or spinal cord and no late grade 4 or 5 oesophageal toxicity has been observed since 1991. Severe and persistent oesophageal ulceration in some patients had previously occurred because of high doses per fraction (2.5–3.7 Gy) related to limited beam availability. Since 2001, pure proton therapy treatments have been delivered in a purpose built Centre using rotating gantries and at fractionated doses of 1.8–2 Gy per fraction: the results of these studies are awaited with interest. Software for proton dose–volume histograms (DVH) have only been available recently at Tzukuba, so it is not possible to retrospectively analyse the patients treated in their publication. However, on a recently treated patient the DVH for a two field (anterior and posterior fields) 200 MV proton plan has been obtained and Table 1
shows a comparison of this plan with averaged values reported by Cominos et al [1] for their best case scenario of a 4 field plan (taken from Table 2 in their paper). The patient was a 61 year old man with a T1 N1 M0 squamous cell carcinoma of the mid-thoracic oesophagus. He had refused the offers of chemoradiotherapy and surgery. It should be noted that the UK prescribed dose is considerably lower than that used in Japan, yet the cardiac doses are markedly reduced in the latter, despite the anterior beam field traversing through the heart. Furthermore, the 4 field X-ray technique will inevitably result in higher lung exposures than for the 2-field proton technique, which were not reported.
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It would be timely for the UK to acquire proton therapy facilities in order to improve therapeutic dose distributions in a wide variety of anatomical sites. Japan – with a population of c. 160 million – will soon have 8 centres capable of delivering proton or ion beam radiotherapy. To maintain a similar proportion, the UK would need to have 3 centres.
Yours etc.,
1 University Hospital Birmingham, Birmingham, B15 2TH, UK;2 Proton Medical Research Centre, University of Tzukuba, Japan
Received for publication January 3, 2006. Revision received January 18, 2006. Accepted for publication January 18, 2006.
References
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