Proton beam radiotherapy

B Jones, R D Errington
2000 British Journal of Radiology  
The use of proton beam radiotherapy is increasing in several countries. A 1-day symposium was commissioned by the Oncology Committee of the BIR to review progress in this sophisticated area of radiotherapy. In particular, the programme was designed: (1) to consider the results of extensive experience of proton therapy used for eye conditions, mainly ocular melanoma, at the Clatterbridge cyclotron; (2) to consider the most recent results of CNS proton therapy, mainly for clival chordomas, in
more » ... on, USA; and (3) to encourage awareness of European developments, especially in France, where similar treatments are now available. Dr Errington provided a brief history of the Clatterbridge cyclotron, and of its conversion from a neutron-generating unit to a ®xed 60 MeV proton beam capable of delivering a Bragg peak at 3 cm depth, which is adequate for treating ocular structures. To date, 960 patients have received proton therapy at Clatterbridge (833 choroidal melanomas, 56 iris melanomas, 26 retinal haemangiomas, 8 conjunctival tumours and 37 patients with macular degeneration). Dr A Kacperek (Clatterbridge) explained the physical advantages of proton beams, the reduced entry and absent distal (or exit) dose beyond the Bragg peak. Variable modulation of the Bragg peak results in a dynamic range of multiple Bragg peaks at different depths, and which summate to give a high dose plateau with an abrupt fall in dose beyond, but at the expense of an increased entry dose [1]. Proton radiotherapy can use all the technical methods of photon radiotherapy (wedges, shielding, conformal three-dimensional (3D) planning, intensity modulation and the recent use of spot scanning of a target by use of multiple pencil beams at PSI-Villigen in Switzerland). Proton dosimetry methods were summarized, including the 10% underestimation of dose errors associated with use of The Faraday Cup at Boston, discovered by excellent international quality control intercomparisons. Mr John Hungerford (Moor®elds Hospital) described the range of available radiotherapy techniques for plaque brachytherapy, which was started by his predecessor Mr Henry Stallard. The range of effective dose varies with the isotope. Iodine plaques have a longer range of 8 mm and have different clinical effects when compared with ruthenium plaques. 125 I results in greater loss of visual acuity, but provides better control of larger tumours. The choice of radioisotope may therefore be determined by the quality of vision present in the contralateral eye. 106 Ru is the preferred isotope in most situations. Mr Hungerford described the Clatterbridge technique for proton treatment of ocular melanoma. The tumour edges are marked by radioopaque tantalum clips and 3D planning software is used to reconstruct the eye, based upon accurate ultrasound measurements. The treatment plan optimizes the dose distribution to reduce dose to the lens and optic nerve. Proton doses of 53 Gy in four fractions are used. More beam modulation is required for treatment of large tumours (those beyond the effective range of plaque brachytherapy). This can result in a higher entry dose and a larger treatment volume, with a resultant painful eye due to neovascular glaucoma, which is caused by persistent retinal detachment rather than the radiation therapy. Indications for proton therapy for ocular melanoma include: (1) tumours that are too large for plaque brachytherapy but have no evidence of retinal detachment prior to treatment; and (2) posteriorly situated tumours that are small enough for a plaque but are too near the optic disc. An audit of the treatment results of patients treated at the Clatterbridge cyclotron undertaken at Moor®elds Hospital in 1998 showed that 95% of eyes were retained after proton therapy, with 89% and 96% of eyes retained after 106 Ru and 125 I, respectively [2] . In terms of retained visual acuity, protons are best, followed by Ru and I plaques, respectively. The complication of rubeosis following proton beam therapy appears to be con®ned to large sized tumours with pre-treatment retinal detachment [3] . It is now clear that this complication is a consequence of prolonged detachment rather than the radiation per se. Generally, protons are more likely to cause
doi:10.1259/bjr.73.872.11026853 pmid:11026853 fatcat:ifumtdcetne25kk6qod2d6hvtu