Cyclotrons in radiotherapy

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Radiation therapy is one of the most frequently used methods for treating cancer patients. Therefore improving the quality of the treatment by using new irradiation technologies is one of the radiotherapist's constant concerns. The use of accelerators for delivering protons, heavier charged particles and secondary beams such as neutrons brings further improvement in dose distribution and in interesting biological properties for radiations producing a high Linear Energy Transfer (LET). Cyclotrons have been present in this field since the early history of radiotherapy and they could play a major role for future dedicated hospital-based facilities which include not only an accelerator but also specific beam-transport and beam-delivery systems. Modern cyclotron technology exists today to meet all of the requirements within a reasonable budget. Specific performances and several cyclotron designs are presented. 1 . INTRODUCTION Heavy particles (protons, light ions, neutrons) offer a unique opportunity for improving the quality of radiation therapy. Their use has been pioneered over four decades at Berkeley where there was a very active collaboration between nuclear physicists, cyclotron designers, physicians and biologists and this fact is well illustrated by the early history of the cyclotron and its application in radiotherapy. Ernest Lawrence, inventor of the cyclotron, started in the late thirties a collaborative work with his brother John [1], a young physician, Paul Aebersold and Richard Stone [2] on the use of neutrons in the treatment of cancers. But there was at this time no basic biological information available to provide a rationale for the use of neutrons in preference to X-rays. Therefore an unexpectedly high incidence of late morbidity appeared. A renewed interest occurred in the early sixties following a great deal of radiological work carried out by M. Catterall [3] and D. Bewley in the Hammersmith Hospital in London. This work explained the radioresistance of tumours and the role of the LET. Therefore the specifications required for neutron therapy are now well known and modern cyclotrons are the standard tool for this application. In 1946, Robert Wilson [4] mentioned that the properties of beams of mono-energetic charged particles such as protons and ions (deposition of a large fraction of their kinetic energy in a small volume at the end of their range (Bragg peak and distal dose fall off), small lateral scattering) could lead to a new radiotherapeutic tool. Again the high energy cyclotrons in Berkeley [5-6] were used for the first treatment with charged particles. Table 1 presents a short review of cyclotron-based facilities running a medical programme (most of them part time). The figures on patient statistics are from the Newsletter Particles issued in January 1994, which is sponsored by the proton therapy co-operative group (PTCOG) and edited by Janet Sisterson from the Harvard Cyclotron Laboratory. The type and the characteristics of the cyclotrons have been added in the two last columns. Table 1 Operating cyclotron facilities Institution, Place First treatment Treated patients (date of total) Accelerator type Maximum energy (MeV) Harvard, Mass. (USA) 1961 6010 Dec. 1993 Synchrocyclotron 160 LBL 184 inch (USA) (1) 1954 220