Why cyclotrons are of little use for high-performance proton therapy ?
Nowadays different companies all over the world have already developed and are still developing accelerators for proton and ion therapy based on cyclotron.
In our opinion this type of accelerators is of little use for the new high performance technology of treatment of oncological diseases.
The point is that principle of functioning of cyclotrons is production of beams with fixed particle energy which they are designed for. Meanwhile, beam therapy requires wide range of energy change. To change energy on existing cyclotron installments the beam is sent through the energy absorber where it retards till the necessary quantity. At the same time due to the interaction with the substance of the absorber not only energy loss happens but also particle dispersion, so the narrow beam comes out of the absorber as a fan. Subsequently this beam disperses more by means of complementary disperser. In order to irradiate the tumor only a wide beam goes through collimator which has a tumor shaped hole from the given angle. Besides, to stop the beam at the required depth a compensator should be placed after the collimator (see the scheme).
What is a disadvantage of this technology?
It is obvious that in this irradiation plan some part of healthy tissue receives an increased dose which is equal to the tumor dose.
It is necessary to produce collimators and compensators which requires mechanical workshop in a hospital.
Collimators are quite heavy items and require supplementary staff for carrying and installation.
Collimators become radioactive after use and should be kept in a radiation protected room during several months until further utilization.
The major part of primary accelerated beam is retarded by collimators in which, as a result of nuclear reactions, neutrons are generated. These neutrons penetrate into the patient and may cause secondary cancer.
The main drawback is that with this technology it is practically impossible to irradiate the tumor from different directions since it would have required manufacturing individual collimators for each direction.
Even with existing technologies it is also impossible to use IMPT mode, especially optimized IMPT which also does not allow having a limit high relation of the dose in the tumor to the dose in healthy tissue.
Thus, all the listed drawbacks (especially 6 and 7) denote that usage of cyclotron accelerators in the new high performance therapy is not perspective.