University of Liverpool

Halo-Dose correlation in a medical accelerator 

Trainee: Jacinta Yap
Supervisor: Carsten P Welsch

Online beam monitoring in medical accelerators is an essential part of the patient’s safety and also assuring the high quality and efficacy of cancer treatment. Charged particle beams interact with patient’s tissue depositing their kinetic energy in many elastic and inelastic collisions demonstrating the highest energy transfer at the end of their path. Hence beam energy and energy spread; position and lateral profile of the beam as well as the beam current have to be precisely determined and recorded. 

In clinical practice ionisation chambers provide users with information on the dose rate. These are, however, interceptive devices, degrading both the beam profile and its energy spread. A new non – interceptive method of online beam monitoring would be highly desirable and shall be developed in the frame of this project.

The LHCb VELO detector is a position sensitive silicon detector, which has been used for tracking vertices originating from collisions at the LHCb experiment at CERN. Its advantageous semi-circular design enables approaching the core of the beam, however, without interfering with it. Thus, it allows precise measurements of the surrounding beam ‘halo’ which can also provide information about the active delivery of the beam. The halo is associated with nuclear and secondary products which can result in additional and unwanted dose being delivered. In a clinical setting however, the beam passes through several components in the delivery system which shape and adapt the beam for the needs of the treatment. Accordingly this excess beam is mostly collimated out however, it is possible to utilise the detection of these peripheral particles with the VELO sensors to determine the parameters of the rest of the beam which is delivered to the patient.

In the frame of this project the stand alone operation of the VELO detector as an online monitor at the Clatterbridge Cancer Centre, UK (CCC) is being investigated. Correlation of the halo to the core of the beam will yield treatment beam ‘halo’ maps, benchmarked against simulation studies into beam transport and particle tracking. As accurate models are necessary to understand beam behavior and integrate the system, a full characterisation of the CCC beamline and the validation of Monte Carlo simulations will be performed in collaboration with UCL, RHUL and CCC. This will also facilitate related studies into LET, DNA damage, repair and radiobiological studies with North West Cancer Research, UK.