Distal penetration of proton beams: the effects of air gaps between compensating bolus and patient

These Monte Carlo studies have shown that, if the distal end of range of the proton beam is to be tailored tightly to the target volume, sophisticated treatment planning programs, beam delivery and patient immobilisation techniques have to be developed and details, such as having the compensating bolus fit snugly to the patient's surface, have to be considered. Although the magnitudes of the effects reported here are specific to the components and beam quality in this Harvard Cyclotron Laboratory proton beam line, the principles are general and apply to all heavy charged particle beams. Thus, in the design of a proton beam delivery system, every effort should be made to allow the beam-modifying devices to get very close to the patient or to devise some other means to minimise the separation of patient and bolus.