Consideration of radiation quality in treatment planning with p(66)/Be(40) neutrons

A thorough microdosimetric investigation of a p(66)/Be(40) neutron therapy beam has been performed employing a commercial tissue equivalent proportional counter. Using y* as a monoparameter of radiation quality, variations in the potency of the beam have been discerned with spatial position and for different field sizes in a water phantom. The identified variations with depth are attributed solely to changes in the character of the neutron component of the beam. When increasing the field size, neutron effects are tempered by an increasing photon component. To accommodate these quality changes in the prescription of absorbed dose, the concept of effective dose is invoked and elementary expressions are proposed to assess this quantity. A simple four-field treatment plan is calculated to show that if unaccounted for, the quality changes with depth alone would result in a 4% discrepancy between the prescribed and effective dose delivered to the tumor volume. The implications for optimum dose delivery in clinical practice are discussed in terms of these findings.