Diamond detector measurements near simulated air channels for narrow photon beams

Background and purpose: To evaluate the combined effect of increased photon transmission, reduced photon scatter, increased secondary electron range a nd loss of electronic equilibrium for narrow 6-MV beams in and around a sim ulated air channel. Materials and methods: A measuring method was developed in-house for relati ve dose measurements near simulated air-like/soft-tissue interfaces in an a utomated water phantom. A Styrofoam(TM) cylinder (density 0.03 g/cm(3)) of 2-cm diameter was submersed in the water phantom and irradiated with small rectangular radiation fields. The field length was fixed at 10 cm and the f ield widths ranged from 1 to 4 cm. The axis of the foam cylinder and the lo ng side of the field were parallel. A water layer of 2 cm was realised upst ream of the cylinder. Relative depth dose and profiles behind the foam cavi ty were assessed using a diamond detector with a sensitive crystal thicknes s of 0.21 mm located at 1 mm from the top of the encapsulation. Results: The dose at central axis 1.1 mm behind the cavity was found to be 92 and 74% for a field size of 10x2 and 10x1 cmi, respectively. The highly convex dose profile of the 10 x 1-cm(2) field, characterising the homogeneo us case, is flattened. Conclusions: The diamond detector is an excellent choice as a detector in s mall photon fields with high-dose gradients as they occur near air channels , provided the orientation of the detector is appropriate. Doses near air c hannels are subject to significant local variations as a function of small changes of field width, and local underdosing may occur in particular cases . (C) 1999 Elsevier Science ireland Ltd. All rights reserved.