Sci—Thurs PM: Planning—03: The Effect of Small Cylindrical Air Cavities on Circumferential Dose Distributions Due to Small 6 MV Photon Fields

The purpose of this work is to evaluate the accuracy of our radiotherapy treatment planning system, Pinnacle3, for calculating the dose distribution around small cylindrically shaped air cavities irradiated by small 6 MV photon field segments. Measurements under these conditions carried out by various investigators have been of limited precision or non‐ideal geometry and have been confined in angular extent around the cavity circumference. In this work, we investigate the use of radio‐chromic film (Gafchromic® EBT) in a novel geometry whereby individual strips of film are formed into cylinders of various diameters and positioned concentrically about a 2.5 cm diameter cylindrical air cavity centred in a 10 cm diameter water bottle phantom. Three fields are investigated: 4 cm square; 1 cm by 8 cm; and 8 cm by 1cm. The film response is carefully calibrated and characterized to yield both the angular and radial distribution of absolute dose. The measurements are compared with Pinnacle3 calculations of the corresponding geometry. For all fields, calculations agree with measurement to within experimental error of about 3% for regions beyond 3 mm of the cavity surface. At the surface of the cavity, however, calculations are between 5 and 16% high, depending on what proportion of the beam area is subtended by the air cavity. This discrepancy is present not only on the axis of the beam but on the entire circumference of the cavity. The results of these measurements must be appreciated when evaluating clinical IMRT plans of sites such as larynx.