SU-FF-T-333: Orthovoltage Dosimetry: Percent Depth Dose Multiplicity From Two Energies

Abstract

Purpose: To produce a multiplicity of depth dose curves on a two‐kVp orthovoltage system by appropriate combination of the 75 kVp and 225 kVp x‐ray beams. Method and Materials: Using tabulated percent depth dose (PDD) curves at 75 and 225 kVp for five rectangular applicators (4×6, 6×8, 8×10, 10×15, and 15×20 cm2) for our Philips RT‐250 unit, we calculated the mixed‐beam PDDs for each applicator using w 75 ⋅ PDD 75 + w 225 ⋅ PDD 225 , where w denotes the beam component weight, the subscript indicates the component kVp, and w 75 +w 225 =1 . By specifying a dose D0 at the surface and an isodose level at a particular depth, we calculated the required beam weights; incorporating the tissue dose rates then allows us to calculate the beam‐on times for the 75 and 225 kVp beams, respectively. All single‐ and mixed‐energy PDDs were verified in 20×20 cm2 blocks of solid water (Gammex RMI, Middleton, WI) using a field standard electrometer and ion chamber set. The ion chamber fit snugly at the centre of a 2.0 cm thick block. The depth doses were measured at solid water depths of 1.0 to 15.0 cm keeping the source‐to‐surface distance constant at 50.0 cm, and using 10.0 cm of backscatter material. The PDD values were calculated by scaling the measurements relative to the tabulated or theoretical values at a depth of 1.0 cm. Results: The ion chamber measurements at 75 and 225 kVp were within 2 percentage points of the tabulated values for all rectangular applicators. Similarly, the mixed‐energy PDD curves for f 75={0.25, 0.5, 1.0} were also within 2 percentage points of the theoretical expectations. Conclusion: A firm theoretical formalism to create PDDs combining the 75 kVp and 225 kVp beams was formulated and verified experimentally using ion chamber measurements. Our mixed‐energy methodology allows one to synthesize a multiplicity of PDDs between any two orthovoltage energies.