Room: Room 207
Purpose: The distribution of radiation through a patient is fundamental to image creation, radiation management, and evaluation of potential radiation risk to patients. Other investigators have generally demonstrated how beam factors such as kilovoltage (kV) and half-value layer (HVL) affect central axis depth dose. To determine the relative importance of these two factors in determining depth dose, we investigated how depth dose depends on kV for constant HVL and on HVL for constant kV.
Methods: A Philips Super 80 CP RF system and a cubic 30-cm 1D water tank (CIVCO, Orange City, Iowa) were used to assess depth dose. Ionization charge was collected using an 0.62-cc Exradin A19 (Farmer type) waterproof ionization chamber (Standard Imaging, Middleton, WI, USA) and a PC Electrometer (Sun Nuclear Corp. Melbourne, FL, USA). An independent free-in-air reference chamber / detector (either RadCal 6 cc ion chamber or Raysafe X2 RF detector) was placed in the periphery of the X-ray field and used to correct for any fluctuations in radiation output. The tank was filled with water to at least 25 cm and placed on a cart. Standard measurements were made with the following configuration: 100 cm source to water surface distance and 20 cm2 field size on water surface.
Results: Depth dose curves are shown in the supporting documentation and demonstrate classic exponential-like decrease in percent dose with increasing depth.
Conclusion: Central-Axis Depth Dose demonstrates a much greater dependence on HVL than on kV. Buildup in the first 10-mm is evident and curves should be normalized to dose at Dmax.