Factors Affecting Depth-Dose Accuracy in Modern Diagnostic X-Ray Imaging

Purpose: Distribution of radiation in and through a patient is fundamental to image creation, radiation management, and evaluation of potential radiation risk to patients. While other investigators have generally demonstrated how beam factors such as kilovoltage (kV), half-value layer (HVL) and field size affect central axis depth dose, no one has demonstrated the relative importance each of these factors in characterizing the beam. Additionally, source-to-skin distance (SSD) affects depth dose due to beam divergence while radiation buildup depends on beam energy. Further, the relative effect on depth dose from the use of copper versus aluminum as filtration material has not been investigated. This research investigates the relative importance of these factors by measuring how central-axis depth dose independently depends on each factor when the others are held constant. 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 a 0.02-cm3 plane-parallel Advanced Markus chamber (PTW-Freiburg, Freiburg, Germany), a 0.62-cm3 Exradin A19 (Farmer type) waterproof ionization chamber (Standard Imaging, Middleton, WI, USA), and a PC Electrometer (Sun Nuclear Corp. Melbourne, FL, USA). Measurements at depths of 10 millimeters and less were made with the parallel plate chamber to assess effects of build-up and field size on dose at shallow depths. Measurements at 10-mm and deeper depths were made with a farmer-type chamber to improve the precision of those measurements. 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-cm x 20-cm field size on water surface. Results: To assure accuracy in determining depth dose for modern diagnostic beams, the data demonstrate that HVL, field size and SSD should be carefully specified. Mayneord's correction can be used to adequately adjust data for variances introduced by measurements at different SSDs. The kV for diagnostic beams need only be generally specified. The effects of scatter radiation and buildup at shallow depths contribute to the shoulder of the depth-dose curve at higher beam qualities but should not adversely affect the normalization of depth-dose measurements. For equal HVL, depth-dose has no discernable dependence on whether copper or aluminum is used to filter the beam.