Tolerance levels for quality assurance of electron density values generated from CT in radiotherapy treatment planning

Abstract

Methods are described which relate the uncertainty in relative electron density derived from CT numbers to the uncertainty in treatment plan calculation for both photon and electron beams. These relationships are used to generate tolerance levels for electron density quality assurance measurements. These tolerance levels are dependent on treatment beam energy and tissue thickness, and are generally broader than current recommendations. The predicted treatment plan errors associated with these tolerance levels are shown to be consistent with calculations made using two commercial treatment planning systems. The tolerance levels are also shown to be practical by comparison against quality assurance measurements made on a conventional CT scanner and a treatment simulator CT system over a 12-month period. The results demonstrate that broader tolerances than are currently recommended can be justified in all situations except for electron beam treatments in which the therapeutic range falls within lung tissue.

Computed Tomography as a Source of Electron Density Information for Radiation Treatment Planning

“…It should also be noted, that for the same scanners and the same voltages (but different phantom, or different settings) the differences were lower than 0.05. All the results obtained for general-purpose CT scanners operating at 120 kV or 140 kV fall within tolerance levels proposed by Kilby et al [10], which are based on calculation of impact of inaccuracies on dose calculations. The results suggest that relationships implemented in TPS can generally be used for general-purpose CT systems operating at voltages close to 120 kV.…”

Treatment Planning System Calculation Errors Are Present in Most Imaging and Radiation Oncology Core-Houston Phantom Failures

“…However, there are a host of reasons why a computational error may arise. Beam modeling is a clearly known problem 12 , but poor commissioning data, the dose grid spacing, 16 and the HU density curve 17 could also contribute. Ultimately, to meet the goal of IROC Houston identifying the underlying cause(s) of a phantom failure, more refined information is necessary.…”

An evaluation of three commercially available metal artifact reduction methods for CT imaging

“…This 40 HU threshold was chosen because it corresponds to approximately a 0.03 g/cm 3 density assignment error for water. This density assignment error was found to result in approximately 1%-2% dose calculation errors for 6MV photon treatments and is the electron density tolerance level recommended by Kilby et al (2002). For each phantom image set analyzed, the percentage of bad pixels (pixels with HU error > 40 HU) was calculated.…”

“…In radiation therapy treatment planning, CT images are used for delineating targets and critical organs, defining the treatment geometry, and assigning densities for heterogeneous dose calculations. For treatment planning, CT imaging artifacts make it difficult for the physician to confidently delineate the tumor and surrounding organs and cause errors in CT numbers (expressed in Hounsfield units [HU]), which can propagate to density assignment errors and subsequently dose calculation errors (Chu et al , 2000; Kilby et al , 2002; Papanikolaou et al , 2004). …”