Prog Med Phys.  2021 Sep;32(3):59-69. 10.14316/pmp.2021.32.3.59.

The Crucial Role of the Establishment of Computed Tomography Density Conversion Tables for Treating Brain or Head/Neck Tumors

• Yang SC
• Lo SH
• Shie LT
• Lee SW
• Ho SY

• KMID: 2521093
• DOI: http://doi.org/10.14316/pmp.2021.32.3.59

Abstract

Purpose
The relationship between computed tomography (CT) number and electron density (ED) has been investigated in previous studies. However, the role of these measures for guiding cancer treatment remains unclear.
Methods
The CT number was plotted against ED for different imaging protocols. The CT number was imported into ED tables for the Pinnacle treatment planning system (TPS) and was used to determine the effect on dose calculations. Conversion tables for radiation dose calculations were generated and subsequently monitored using a dosimeter to determine the effect of different CT scanning protocols and treatment sites. These tables were used to retrospectively recalculate the radiation therapy plans for 41 patients after an incorrect scanning protocol was inadvertently used. The gamma index was further used to assess the dose distribution, percentage dose difference (DD), and distance-to-agreement (DTA).
Results
For densities <1.1 g/cm³ , the standard deviation of the CT number was ±0.6% and the greatest variation was noted for brain protocol conditions. For densities >1.1 g/cm³ , the standard deviation of the CT number was ±21.2% and the greatest variation occurred for the tube voltage and head and neck (H&N) protocol conditions. These findings suggest that the factors most affecting the CT number are the tube voltage and treatment site (brain and H&N). Gamma index analyses for the 41 retrospective clinical cases, as well as brain metastases and H&N tumors, showed gamma passing rates >90% and <90% for the passing criterion of 2%/2 and 1%/1 mm, respectively.
Conclusions
The CT protocol should be carefully decided for TPS. The correct protocol should be used for the corresponding TPS based on the treatment site because this especially affects the dose distribution for brain metastases and H&N tumor recognition. Such steps could help reduce systematic errors.

Keyword

Computed tomography number; Electron density tables; Dose distribution; Gamma index; Imaging protocols

Figure

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  Fig. 1 The cylindrical Cheese Virtual WaterTM phantom.

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  Fig. 2 The computed tomography (CT) number–electron density conversion tables were split into two ranges: <1.1 and >1.1 g/cm3 densities. A straight-line fit was constructed for each range. The mean and standard deviation for each range and set of scanning conditions were used as a measure of the CT number relative to the CT scanning parameters.

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  Fig. 3 The computed tomography (CT) number–electron density conversion tables using different tube voltages. For densities >1.1 g/cm3, the standard deviation for the CT number was ±21.2%. For densities <1.1 g/cm3, the standard deviation for the CT number was ±0.6%.

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  Fig. 4 Local gamma passing rates with various passing criteria calculated from the PTW–VeriSoft® dosimeter measurements are shown for (a) the 80 and 140 kVp plans and (b) the 120 and 140 kVp plans. Global gamma passing rates with various passing criteria calculated from the PTW–VeriSoft® dosimeter measurements are shown for (c) the 80 and 140 kVp plans and (d) the 120 s and 140 kVp plans. The dosimetric effects for the four treatment sites (brain metastases plans, head and neck cancer plans, lung cancer plans, and prostate cancer plans) were analyzed. The error bars above the histogram indicate the standard deviation.

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  Fig. 5 Sagittal view of a brain metastases treatment plan (a) parallel to the IOM line and across the frontal lobe (the top of the ROI), (b) parallel to the IOM line and across the corpus callosum (the middle of the ROI), and (c) parallel to the IOM line and across the cerebellum (the end of the ROI). The local gamma passing rates for various passing criteria were compared for 1%/1 mm and show (d–f) ED calibrations of 80 and 140 kVp and (g–i) ED calibrations of 120 and 140 kVp. Passing criteria: gamma ≤1. Green 90.0%–100.0%, yellow 75.0%–90.0%, and red 0.0%–75.0%. IOM, inferior orbitomeatal; ROI, circular region of interest; ED, electron density.

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  Fig. 6 Sagittal view of H&N tumor treatment plan system (a) parallel to the line and across the maxillary sinus (the top of the ROI), (b) parallel to the IOM line and across the mandible (the middle of the ROI), and (c) parallel to the IOM line and across the C7 (the end of the ROI). The local gamma passing rates for various passing criteria were compared for 1%/1 mm and show (d–f) ED calibrations of 80 and 120 kVp and (g–i) ED calibrations of 120 and 140 kVp. Passing criteria: gamma ≤1. Green 90.0%–100.0%, yellow 75.0%–90.0%, and red 0.0%–75.0%. IOM, inferior orbitomeatal; ROI, circular region of interest; ED, electron density.

Reference

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