Progress in Medical Physics (한국의학물리학회지:의학물리)

Korean Society of Medical Physics (한국의학물리학회)
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The Crucial Role of the Establishment of Computed Tomography Density Conversion Tables for Treating Brain or Head/Neck Tumors

  • Yang, Shu-Chin (Department of Radiation Oncology, Chi Mei Medical Center) ;
  • Lo, Su-Hua (Chiu Ho Medical System Co., LTD) ;
  • Shie, Li-Tsuen (Department of Radiation Oncology, Chi Mei Medical Center) ;
  • Lee, Sung-Wei (Department of Radiation Oncology, Chi Mei Medical Center) ;
  • Ho, Sheng-Yow (Department of Radiation Oncology, Chi Mei Medical Center)
  • Received : 2021.05.17
  • Accepted : 2021.08.06
  • Published : 2021.09.30
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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.

Keywords

Acknowledgement

The authors are greatly thankful to Mrs. Lo of Chiu Ho Medical System Co., LTD, Taiwan, for the knowledge on Pinnacle TPS. This study was supported by the Chi Mei Medical Center, Liouying, from January 2020 (grants CLFHR10914).

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