Phantom Study of Radiotherapy Planning Using PET/CT

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Other Title
  • PET/CTを用いた放射線治療計画の基礎的研究
  • PET/CTを用いた放射線治療計画の基礎的研究 : 不均一腫瘍モデルでのGTVの輪郭描出
  • PET/CT オ モチイタ ホウシャセン チリョウ ケイカク ノ キソテキ ケンキュウ : フキンイツ シュヨウ モデル デ ノ GTV ノ リンカク ビョウシュツ
  • 不均一腫瘍モデルでのGTVの輪郭描出
  • Delineation of Gross Tumor Volume of Inhomogeneous Tumor Models

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Abstract

Purpose: We have been investigating automatic contour delineation of the target volume using PET/CT images, but there are several problems with the method using the SUV value. We performed a basic study on contour delineation of the target volume with inhomogeneously distributed 18F-FDG. Methods and Materials: We prepared and investigated a simple model (acrylic sphere) of inhomogeneous 18F-FDG distribution. As a tumor model encountered in clinical practice relatively frequently, the layer thickness and 18F-FDG concentration were changed assuming the following conditions, and changes in the SUV value and target volume contour were investigated: 1) A layer without 18F-FDG accumulation (necrotic layer) is present in the center of a sphere assumed to be a tumor and 2) the 18F-FDG concentration distribution is uneven in the sphere. Since the size of each sphere is known, the rate (%) of the SUV to its maximum agreed with the size was determined to evaluate contour delineation. Results: 1) In the model containing a necrotic layer in the center of the sphere, when the necrotic layer was enlarged and the region incorporating 18F-FDG was narrowed, the SUV value was underestimated. The contour could be delineated agreeably with the acrylic sphere size regardless of the thickness of the necrotic layer by setting the SUV value at 30-50% of the maximum. 2) In the model with an uneven 18F-FDG concentration distribution in the sphere, the contour was distorted and different from the actual size and shape of the acrylic sphere. Conclusion: We prepared a phantom model of inhomogeneous 18F-FDG distribution. When a necrotic layer was present in the center of the sphere, the SUV value tended to be underestimated when the region incorporating 18F-FDG was narrowed. The contour could be accurately delineated by setting the SUV value at 30-50% of the maximum when the 18F-FDG concentration in the outer layer was high, but attention should be paid when the 18F-FDG concentration distribution is uneven.

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