Treatment plan of EUS-guided interstitial brachytherapy

The morphology and location of the tumor were scanned by EUS carefully, and the maximum cross-section of the tumor was determined. The maximum and minimum diameters of the tumor were measured, and the important structures within and around the target were marked. In general, the maximum cross-section was first chosen to implant the radioactive seeds.

In the process of EUS-guided brachytherapy, the transformation of the puncture section was achieved principally through rotation of the EUS probe. In the appropriate puncturing plane, a target puncture area was determined, which was positioned at about 6 to 7 o’clock on the EUS image. In contrast to other implantation methods, the radioactive seeds could not be implanted in the region outside the target area through EUS guidance. The radioactive seeds were arranged along the puncture path at a certain interval. Assuming the target tumor was a spherical mass close to the digestive tract, the differences between EUS-guided puncture and traditional prostate brachytherapy are shown in Figure 6A-D. The absorbed doses at the edge of the tumor on the EUS image were calculated, and the lowest value was defined as the minimal peripheral dose. The proper activity of a single seed and the seed arrangement were calculated to ensure that the minimal peripheral dose was not lower than the therapeutic dose.

Differences were observed between EUS-guided puncture and traditional prostate brachytherapy, including rotation transformation of puncture section under EUS-guided brachytherapy (A.), irregular arrangement of radioactive seeds in an EUS section (B.), parallel transformation of puncture section under traditional prostate brachytherapy (C.), and regular arrangement of radioactive seeds in an section (D.) The black dotted line represented the puncture section. The radioactive seeds (black dots) were arranged at equal distances along straight lines. E. Transformation of puncture plane was through a rotation (α⁰ Degree) of the EUS probe. R was the distance from the point A to EUS probe. H indicated the vertical movement when the minimum peripheral doses at point A reached half the therapeutic dose. The target tumor was assumed to be a spherical mass proximal to the digestive tract.

The moving distance of the EUS puncture section in the vertical direction was recorded when the minimal peripheral dose reached half of the therapeutic dose. The rotation angle of the EUS probe was then calculated according to the following modified formula (Figure ​(Figure6E):6E): α=H × 360/πR, where R was the maximum distance from the probe to the tumor edge in proper EUS section, the vertical movement of the plane was H when the minimum peripheral doses reached half the therapeutic dose at the corresponding point. An excessive rotation angle would result in a wide cold area with an insufficient radiation dose. However, the angle of rotation should not be too small, which would increase the time of punctures and the risk of complications.

The region of the cold area in which the cumulative absorbed dose was less than the therapeutic dose was calculated. Additional radioactive seeds should be replanted within or near the cold area to supply the necessary radiation. Therefore, the absorbed dose in the important anatomical structures around the target area, such as the pancreatic duct and blood vessels, could be calculated. The local distribution and amount of radioactive seeds were adjusted to avoid irradiation complications.