Precision dose measurement of ¹²⁵I seed in solid water through SPECT/CT detecting

The aim of the study is to study the feasibility of gamma-ray-detection-based precision dose measurement of ¹²⁵I seed brachytherapy in solid water.

Seven group ¹²⁵I seeds with different activities were put into a hole in the center of solid water individually. Each group had ten seeds, and the seed activity ranged from 1.48 × 10⁷ Bq to 3.7 × 10⁷ Bq. Single-photon emission computed tomography/computed tomography (SPECT/CT) was used to scan the seeds perpendicular to the long axis of the seed, with a slice thickness of 3.75 mm. The radioactive count values (x) of the radioactive concentration around the seeds were collected at a distance of 1-15 mm from the center of the seeds, while the corresponding doses (Y) (Gy) were calculated. SPSS 18.0 was used to analyze the relationship between the count value and the dose.

With the same seed activity, the count values became smaller according to the distance from the center of the seeds. The count values at the same point had an increasing trend according to the activity. This is similar to the doses calculated at the same point. There was an exponential relationship between the dose around the ¹²⁵I seeds, and the radioactive count value detected by SPECT/CT. Correlative curves between the dose and radioactive count value detected by SPECT/CT of different-activity ¹²⁵I seeds were fitted. The formulas of the dose and radioactive count with different seed activity were in the form of Y = b₀ (b₁)^x. The constant b₀ ranged from 1.48 to 3.93, according to the seed activity, while b₁ was 1.006 for every seed's activity.

The count value around the ¹²⁵I seed can be detected accurately by SPECT/CT, and then can be quantified. This study provided useful experiment data for the precision measurement of ¹²⁵I seed implantation. Radiation detection-based dose measurement may become a new noninvasive technology for the dynamic dosimetry verification method after brachytherapy.