Small bowel dose in subserosal tandem insertion during cervical cancer brachytherapy

Cervical cancer patients may sometimes experience subserosal tandem insertions during brachytherapy, which can lead to increased but unnoticed irradiations to the small bowel (SB). In this study, we aimed to quantify and further predict individual SB dose increase and to increase focus on the SB in subserosal tandem insertions. Images and dosimetry data of cervical cancer brachytherapy with subserosal insertion (SI) were reviewed. The percentage increases in the SB dose compared with intracavitary insertion (II) at 8 points of D(x)cc with 10 cc intervals were assessed. SI was classified into anterior and posterior SI according to the insertion site. The differences in minimum distance from the tandem tip to the SB on the axial view between these 2 insertions were tested using the Mann-Whitney test. The distance and D(x)cc were involved in the individual dose increase model by linear regression as prediction factors. A total of 27 insertions were evaluated, including 8 insertions with SI and 19 insertions with II. The median percentage increases in the normalized SB dose for all SI showed a logarithmic trend with a 55.4% increase at the hotspot. In contrast to posterior SI, anterior SI demonstrated a more significantly logarithmic trend, which featured highly increased doses at the hotspot (79.1% for the absolute SB dose and 137.8% for the normalized SB dose). The prediction models can predict the percentage dose increases in SI: Increased D(x)cc [%] = 31.370 - 7.865 ln(distance) - 3.949 ln(x) (absolute SB dose), and Increased D(x)cc [%] = 55.618 - 18.591 ln(distance) - 7.232 ln(x) (normalized SB dose). We developed prediction models for individual SB dose increase in SI in our study. SB hotspots in anterior SI require greater attention during cervical cancer brachytherapy. The models are new ones and are given for the first time.