Background: This study explored the effects of air cavity on the dose distribution of radiotherapy in patients after extensive hysterectomy. In patients who have an air cavity in the intestines near the planning target volume (PTV), the photon beams may interact with the air cavity to cause electron disequilibrium (ED), resulting in a reduction in the absorbed dose by the surrounding tissues. In this paper, the electron density assignment (EDA) of the air cavity was used to simulate the disappearance of the intestinal gas, and the effects of the intestinal air cavity on the volume modulated arc therapy (VMAT) results were examined.
Methods: A total of 21 patients who underwent VMAT after extensive hysterectomy were enrolled in this retrospective analysis. The dose parameters from the selected treatment plan were used the experimental reference. The treatment plan of the reference group was copied, and the intestinal air cavity structure was identified using the computed tomography (CT) simulation image. The electron density value of the intestine near the cavity was measured and averaged according to the intestinal electron density value recommended by International Commission Radiological Units (ICRU) report No. 46. The averaged value was assigned to the air cavity structure. Subsequently, the treatment plan was re-calculated without changing other parameters, and the resulting treatment plan was defined as the experimental group. The dose parameters of the PTV and organs at risk (OAR) in the 2 groups were assessed, and the influence of the intestinal air cavity on the VMAT dose distribution in cervical cancer patients was analyzed.
Results: The minimum dose (D98) and the maximum dose (D2) of the PTV was significantly different between the experimental group and the control group (P<0.05), however, mean dose of the PTV was comparable between the 2 groups. The dose parameters of OARs were not significantly different between the two groups except the bone structural organs.
Conclusions: When the intestinal air cavity is large and related to the target area, the intestinal air cavity should be intervened, and the patient should be treated with radiotherapy after the intestinal gas decreases.
Keywords: Volume modulated arc therapy (VMAT); air cavity; cervical cancer; dose distribution.