A comparison of tandem ring and tandem ovoid treatment as a curative brachytherapy component for cervical cancer

Abstract

Purpose: The standard treatment for locally advanced stage cervical cancer is definitive radiotherapy, the quality of which affects both survival and side effects. Brachytherapy is a major component of definitive radiotherapy; it is administered using different techniques and applicators. The purpose of this study was to dosimetrically compare tandem ovoid (T-ovoid) and tandem ring (T-ring) brachytherapy treatments.

Material and methods: Both applicator sets were applied to the same 20 patients, and treatment plans were made three-dimensionally (3D), with high-risk clinical target volume (HR-CTV) and organs at risk contoured. The HR-CTV was defined according to post-external magnetic resonance results. The patients with residual tumors not exceeding one-third of the parametrium were included in this study, while patients with larger masses were excluded and received interstitial therapy. The doses were calculated for both plans. Optimization for the HR-CTV was made with the aim that the equivalent dose according to 2 Gy (EQD₂) of 90% of the HR-CTV (D₉₀) would be higher than 85 Gy, without exceeding the maximum dose for organs at risk. Then, pairwise dosimetric comparisons were performed.

Results: Plans were compared dosimetrically according to the HR-CTV, point A and B doses, and organs at risk. Although the point A and B doses were higher with T-ovoid use, the 3D HR-CTV coverage was statistically better with T-ring application (EQD₂ of HR-CTV D₉₀: 97.46 Gy for T-ring and 88.44 Gy for T-ovoid; p < 0.0001). In addition, the rectum and bladder doses were statistically lower with T-ring usage (EQD₂ of rectum, 2 cc; T-ring, 63.10 Gy; T-ovoid, 74.99 Gy; p < 0.0001; EQD₂ of bladder, 2 cc; T-ring, 85.26 Gy; T-ovoid, 89.05 Gy; p < 0.0001).

Conclusions: In our study with a limited number of samples, T-ring applicator seems to offer better 3D brachytherapy dosimetry for both HR-CTV and nearby organs at risk.

Keywords: brachytherapy; high-risk clinical target volume; ovoid; ring; three-dimensional planning.

Fig. 1

T-ring (right side, three images) and T-ovoid (left side, three images) applications and plans in the same patient: axial, coronal, and sagittal planes images (images used with a permission from the patient)

Fig. 2

Dose-volume histogram of the high-risk clinical target volume using the T-ring and T-ovoid applicators (one fraction). The mean doses to 100%, 99%, 97%, 95%, 90%, 80%, 70%, 60%, 50%, 40%, 30%, 20%, and 10% volumes were calculated in each group, and a cumulative dose-volume histogram was obtained. Blue: T-ring group; orange: T-ovoid group