Dosimetric comparison of ring and ovoid applicators

To compare the dosimetry of different vaginal applicators used in the postoperative vaginal cuff irradiation.

In this model dosimetric study, standard ovoid applicator sets with 3 cap sizes (small/15 mm, medium/20 mm, and large/25 mm), and ring applicator sets with 3 diameter sizes (small/26 mm, medium/30 mm, large/34 mm) each with 3 different angles (30, 45, and 60 degrees) were used. Calculations were performed in Plato TPS (Treatment Planning System) as dwell positions of 2.5 mm step and equal dwell time for the sources. For ovoid applicators, the source positions 1-5 were loaded as active on each catheter, and in ring applicators the source positions 1-12, and 18-29; 1-14, and 19-32; 1-17, and 22-37 were loaded for small, medium and large ones, respectively. In addition to ICRU rectum reference point (R(1)), 4 different rectum reference points (RP(s)) (R(2)-R(5)) were defined 1 cm apart. The vaginal cuff RP (V(c)) was defined at 5 mm depth from the vaginal surface. The reference volume dose distribution was obtained according to the V(c) (100%) reference point. The width (w), max-min length (h, h'), and max-min thickness (t, t') of the reference isodoses were measured and max-min reference volume (v, v') was calculated.

R(1) dose was higher for ovoids (43.1%) than ring (28.6%), in all applicators. The increase of R(1) dose was parallel to the increase of ovoid/ring size (70-89%). As the applicator (ovoid-ring) size increased, max thickness of the reference isodose also increased (19-50%). For the largest size, the max increase was 62% (74.1-119.88 cm(3)) and 93% (59.15-114.24 cm(3)) for v and v', respectively. However, the magnitude of the reference volume was independent of the ring angle.

In the present study it was found that when ring applicators were used according to recommended dwell positions, max rectum reference dose decreased in comparison to ovoid applicators; on the other hand, it should be noted that the size of the reference volume also decreased significantly.