DOSIMETRIC COMPARISON OF RING AND OVOID APPLICATORS

AIM: It was aimed to compare the dosimetry of different vaginal applicators used in to postoperatif vaginal cuff irradiation. METHODS: In this model dosimetric study, standard ovoid applicator set with 3 cap sizes; small: 26 mm, medium: 30 mm, and large: 34 mm, and ring applicator sets (each of which has angles of 30, 45, and 60 degrees) were used Calculations were performed in Plato IPS (Treatment Planning System) as dwell positions of 2.5 mm step and equal dwell time for the sources. For ovoids the source positions 1 -5 were loaded as active on each catheter, and in ring applicators 1-12; and 18-29; 1-14; and 19-32; and 1-17 and 22-37 were loaded for small, medium and large respectively. In addition of ICRU 38 rectum reference point (R.,) four different rectum reference points (RPs) (R2-Rs) were defined 1 cm apart. The vaginal cuff RP (Vc) was defined at 5 mm depth of vaginal surface. The reference volume dose distribution was obtained according to the Vc 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. RESULT: R1 dose was higher for ovoids (43.1 %) than ring (28.6%), in all applicators. The increase of R, dose was parallel to the increase of ovoid/ring size (70-89%). When the aplicator (ovoid-ring) size increased, referance isodose's maximum thickness rised (19-50%). For the largest size, the max increase was 62% (74.1-119.88 cc) and 93% (59.15-114.24 cc) for v and v', respectively. However, the magnitude of the reference volume was independent of ring angle. CONCLUSION: It's known that type of applicator, source arrangement and optimization techniques effect the size of the reference volume and dose to organs at risk. In practice, different applicator types are used in order to tailor treatment according to patient specific factors (tumour size, organ anatomy, etc.). In the present study it was found that when the ring applicator was used, max rectum reference dose decreased in comparison to ovoids, on the other hand it should be noted that the size of reference volume also decreased significantly.

Keywords: intracavitary brachytherapy, applicator, dosimetry