PO0109

Since the introduction of three-dimensional computed tomography-based brachytherapy in the Institute of Oncology “Prof. Dr. Ion Chiricuta” Cluj-Napoca (Romania) in 2016, over 1642 patients were treated in 4040 sessions of vaginal brachytherapy. The applicator used transitioned from classical vaginal cylinder to unshielded vaginal cylinder and since 2022 to stump universal cylinder, all with different geometries when it comes to the first position of the iridium 192 source compared to the surface of the cylinder's tip. We aimed to find what applicator gives the minimal difference between target volume and irradiated volume, what are the differences in applicator dosimetry, rectal and bladder doses by creating three plans for the same patients using all three types of cylinders. For 24 selected patients (we selected 3 patients per year) that had treatment plans done using one of the mentioned cylinder with the diameter of 30 mm, and we created three treatment plans and measured distance from first source position to surface of the tip cylinder, width of the 100% isodose at the level of the first source position, the D90 for the target volume, 2cc for rectum and bladder and overall nominal treatment time. For each par We found that the distance between the first source position and surface of the cylinder tip was 1.08, 0.77 and respectively 0.30 cm for the white classic cylinder, unshielded cylinder respectively the stump cylinder. Regarding the width of the 100% isodose line was median of 5.28, 4.60 respectively 3.86 cm for the white classic cylinder, unshielded cylinder respectively the stump cylinder. In regards of doses, the median value of the D90 for the target volume was 42.8, 39.3 respectively 36.7 Gy for the white classic cylinder, unshielded cylinder respectively the stump cylinder. The median volume of the target was 66.6 cc and when compared to the irradiated volume we found that for the white classic cylinder, unshielded cylinder respectively the stump cylinder the median 100% irradiated volume generated the values were median 81.53 (120.9cc), 50.90 (100.50 cc) respectively 32.73 (32.73 cc) % higher than the target volume. For the organs at risk we found that the median dose to the 2cc for the rectum was 127, 104 respectively 85 % of the prescribed dose in case of the white classic cylinder, unshielded cylinder respectively the stump cylinder; and for the 2cc of the bladder the median values were 131, 95 respectively 78 % for the white classic cylinder, unshielded cylinder respectively the stump cylinder. In conclusion we can say that the white classic cylinder witch has the longest distance between first source position and the surface of the cylinder's tip, has also the largest 100% isodose width, the largest irradiated volume (almost 82% more) when compared to target volume, and of course the highest percentage of doses delivered to organs at risk. Our findings show that geometry wise, the new and improved stump cylinder is better when we take into account the over irradiation offered with the old cylinders. We consider that further investigation should be done in order to see how these dosimetry findings can be translated to overall survival, disease free survival and organs at risk complications.