ESTRO 36 Abstract Book

S965 ESTRO 36 2017 _______________________________________________________________________________________________

Purpose or Objective To evaluate if EQD2 (α/β=3) at 0.1cc, 1cc and 2cc of vagina in cylinder vaginal-cuff brachytherapy (VBT) ±external beam irradiation (EBI) is associated with G2 toxicity in postoperative endometrial carcinoma (P-EC). Material and Methods From June 2014-November 2015, 67 consecutive P-EC patients received VBT±EBI: 54 EBI (median 45Gy, range 44- 50.4)) +VBT (7Gy) and 13 exclusive BT (6Gy x 3 fractions). 2.5cm of vagina was delineated after CT for 3D treatment planning. The active source length was 2.5cm. The BT dose was prescribed at 5mm from the applicator surface. Patients were treated with HDR 192 Ir source using a MicroHDR source projector (Nucletron®). D90, V100 and EQD2 (α/β=3) at 0.1cc, 1cc and 2cc were calculated. The mean follow-up was: 23.2 months (range 7.6-46.8). D90 (cc): median 7.8 (range 4.6-8.9); V100 (Gy): median 7.9 ( range 4.4-10.8). Vaginal toxicity was prospectively assessed using objective LENT-SOMA scores. Late vaginal toxicity: 17/67 (25%) 8 with G1 and 9 G2. For this analysis G0 and G1 patients were considered as no late toxicity (58/67, Group-1) and 9 patients with G2 (9/67,Group-2) were considered as having late toxicity. Statistics: t-Student test and Chi squared, alpha=5%. Results The median EQD2 (α/β=3) doses were 88.6Gy (62.8-177.6) for 0.1cc, 72.4Gy (57.1-130.4) for 1cc and 69Gy (53-113.4) for 2cc. There were no differences in toxicity and EQD2 (α/β=3) between exclusive VBT vs. EBI+VBT. EQD2 (α/β=3) : The mean EQD2 (α/β=3) : at 0.1cc was 92.9Gy (SD 17.7) for Group-1 and 96.3Gy (SD 31.6) for Group-2 (p=0.62); being 72.3Gy (SD 6) at 1cc for Group-1 and 73.5Gy (SD 5.3) for Group-2 (p= 0.58); and 67.6Gy (SD 6.2) at 2cc for Group-1 and 73.1Gy (SD 10.8) for Group-2 (p=0.03). 20.5% of patients receiving doses ≥68Gy EQD2 (α/β=3) at 2cc of vagina developed G2 toxicity. All patients with G2 toxicity had received doses ≥68Gy EQD2 (α/β=3) at 2cc (p=0.04). Conclusion 68Gy EQD2 (α/β=3) doses at 2cc were related to G2 toxicity in P-EC VBT. In view of these results patients receiving these doses should be informed of their risk and individual characteristics should be considered in treatment planning and follow-up to reduce G2 toxicity. Grant: Spanish Association Against Cancer (AECC) Foundation . EP-1781 statistical and dosimetric analysis of air gaps in vaginal cuff brachytherapy S. Abdollahi 1 , L. Rafat-Motavalli 2 , H. Miri-Hakimabad 2 , M. Mohammadi 1 , E. Hoseinian-Azghadi 2 , N. Mohammadi 2 , N. Rafat-Motavalli 2 , L. Sobhkhiz-Sabet 1 1 Reza Radiation Oncology Center RROC, medical physics, Mashad, Iran Islamic Republic of 2 Ferdowsi University of Mashhad, Physics, Mashhad, Iran Islamic Republic of Purpose or Objective To retrospectively evaluate the incidence, magnitude, and dosimetric impact of any air pockets between the vaginal cylinders and the vaginal mucosa using CT-scan images. Material and Methods 120 postoperative vaginal cuff brachytherapy cases were analyzed for receiving the prescribed dose to 5 mm depth from the cylinder wall. CT-Based treatment planning was performed in each fraction. The incidence, vaginal mucosa displacement and air volume were assessed in each treatment. A Monte Carlo study has also been done to evaluate the dosimetric effect of air pockets around the vaginal cylinder. Results In 50 patients, a total of 90 air pockets were observed in 150 procedures. Four patients had pocket free insertion during the whole treatment sessions. The volume of air pockets ranged between 0.01 cm3 and 4.5 cm3 with average value of 2.5cm3, and the maximum displacement

of vaginal mucosa from cylinder surface was between 0.2 and 2 cm with average value of 0.8cm. Thirty patients had no air pockets on their first fraction but in subsequent fractions. Twenty patients had incorrect applicator insertion as they have an air gap between applicator tip and cylinder dome ranged between 0.3 cm and 1.1 cm with average value of 0.8 cm. The Monte Carlo dosimetry also shows that the average dose reduction to the vaginal surface and 5mm-depth, at the air pocket, is respectively about 9.2% and 7.3% per 1 mm of the air thickness. Conclusion Air pockets between vaginal cylinder and the vaginal mucosa are observed in the majority of treatment fractions, and the probability of occurrence varies from patient to patient and procedure to procedure. The dose reduction effect of air pockets needs to be considered especially around the vaginal cuff using imaged based treatment planning in each fraction and the effect on the clinical outcome should be put under more investigation. EP-1782 Effect of the amount of bladder filling on normal tissue doses in 3D-HDR vaginal vault brachytherapy I. Er 1 , S. Kınay 1 , R. Kandemir 1 , F. Obuz 2 , A.N. Demiral 1 1 Dokuz Eylul Univ. Health Sciences Institute, Radiation Oncology, IZMIR, Turkey 2 Dokuz Eylul Univ. Health Sciences Institute, Radiology, IZMIR, Turkey Purpose or Objective In this retrospective study, it was aimed to compare the dose volume parameters of organs at risk for the bladder filling of 50 cc versus 150 cc in high dose rate (HDR) three- dimensional (3-D) vaginal cuff (VC) brachytherapy (BRT). Material and Methods The dosimetric data of the 8 hysterectomized patients with gynecological malignancy who received postoperative pelvic external radiotherapy (RT) + 3-D HDR VC-BRT between March 2015 and August 2015 were analyzed. Computerized tomography (CT) sectional images obtained for VC BRT treatment planning were used for the study. The proximal 1/3 portion of vagina, drawn as a layer of 0.5 cm thickness from the vaginal mucosa (cylinder surface), had been delineated as clinical target volume (CTV). A total dose of 18 Gy (3x6Gy) had been prescribed to the CTV. For the study, bladder, rectum, sigmoid, bowel and CTV were recontoured in a 3-D manner by the same radiation oncology resident. Then the contours were controlled and corrected first by a staff radiation oncologist and then a staff radiologist. Afterwards, treatment planning was performed by the medical physicist using BRT treatment planning system. Mean CTV dose-volume parameters (D90%, D100%, D50%/D90%, V100%, V150%) for all patients were similar in the treatment plans for two different bladder fillings. Bladder V50%, D50% and D2cc, rectum D2cc, sigmoid D2cc, bowel D50% and D2cc were recorded from the dose- volume histograms obtained in the treatment planning system. Paired comparisons were made for the parameters above for the bladder filling of 50 cc versus 150 cc. Different bladder fillings were compared using Wilcoxon Signed-Rank Test in the SPSS 15.0 statistics program. Results It was observed that as the amount of bladder filling increases, bowel is displaced cranially from the applicator. Bladder D50% decreases (p=0.012) while bladder D2cc increases (p=0.025) in case of 150 cc bladder filling instead of 50 cc. Rectum D2cc shows a statistical trend for increase (p=0.05), however bowel D50% decreases (p=0.012) in 150 cc bladder filling compared to 50cc. Conclusion The statistically significant decrease in bladder and especially bowel D50% parameters supports filling bladder with 150 cc instead of 50 cc in 3D VC-BRT.