ESTRO 38 Abstract book

S86 ESTRO 38

applicator, but from 2007 also with an intravaginal tandem-needle-template (TNT) with no ring. Time trends were visualized by sliding average calculation with 50 pts per frame (Figure 1). The planning aims and DVH hard and soft constraints were evaluated in relation to the Embrace II protocol (Table 1). Equivalent dose in 2 Gy fractions (EQD2) was calculated using α/β=10 for tumour and α/β=3 for organs at risk. Statistical analysis was done with t-test and Chi square. Results The implant technique for IGABT changed significantly as the use of IC/IS went up from 32% to 68%, with 52% of the implants being performed with the TNT device in the last 50 pts (Table 1). The dose contribution of EBRT to D90 of CTV HR was reduced by 7 Gy while the contribution from IGABT enlarged by 11 Gy leading to an overall improvement in the D90 of CTV HR from 89 to 93 Gy. This improvement was realised with no increase in TRAK. For D2cm 3 of bladder, rectum and sigmoid the dose was significantly lowered by 3, 8 and 10 Gy, respectively. The dose to the ICRU recto-vaginal point was decreased by 8 Gy. Recording of D2cm 3 for bowel was initiated after the first 75 pts and stayed constant at about 60 Gy over the observation time. EMBRACE hard and soft constraints were fulfilled in 96% and 66% of the last 50 pts. Conclusion The constraints and planning aims obtained through the GEC ESTRO collaboration on IGABT in LACC has profoundly impacted our treatment practice with significant improvements in the dose-volume parameters. This improvement was primarily obtained by decreasing the dose contribution of EBRT and by use of advanced IC/IS implants for IGABT.

Results Average cumulative EQD2 doses (Gy) for IC/IS plan were the following: HRCTV D90 88.2±2.6; Bladder D2cc 72.1±7.9, D0.1cc 84.3±10.7; RectumD2cc 58±6.8, D0.1cc 66.7±11.3; Sigmoid D2cc 63±5.3, D0.1cc 73.9±8.6). Average cumulative EQD2 doses (Gy) for IC plan were the following: HRCTV D90 86.6±3.4; Bladder D2cc 74.4±9.9, D0.1cc 91±19.5; RectumD2cc 59.2±7.7, D0.1cc 69.5±13.7; Sigmoid D2cc 64.1±6.3, D0.1cc 76.4±10.4). Difference between IC/IS and IC plan was significant for all ROIs and dose points (wilcoxon test). Average cost function for IC/IS or IC plan was 0.8±0.1 and 0.7±0.2 respectively (p< 0.0001). Quartile analysis of the cost function plot (Fig.2) shows that the difference between IC/IS and IC plans is not significant just in the 4th quartile.

Conclusion IC/IS plans achieved a significant dosimetric gain in a proportion of patients larger than expected. Given the comparable costs and complication rates between IC and IC/IS, a more extensive use of IS component in small tumors seems justified OC-0174 Advancement of brachytherapy for locally advanced cervical cancer in the era of image guidance J.C. Lindegaard 1 , L.U. Fokdal 1 , P. Petric 1 , S.K. Nielsen 1 , K. Tanderup 1 1 Aarhus University Hospital, Department of Oncology, Aarhus, Denmark Purpose or Objective Two decades ago standard treatment for locally advanced cervical cancer (LACC) involved 4-field box external beam radiotherapy (EBRT) and 2D brachytherapy aiming for 80- 85 Gy to point-A. In 2005 image guided adaptive brachytherapy (IGABT) was introduced in our department and we joined the newly formed international GEC ESTRO collaboration on IGABT in LACC. The aim of the present study was to evaluate the impact of this collaboration in terms of developments of BT techniques and dose-volume 400 consecutive pts treated 2005-2018 using EBRT ±concomitant cisplatin and IGABT were analysed. FIGO stage distribution was I-IIA 9%, IIB 61% and III-IV 30%. EBRT dose was 45-50 Gy delivered in 25-30 fx. A simultaneous integrated boost to 50 Gy (stage IIB) or 60 Gy (stage III-IV) for the primary tumour was used in the early years, but was gradually phased out and the EBRT dose reduced to 45 Gy/25 fx for all pts. IGABT was performed with PDR and was based on MRI with a gradual introduction of planning aims and DVH constraints from our collaborative GEC ESTRO experience as they became available. The Embrace II protocol (www.embracestudy.dk) was formally implemented in 2016. Combined intracavitary/interstitial technique (IC/IS) was available from 2006, initially beside a tandem-ring parameters over time. Material and Methods