ESTRO 36 Abstract Book

S980 ESTRO 36 _______________________________________________________________________________________________

192, with a good coverage of the PTV and excellent results as for acute toxicity. The greater toxicity was observed immediately after the treatment was finished reducing in an important way at the third month after treatment. EP-1784 Needle use in cervical cancer brachytherapy using the Utrecht applicator: a single center experience M. Smolic 1 , C. Sombroek 1 , M. Bloemers 1 , B. Van Triest 1 , M.E. Nowee 1 , A. Mans 1 1 Netherlands Cancer Institute Antoni van Leeuwenhoek Hospital, Radiation Oncology, Amsterdam, The Netherlands Purpose or Objective The Utrecht applicator (Elekta, Veenendaal, the Netherlands) used in brachytherapy (BT) for cervical cancer can include up to 10 interstitial needles along with the intra-uterine and ovoid channels. The aim of this study is to examine the clinical use of needles at our institute, and to investigate whether the two needles with largest discrepancy between application and use are essential to treatment planning. Material and Methods The study included 22 cervical cancer patients treated with 3 fractions of BT. The application of needles per fraction was based on consensus amongst radiation oncologists, medical physicists and RTTs, using the available pre-treatment imaging, physical examination and MRI scan made in the week before BT. We examined how often each of the 10 possible needles (Figure 1) was applied and the frequency of its subsequent use in treatment planning, as well as the average intensity of each needle’s use, given by the average ratio of needle channel dwell time to total treatment time. We investigated whether the two needles with lowest frequency and intensity are essential for achieving the planning aims while respecting OAR constraints and yielding an acceptable conformal dose distribution. We did this by removing these needles from the optimized clinical plans (CP) and re-optimizing using the other available needles, intra-uterine and ovoid channels (RP). We aimed to obtain identical HR-CTV D90 values while still trying to achieve similar OAR planning aims and dose distribution conformality as achieved in the CP. We compared RP and CP for DVH parameters (HR-CTV D90 and OAR D2cc’s, as well as the ratio D2cc OAR to D90 HR-CTV as a measure of DVH parameter favorability) and dose distributions, characterized by high dose volumes HR-CTV V200% and V300%, the dose homogeneity index (DHI = 1 – HR-CTV V150%/HR-CTV V100%) and conformal index (COIN = HR-CTV V100 (cc)/HR-CTV Volume (cc) x HR-CTV V100 (cc)/Implant V100 (cc)). Results Needles C, H, J and A are applied most often (in 89%, 89%, 71% and 67% of the cases, respectively) while the needles with the lowest frequency of subsequent use are A (64%), J (68%), F (68%), E (77%). Needles contributing the least to the total treatment time are J (2.9%), E (2.9%), A (3.0%) and F (3.5%). Needles A and J are thus applied often but have the lowest frequency and intensity of subsequent use. Of the 66 clinical treatment plans in this study, 25 made use of both needles A and J. Re-optimizing these clinical plans without using needles A and J leads to minimal differences in DVH parameters and dose distributions between CP and RP (Table 1)

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Conclusion Needles C and H are applied and used most frequently and intensely in our clinic. Needles A and J are applied often but have the lowest frequency and intensity of subsequent use. We managed to obtain equally clinically acceptable plans without these needles, indicating that they are not essential to treatment planning and may be discarded in future. EP-1785 use of rectal tube in vaginal cuff HDR- brachytherapy: an unexpected advantage A. Cerrotta 1 , C. Tenconi 2 , B. Pappalardi 1 , T. Giandini 2 , S. Naimo 1 , D. Mazzeo 2 , E. Mazzarella 2 , S. Grisotto 2 , M. Borroni 2 , C. Fallai 1 , M. Carrara 2 1 Fondazione IRCCS Istituto Nazionale dei Tumori, Department of diagnostic imaging and radiotherapy- Radiotherapy 2, Milan, Italy 2 Fondazione IRCCS Istituto Nazionale dei Tumori, Department of diagnostic imaging and radiotherapy- Medical Physics Unit, Milan, Italy Purpose or Objective Rectal toxicity, both early and late, is a substantial problem in gynecological brachytherapy (BT) , occurring in the majority of patients within the first 2 years after the end of treatment. Specific strategies and efforts are needed to limit high doses to the rectal mucosa without reducing the overall target coverage. The aim of our study, following an occasional observation, is to evaluate if the rectal tube we use in our Institute for in vivo dosimetry allows also a reduction of the rectal dose. Material and Methods In our Institute, adjuvant vaginal BT is always Image Guided (CT/MRI) with a multichannel endovaginal applicator, chosen taking into account both the comfort of the patient and the best contact of the applicator surface with the vaginal mucosa. For all patients, rectal wall in vivo dosimetry is performed with a dedicated rectal tube integrated with dosimeters and rigidly fixed to the endovaginal applicator. Over the time, for technical/ clinical problems, two CT scans (acquired within a time interval of 10 minutes) were obtained in 6 patients : one without and one with rectal tube. Each treatment plan was generated and delivered using CT-scan with rectal tube in order to calculate dose to target and OARs (TPS: Oncentra Brachytherapy System). Vaginal cuff, bladder,