ESTRO 37 Abstract book

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ESTRO 37

EP-2217 Dose to Intermediate Risk CTV in cervical brachytherapy: A retrospective study at a UK centre. L. Hawley 1 , A. Cook 2 , G. Lockwood 2 1 Cheltenham Oncology Centre, Oncology, Bristol, United Kingdom 2 Cheltenham Oncology Centre, Oncology, Cheltenham, United Kingdom Purpose or Objective To retrospectively investigate the dose received by the Intermediate Risk CTV (IR-CTV) during brachytherapy for cancer of the cervix, following change in protocol with a reduction of EBRT dose and tighter dose constraints on organs at risk. Material and Methods

IR-CTV may not get the correct dose. In addition all treatments will be affected by the introduction of tighter 'soft” constraints for the OARs and the use of 45Gy in EBRT, as recommended by the EMBRACE-II protocol to reduce toxicity. Ideally all cases should have IR-CTV outlined; however in reality there are constraints (time, staffing) which mean this may not be practical. Therefore we advise that, where there is a bulky tumour at diagnosis (particularly with vaginal extension), with a good response to EBRT, it would be prudent to outline the IR-CTV at the time of brachytherapy planning. EP-2218 The results from the 3D and 2D-guided brachytherapy planning in localy advanced cervical carcinoma. E. Titovich 1 , D. Kazlouski 1 , Y. Kazlouskaya 1 , A. Saroka 1 , V. Suslava 2 1 National Cancer Center of Belarus, Radiotherapy engineering and medical physics, Minsk, Belarus 2 National Cancer Center of Belarus, Radiology-3, Minsk, Belarus Purpose or Objective To perform dose-volume statistics comparison of the MRI and fused IBU-ultrasound-guided brachytherapy treat- ment plans in locally advanced cervical carcinoma. Material and Methods From January to October 2017, 20 patients with locally advanced carcinoma of the cervix were treated in our center. All patients underwent 50Gy/25 fractions to the entire pelvis region (3D CT-based treatment planning). After that, all patients received 5Gy/fraction intracavitary brachytherapy (5 fractions in 3 weeks). All applications were performed under ultrasound control. The bladder was filled with 100 ml saline to ensure good visualization and to avoid the bulb. IU-channel for the ring applicator was selected according to the size of the uterus (obtained using ultrasound imaging). US-guided treatment planning enables visualization of the cervix and uterus and allows sparing of the normal tissues. This planning is aimed to cover a whole cervix volume with 100% of the prescribed dose. X-ray imaging was performed using IBU-Digital. According to GEC-ESTRO recommendations High Risk CTV (HR CTV) was identified using the MRI image. The bladder, rectum and sigmoid were outlined as OARs. US with IBU-based calculated treatment plans were transferred to the MRI scans to define D 2cc OARs and D 90 HR CTV. Treatment plans based on the fused US and IBU-guided images were compared with the MRI treatment plans. The total accumulated dose value for EBRT and brachytherapy boost were evaluated in terms of equivalent dose in 2 Gy per fraction (EQD 2 ), using a/b = 3 Gy for OARs and a/b = 10 Gy for HR CTV. Results The average dose coverage with 100% isodose was 93.59% for IBU-US guided plans and 93.54% for MRI based plans. Dose D 2cc for rectum, sigmoid and bladder were 2.72Gy, 3.26Gy and 4.24Gy respectively for IBU-US-guided plans and 2.83Gy, 3.18Gy, 4.05Gy respectively for MRI-guided plans. Conclusion Using ultrasound in gynaecologic brachytherapy to guide the applicator placement allows to avoid perforation and optimize the applicator position within the uterine canal, and thus to improve the quality of implants. Fused ultrasound and IBU-guided brachytherapy planning in comparison with MRI-based planning has a good alternative to 3D treatment planning in localy advanced cervical carcinoma witout parametrical and vaginal involvement.

Original plans were analysed for organs at risk and the high risk CTV (HR-CTV). IR-CTV was outlined using MRI images from diagnosis and post EBRT. The IR-CTV was delineated using the guidelines from the ICRU report 89: IR-CTV = HR-CTV+margin +initial tumour volume prior to EBRT. The total EQD2 for the IR-CTV was then calculated for EBRT doses of 50.4Gy and 45Gy. Results 11 patients were included in the study; 4 received an EBRT dose of 45Gy and 7 received 50.4Gy. All patients were prescribed brachytherapy dose of 7Gy to the 100%, per fraction, for their first 3 fractions. The fourth treatment was modified according to dose to OARs. 2 were prescribed 7Gy, 8 were prescribed 3-6.5Gy and 1 patient did not receive a fourth dose. The calculations of total dose to IR-CTV, using the patients’ EBRT dose received (either 45Gy or 50.4Gy) plus brachytherapy dose, showed that 10 out of 11 patients would have received the required dose of at least 60Gy to the IR-CTV D90. The patient who did not receive 60Gy to the IR-CTV had a large tumour at diagnosis which distorted anatomy; she then had a good response to EBRT which meant there was a large difference between sizes of the HR-CTV and IR-CTV. It was noted that the patients who received 45Gy EBRT had lower total dose to the IR-CTV than those who received 50.4Gy EBRT; this coincided with a reduction in OAR constraints. Conclusion Our study demonstrates that using 45Gy in 25 fractions EBRT and the maximum brachytherapy dose that OAR will allow (aiming 28Gy in 7 fractions), achieves the required EQD2 to the IR-CTV D90 in most cases. We found that 10 out of 11 IR-CTV volumes retrospectively outlined had actually received 60Gy or more. However, it is important to formally outline the IR-CTV, as there are a proportion of patients where this is not the case. The cases most at risk of this are where initial tumour volume is so large it distorts anatomy, and means that the IR-CTV volume is in turn very large. This problem is amplified when there has been a good response to EBRT, giving a much smaller HR- CTV. In this situation there is a high chance that if the treatment plan is only constructed using the HR-CTV, the