ESTRO 37 Abstract book

S1233

ESTRO 37

Oncology, TFYR, Macedonia Former Yugoslav Republic 10 International Atomic Energy Agency, NAHU- Nuclear Sciences and Applications, Vienna, Austria Purpose or Objective Seven trials of combined external beam (EB) plus several high-dose-rate (HDR) brachytherapy treatments of cervix cancer, each trial comprising ≥2 HDR arms, were analysed using the LQ (linear-quadratic) model to compare outcomes using generic parameter values and to assess a resource-sparing aim of using only two HDR fractions. Material and Methods Six published trials used 2-15 HDR fractions. The seventh (paper in preparation) was a 7-institution International Atomic Energy Agency (IAEA) trial comprising 601 patients with stages IIB and IIIB cervical carcinoma. All patients received 46 Gy EB in 23-fractions (5/week) to the pelvis. In arm A, 4 HDR applications of 7 Gy were given to point- A, one at the end of weeks 3,4,5,6. In arm B, 2 HDR applications of 9 Gy were given at the end of weeks 5,6. Arms CD were replicates of AB plus cisplatin (40 mg/m 2 ) in weeks 1 through 5, with the same planned 40-day overall treatment time. Locoregional tumour control (LRC), survival, acute/late adverse events (AE, CTCv3) were compared between arms. Generic parameter values of a/b=10 Gy for tumour control and 3 Gy for late adverse events (AE) were used in the LQ modelling. Results Values of BED 10 for tumour control ranged between 83.7 and 102.8 Gy among the 7 trials, with LRCs of 70-90%. In the IAEA trial, BED 10 =102.8Gy and BED 3 =128.3Gy for arms- AB (4x7 Gy), and lower by 10-13% for arms-CD (2x9 Gy). Equivalence for tumor control would be predicted using 2x11.2 Gy, or for late AE 2x10.6 Gy, but clinical perception of potential higher AE associated with high doses per fraction reduced the prescription dose to 2x9 Gy as used in one previous trial. Tumour control was significantly lower by ~7-16% in the 2x9 Gy +/- cisplatin arms CD compared to the 4x7 Gy +/- cisplatin AB arms, and AE showed a non-significant lower tendency by ~6- 11%. There was not enough statistical power to detect any significant effect of cisplatin among arms. If the 2- fraction arm had been 2x10.6 Gy, the EQD2 3 (Equivalent- dose in 2-Gy fractions) would be ~77.0 Gy and similar AE rates to arm A would be predicted. Also, the EQD2 10 would be ~82.4Gy not 85.7 Gy i.e. ~4% less or ~3 percentage points less tumour control would be The results of these 7 trials are fairly consistent with each other on a LQ/BED modelling basis, considering the different numbers of patients involved and inter- institutional technical differences. The IAEA trial showed that 2x9 Gy was inferior to 4x7 Gy HDR BT, as expected from the LQ/BED values. However, 2x9 Gy HDR still may be useful in situations with severely limited resources. The modelling also predicted that EB plus 2x10 Gy HDR BT may restore the LRC rate to near that after 4x7 Gy, and with similar late-AE rates, but that would need trialling. EP-2231 Radiobiological plan evaluation in brachytherapy using two different cell survival models D. Arous 1 , E.O. Pettersen 1 , E. Malinen 2 , T.P. Hellebust 2 1 University of Oslo, Department of Physics, Oslo, Norway 2 University of Oslo - Oslo University Hospital, Department of Physics - Department of Medical Physics, Oslo, Norway Purpose or Objective The validity of the linear quadratic (LQ) cell survival model for high doses has been questioned, and the universal cell survival curve (USC) has emerged as a possibly more suitable model. Here, brachytherapy plans predicted. Conclusion

Conclusion As of now GEC ESTRO defined target volumes can be safely used for image-based brachytherapy, DWI along with ADC maps should only be used as a supplement for target delineation, only to probably increase the HRCTV contours for which the probability is less, based on results from our study. DWI or its derived ADC maps should never be used as a standalone imaging for planning or contouring due to its poor anatomical visualization and distortions. This can result in unacceptable uncertainties in applicator reconstruction which will give rise to dose uncertainties and a possible inferior treatment. EP-2230 Modelling combined external beam and high- dose-rate brachytherapy of cervix cancer in 7 trials J. Hendry 1 , A. Polo 2 , G. Jones 3 , U. Mahantshetty 4 , G. Sarria 5 , L. Van Wijk 6 , N. Da Motta 7 , N. Benjaafar 8 , S. Smickoska 9 , M. Abdel Wahab 10 , E. Zubizarreta 2 1 Christie Hospital, Medical Physics, Macclesfield, United Kingdom 2 International Atomic Energy Agency, ARBR- NAHU- Nuclear Sciences and Applications, Vienna, Austria 3 The Cancer Centre Eastern Caribbean- Antigua & Barbuda, The Cancer Centre, Antigua & Barbuda, Antigua and Barbuda 4 Tata Memorial Hospital, Radiation Oncology, Mumbai, India 5 National Cancer Institute, Radiation Oncology, Lima, Peru 6 Groote Schuur Hospital, Radiation Oncology, Cape Town, South Africa 7 Irmandade de Santa Casa de Misericordia, Radiation Oncology, Porto Alegre, Brazil 8 National Institute of Oncology Sidi Mohamed Ben Abdellah, Radiation Oncology, Rabat, Morocco 9 Institute of Radiotherapy and Oncology, Radiation