Isodose surface volumes in cervix cancer brachytherapy: Change of practice from standard (Point A) to individualized image guided adaptive (EMBRACE I) brachytherapy

Purpose: To investigate the isodose surface volumes (ISVs) for 85, 75 and 60 Gy EQD2 for locally advanced cervix cancer patients. Materials and methods: 1201 patients accrued in the EMBRACE I study were analysed. External beam radiotherapy (EBRT) with concomitant chemotherapy was followed by MR based image-guided adaptive brachytherapy (MR-IGABT). ISVs were calculated using a predictive model based on Total Reference Air Kerma and compared to Point A-standard loading systems. Influence of fractionation schemes and dose rates was evaluated through comparison of ISVs for alpha/beta 10 Gy and 3 Gy. Results: Median V85 Gy, V75 Gy and V60 Gy EQD210 were 72 cm(3), 100 cm(3) and 233 cm(3), respectively. Median V85 Gy EQD210 was 23% smaller than in standard 85 Gy prescription to Point A. For small (<25 cm(3)), intermediate (25-35 cm(3)) and large (>35 cm(3)) CTVHR volumes, the V85 Gy was 57 cm(3), 70 cm(3) and 89 cm(3), respectively. In 38% of EMBRACE patients the V85 Gy was similar to standard plans with 75-85 Gy to Point A. 41% of patients had V85 Gy smaller than standard plans receiving 75 Gy at Point A, while 21% of patients had V85 Gy larger than standard plans receiving 85 Gy at Point A. Conclusions: MR-IGABT and individualized dose prescription during EMBRACE I resulted in improved target dose coverage and decreased ISVs compared to standard plans used with classical Point A based brachytherapy. The ISVs depended strongly on CTVHR volume which demonstrates that dose adaptation was performed per individual tumour size and response during EBRT. (C) 2018 Elsevier B.V. All rights reserved.