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

Abstract

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 α/β 10 Gy and 3 Gy.

Results: Median V85 Gy_, V75 Gy and V60 Gy EQD2₁₀ were 72 cm³, 100 cm³ and 233 cm³, respectively. Median V85 Gy EQD2₁₀ was 23% smaller than in standard 85 Gy prescription to Point A. For small (<25 cm³), intermediate (25-35 cm³) and large (>35 cm³) CTV_HR volumes, the V85 Gy was 57 cm³, 70 cm³ and 89 cm³, 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 CTV_HR volume which demonstrates that dose adaptation was performed per individual tumour size and response during EBRT.

Keywords: Cervical cancer; Isodose surface volume; Magnetic resonance image guided brachytherapy; Point A; Standard loading brachytherapy; TRAK.