From point A to the sculpted pear: MR image guidance significantly improves tumour dose and sparing of organs at risk in brachytherapy of cervical cancer

Background and purpose: Brachytherapy in locally advanced cervical cancer is still widely based on 2D standard dose planning, although 3D image guidance is available. The purpose of this study was to compare point doses to 3D dose volume parameters for tumour and organs at risk (OARs), and to evaluate the improvement of dose parameters with MR image guided adaptive brachytherapy (IGABT).

Material and methods: MRI-based IGABT was performed in 72 consecutive patients. HR-CTV, IR-CTV, bladder, rectum and sigmoid were contoured according to GEC-ESTRO recommendations. BT standard dose planning was compared to MRI-based dose optimisation.

Results: HR-CTV dose (D90) was highly variable in standard plans with point A dose prescription. In small tumours (<31 cc) HR-CTV was well covered by standard plans in 94% of patients, while OAR constraints were exceeded in 72% of patients. Optimisation decreased violation of OAR constraints to only 6% of patients while maintaining excellent target coverage. In large tumours (>31 cc) the dose optimisation improved the HR-CTV D90 by a mean of 7 Gy resulting in full coverage in 72% of patients as compared to 25% for standard plans, even while reducing violation of OAR constraints.

Conclusion: Point A dose is a poor surrogate of HR-CTV dose, and the use of 3D image-based dose planning is encouraged. MRI-based IGABT significantly improves target coverage and OAR dose.