Inter Fraction Target Dynamics and Variability Between T1 and T2-Weighted MRI During Surgical Cavity Hypofractionated Stereotactic Radiotherapy: Prospective Serial Imaging Study

Abstract

Hypofractionated stereotactic radiotherapy (HSRT) to surgically resected brain metastases is a standard option to reduce local recurrence and spare the toxicity of whole brain radiotherapy. This study examined changes in the clinical target volume (CTV) and associated variations in brain parenchyma dosimetry on a 1.5T MRI linear accelerator. In addition, the suitability of using T2/FLAIR (T2f) as a surrogate to define CTV was explored by assessing contouring variability between different standard MRI sequences during treatment.Fifteen patients were prospectively imaged with gadolinium-enhanced T1 (T1c) and T2f axial sequences at the time of planning (FxSim) and at fraction 3 (Fx3) of HSRT to the cavity. Clinical target volumes (CTV) were contoured at both time points on T1c and T2f sequences. All patients were treated with either 27.5Gy or 30Gy in 5 fractions. The total volume of brain minus CTV receiving 30Gy (V30Gy) was obtained at both time points on T1c sequence. Inter-fraction surgical cavity dynamics and CTV delineation variability were quantified using absolute volume, Dice similarity coefficient (DC) and Hausdorff distance (HD) metrics.The median time from surgery to HSRT was 22 days. The median CTVs on T1c and T2f sequences at FxSim were 12cm3 (range, 1.2-30.1) and 10.2cm3 (range, 2.9-27.9), respectively. At Fx3, the median CTV decreased slightly in both sequences to 9.7cm3 (range, 3.7-25.9) and 8.7cm3 (range, 2.9-24.9), translating to a median % reduction in CTV compared to FxSim of 11.7% on T1c and 8.9% on T2f, respectively (P < 0.05). The mean V30Gy for all patients at FxSim and Fx3 were 6.14cm3 (range 0.11-15.7) and 7.17cm3 (range 0.15-20.1), respectively (P < 0.05). We observed a reduction in CTV volume in 12/15 (80%) patients on T1c at Fx3 relative to FxSim. Two patients that had a reduction in CTV volumes at Fx3 developed radionecrosis (RN) on follow up MRI and recorded a mean V30Gy of 10.8cm3 at FxSim and 13.6cm3 at Fx3. The mean DC was 0.831 (range 0.688-0.905) and the mean HD was 6.32mm (range 2.4-12.3) when comparing between CTV volumes contoured on T1c and T2f, indicating variability of CTV delineation between the two sequences.Clinically meaningful changes in target dynamics were observed during the course of cavity irradiation, supporting the use of MR image guided radiation therapy and treatment adaptation. We also observed variability in CTV delineation between T1c and T2f sequences, highlighting the importance of further work in this field.