Assessment of early treatment response after CT-guided radiofrequency ablation of unresectable lung tumours by diffusion-weighted MRI: a pilot study

The aim of this study was to evaluate prospectively the early treatment response after CT-guided radiofrequency ablation (RFA) of unresectable lung tumours by MRI including diffusion-weighted imaging (DWI). The study protocol was approved by the ethics committee of our hospital and signed consent was obtained from each patient. We studied 17 patients with 20 lung lesions (13 men and 4 women; mean age, 69±9.8 years; mean tumour size, 20.8±9.0 mm) who underwent RFA using a LeVeen electrode between November 2006 and January 2008. MRI was performed on a 1.5T unit before and 3 days after ablation. We compared changes in the apparent diffusion coefficient (ADC) on DWI and response evaluation based on subsequent follow-up CT. 14 of the 20 treatment sessions showed no local progression on follow-up CT, whereas 6 treatment sessions showed local progression (range, 3–17 months; mean, 6 months). For the no-progression group, the ADC pre- and post-RFA were 1.15±0.31 × 10⁻³ mm² s⁻¹ and 1.49±0.24 × 10⁻³ mm² s⁻¹, respectively, while the respective ADC values for those that showed local progression were 1.05±0.27 × 10⁻³ mm² s⁻¹ and 1.24±0.20 × 10⁻³ mm² s⁻¹. The ADC of the ablated lesion was significantly higher than before the procedure (p<0.05). There was a significant difference in the ADC post-RFA between no-progression and local progression groups (p<0.05). Our prospective pilot study showed that the ADC without local progression was significantly higher than with local progression after RFA, suggesting that the ADC can predict the response to RFA for lung tumours.After the first report in 2000 1], lung radiofrequency ablation (RFA) is now considered effective in the treatment of lung cancer, which is traditionally considered unresectable owing to compromised pulmonary function or advanced age. In general, complications associated with lung RFA are minimal, and favourable local control has been reported in a number of studies of tumours with a diameter of 30 mm or less 1–5]. However, only a limited number of studies have been published regarding the treatment outcome after lung RFA 6–10]. In this process, a layer of normal lung tissue surrounding the tumour is also ablated as a safety margin. Inevitably, the ablated lesion depicted on a CT scan immediately after the procedure is larger than the original tumour mass. However, this region of increased density shrinks with time, but follow-up CT may still show the ablated lesion being as big as, or larger than, the tumour size before the procedure 6, 7]. Thus, radiologists sometimes encounter difficulty in distinguishing scarred tissue from a tumour residue/local progression when the size of the lesion remains the same. Accurate assessment of RFA outcome would have important consequences, as recurrent tumours can be treated again if detected at an early stage. Different modalities of early-stage follow-up examination, such as contrast-enhanced CT 8] and fluorodeoxyglucose positron emission tomography (FDG–PET), have been of great interest and their usefulness has been reported by several groups 9, 10]. Another approach — MR diffusion-weighted imaging (DWI) — which is based on the measurement of motion of water molecules, has also been reported as a non-invasive evaluation modality 11–19]. In this method, the apparent diffusion coefficient (ADC) represents the water content and distribution, the cellular density and the cell membrane integrity, suggesting the potential usefulness of an ADC map for estimating tumour viability. Indeed, DWI has been successfully used to assess the efficacy of radiotherapy 11, 12], chemotherapy 13–15] and transcatheter arterial embolisation 16, 17]. To our knowledge, only two studies have reported the use of DWI to evaluate the therapeutic outcome of RFA 18, 19]. A previous study reported that the ADC value of an ablated rabbit tumour model (VX2 tumour) was significantly higher than that of untreated tumours, and that FDG uptake on micro-PET for small animals with ablated tumours was significantly lower than for untreated tumours. These results indicate that DWI at 2 days and FDG–PET at 3 days after RFA are both potentially feasible modalities for monitoring the early effects of the procedure 19]. In this study, we calculated the ADC in tumour lesions before and after clinical lung RFA and examined the usefulness of DWI in the early detection of tumour response to RFA.