The criteria for first-line therapy are an effective treatment that reproducibly delivers the required endpoint within the limits of acceptable toxicity, and crucially better than any of the other therapeutic options, writes Alice Gillams.
An additional consideration is the impact on future lung function and on future therapeutic options for patients who are very likely to have more than one metastatic episode, <70% following successful metastasectomy. Lung ablation of metastases currently fulfils these criteria and is likely to become a first-line therapy for selected patients.
Efficacy
Ablation reproducibly eradicates small lung metastases with minimal local recurrence rates. The factors that determine local failure are well documented allowing appropriate selection of patients for ablation. These are size and location. Whilst metastases up to 3.5cm in diameter can be completely ablated, the very best results are obtained in
Tumour contiguity to larger vessels or bronchi may reduce thermal efficacy and the local recurrence rate rises. Strategies to overcome vessel cooling with high power microwave or careful probe positioning are often, but not always, successful. Peripheral tumours are easier to access, and less likely to lie adjacent to large vessels or bronchi. On multivariate analysis, size remains the dominant criteria.
Ablation is effective in all types of metastases; the most common indications for radical local treatment are colorectal, renal, head and neck and sarcoma metastases.
Five per cent of local recurrence post ablation is competitive with surgical resection data. Although many surgical publications do not differentiate between index tumour control and intrapulmonary recurrence, stump recurrence after wedge resection of colorectal metastases has been reported to be as high as 23%. Local recurrence following stereotactic body radiation therapy (SBRT) is reported to be 20% at two years.
Morbidity
Although lung resection has been used effectively for several years, even with video-assisted thoracoscopic surgery (VATS) techniques, this remains a major procedure and the major morbidity remains high. A multicentre study published in 2014 of more than 1,000 lung resections, reported a mortality of 0.4%, major morbidity 16%, and re-operation rate for complications of 0.9%. Lung ablation, particularly in the context of metastatic disease in patients with normal background lung parenchyma, is a very safe, low morbid procedure. Treatment of single, small, well located tumours can often be performed with sedation as a day-case procedure. The most common side-effect is a pneumothorax, but only 10% require percutaneous, image-guided, intercostal drainage. Of those that require intercostal drainage, most resolve within 18 hours. Complications such as haemorrhage, infection and prolonged air leak are much rarer. Ablation is the least invasive, radical therapy for lung metastases.
Impact on respiratory function
Ablation maximises the preservation of normal parenchyma. Studies of lung function testing before and after ablation show a slight reduction immediately post-procedure but restoration of preprocedural function at three months, and this is maintained at 12 months. The same is not true for surgery where there is usually an irreversible reduction in function. A recent publication showed 7–15% reductions in forced expiratory volume in one second (FEV1) and forced vital capacity (FVC), even following metastasectomy.
Impact on future therapeutic alternatives
Whereas there are limits to how much lung can be resected, and radiotherapy can usually only be applied to an area once, ablation can be used repeatedly. Ideally all small well located metastases would be treated with ablation, reserving resection and SBRT for future metastases that are not suitable for ablation, but can be radically treated with these other modalities. If surgical resection is used for initial, ablation-suitable tumours, then if a metastasis arises that requires surgical intervention, such as one with endobronchial extension, surgery may no longer be possible. Using surgery early on in a patient’s cancer pathway, narrows their future therapeutic options.
Alternative therapies
No other therapy can deliver radical therapy to multiple, small volume tumours with so little morbidity. Chemotherapy can shrink tumours or even render some tumours, imaging occult, for a while, but nearly all tumours will redeclare. SBRT is an effective therapy for one or two tumours but is expensive, cumbersome, requires a degree of intervention in the form of percutaneous fiducial placement and carries both chest wall and mediastinal toxicity. It carries a higher local recurrence rate and as a result, SBRT is more often used for metastases which are not well-located for thermal ablation. Surgical resection, even with VATS, remains highly morbid.
Conclusion
Small, well located metastases are best treated with percutaneous, image guided ablation which will eradicate the metastasis with minimal morbidity, minimal impact on function whilst maintaining all the therapeutic options for future metastatic events. Ablation should be a first-line therapy in appropriate patients.
Alice Gillams is a consultant radiologist at The London and Harley Street Clinics, London, UK. She has reported no disclosures pertaining to this article.
