MTE24.02 Small Lung Tumours: High Risk Lesions and Contraindications to Stereotactic Ablative Body Radiotherapy (SABR)

Abstract

The development of SABR has revolutionised the non-surgical treatment of small, node negative lung tumors, both primary and metastatic. SABR is a convenient, painless, inexpensive outpatient procedure and there is now randomized evidence that it not only results in better local control than conventionally fractionated radiotherapy in patients with inoperable peripheral stage I non-small cell lung cancer, but increases survival as well. The impressive local control rates of over 80% have tempted some investigators to expand the indications for high dose hypofractionated SABR beyond small peripheral tumors. Perhaps the most controversial extended indication is the use of SABR for “central” lung tumours. There is no agreed definition of what constitutes a central tumor, although in the absence of consensus, the “no-fly zone” described by Timmerman is widely used even though it was based on a very small number of events.1 Phase II trials have been interpreted as indicating that SABR of central tumors has acceptable toxicity (RTOG 0813), even though there was a 3% mortality likely resulting from treatment; or conversely as unsafe, with the Nordic HILUS trial reporting around 10% mortality after SABR of tumours close to the main or a lobar bronchus. The European Lung Tech trial should throw more light on the safety of treating central tumors with a “risk-adapted” approach using a more fractionated schedule of 60 Gy in 8 fractions.2 Many patients referred for SABR have poor respiratory or cardiac function making them unsuitable for surgical resection. Does this make them unsuitable for high dose SABR as well? The available evidence suggests not,3 but extreme caution should be taken in patients who have underlying interstitial lung disease.4 The relationship between SABR and cardiac injury, especially in the era of combined immunotherapy and SABR, remains under investigation. This high risk group of patients are currently eligible for the Stablemates trial, which is comparing sublobar resection with SABR. Eligibility includes FEV1 or DLCO < 50% predicted. A patient’s suitability for SABR will also depend on the dose constraints on nearby organs at risk. The chest wall is less concerning than previously, but is it the dose to the neurovascular bundle, rib or the whole musculoskeletal structure that most accurately predicts risk of chest wall pain? The brachial plexus tolerates hypofractionation poorly, and as with other scenarios where there is doubt about the safety of SABR, it is well to recognise that less effective but safer fully fractionated schedules are available. Other considerations, not in terms of risk, but of practical delivery of the treatment include synchronous multiple tumors, large tumors (>5 cm), and visibility for image guidance: Well defined? Too small? Image degraded by implanted fiducials? 1. Timmerman R, McGarry R, Yiannoutsos C, et al. Excessive Toxicity When Treating Central Tumors in a Phase II Study of Stereotactic Body Radiation Therapy for Medically Inoperable Early-Stage Lung Cancer. J Clin Oncol 2006;24:4833-9. 2. Adebahr S, Collette S, Shash E, et al. LungTech, an EORTC Phase II trial of stereotactic body radiotherapy for centrally located lung tumours: a clinical perspective. Br J Radiol 2015;88:20150036. 3. Guckenberger M, Kestin LL, Hope AJ, et al. Is there a lower limit of pretreatment pulmonary function for safe and effective stereotactic body radiotherapy for early-stage non-small cell lung cancer? J Thorac Oncol 2012;7:542-51. 4. Bahig H, Filion E, Vu T, et al. Severe radiation pneumonitis after lung stereotactic ablative radiation therapy in patients with interstitial lung disease. Pract Radiat Oncol 2016;6:367-74. Stereotactic ablative radiotherapy, contraindications