So Far, So Good for Lung Cancer SBRT

Abstract

In this issue of the Journal of Thoracic Oncology, Stephans et al. from the Cleveland Clinic report their review of pulmonary function testing (PFT) before and after stereotactic body radiation therapy (SBRT) for stage I lung cancer.1 Though retrospective, this contribution is meaningful. The “believers” know that SBRT offers a substantial improvement in local control with relatively little toxicity compared with conventionally fractionated radiation therapy for these medically inoperable patients. The “skeptics” are fearful that the delivery of such high doses of radiation therapy may not be safe in the long run for this frail patient population. Stephans et al. show data that are consistent with two other datasets, that forced expiratory volume in 1 second FEV1% and diffusing capacity of the lung for carbon monoxide remain relatively unchanged following SBRT.2,3 Local control rates of 85 to 90% are now expected with SBRT for T1-2N0 lung cancers, provided that biologic equivalent doses of 100 Gy are employed.4 Multiple radiation oncology centers from around the world using multiple delivery technologies have demonstrated this level of effectiveness.5–8 These individual centers have also reported acute and late toxicities, which have been low ( 5%). Until recently, there has been a paucity of data showing the effect of SBRT on PFT. Prospective trials may lend validity to these results. Phase I/II prospective studies have recently been completed by the Radiation Therapy Oncology Group (RTOG 0236)9 and Japanese Cooperative Oncology Group (JCOG 0403). Results from these trials are pending. A phase III randomized trial comparing SBRT to surgery is just underway for operable patients in The Netherlands (ROSEL trial). Further prospective studies evaluating SBRT for lung cancer are either ongoing or are being planned. Completion of these trials is important to verify these results in a multicenter setting and to promote the option of SBRT to pulmonologists, medical oncologists, and surgeons who also manage these patients. Looking more closely at the current manuscript, a few questions are raised that need further evaluation from others. Are there patients with PFT results that are too low to be treated with SBRT? Are patients with central tumors more apt to experience decline in PFTs (i.e., due to centrally located fibrosis or atelectasis)? Does a higher conformality index really result in greater PFT decline? These are important questions that often arise but remain unknown. Stephans et al. report a patient with an FEV1% as low as 15% who is locally controlled and without complications. Larger multicenter databases may provide a larger set of patients on which to base a PFT threshold. Of the 10 patients treated for centrally located tumors, no PFT difference has yet been encountered, though they were treated with a gentler dosing schedule. The RTOG just initiated a separate SBRT trial for patients with centrally located tumors (RTOG 0813). The intent of the RTOG trial is to determine the maximum tolerated dose to a bronchus using five fractions, and to determine the effectiveness of that dose. Also in the current dataset, a higher conformality index resulted in greater PFT loss, implying that there may be a threshold for the number of beams used above which the low-dose volume becomes too high. What should the 5 Gy isodose volume be limited to? As the number of beams increases, so does the 5 Gy isodose volume. This has implications for SBRT planning and delivery.