Investigating Radiation-Induced Injury to Airways and Pulmonary Vasculature in Lung Stereotactic Ablative Body Radiotherapy (SAbR)

Very few recent lung cancer therapies have had as positive an impact on public health as lung SAbR. Lung SAbR involves the precise administration of very high, biologically potent doses (54-70 Gy) in 3-5 fractions. The most recent update of the Radiation Therapy Oncology Group (RTOG) 0236 Phase II multi-center lung SAbR trial showed 5-year primary tumor control >90% in inoperable Stage I NSCLC patients with tumors ≤ 5cm. However, the use of such highly potent doses puts patients at risk for collateral toxicity including radiation pneumonitis and radiation injury to airways, causing stenosis, atelectasis and ultimately fibrosis. A review of 35 early-stage NSCLC SAbR clinical studies found that the reported maximum values of Grade ≥3 toxicities were between 10-28%. In the RTOG 0236 trial, 17% patients presented Grade ≥3 toxicities. Furthermore, toxicity has been shown to increase dramatically for centrally-located and/or larger (> 5cm) early-stage tumors. Due to these concerns, in current clinical practice, the use of high-dose-per-fraction lung SAbR is routinely indicated only for a small percentage (~3 - 4%) of the NSCLC population - inoperable, early-stage patients with small, peripheral lesions. Inoperable patients with early-stage central and/or larger tumors, higher stage and/or multiple lesions, and pulmonary oligometastases (metastatic disease that is limited in number of lesions and sites) are either treated with conventional radiotherapy and/or chemotherapy (~30 - 40% 5-year survival), or not treated (median survival ~1 year).

The segmented structures will be imported into a clinical radiotherapy treatment planning system in order to calculate dose to individual BSS elements. Follow-up CT images and follow-up SPECT V/Q scans will be used to characterize the radiosensitivity of these structures and spatially map the potential radiation-induced loss of lung function. Eventually, using this information, the investigator will investigate treatment planning strategies that help limit radiation dose and consequent damage to BSS elements, thereby reducing lung toxicity. The investigator hypothesizes that - Anatomically variable radiation injury to the bronchial tree and pulmonary vasculature is an important determinant of post-SAbR pulmonary toxicity and residual pulmonary function.