Phase I Trial of Dose-Escalated Five-Fraction Stereotactic Ablative Radiotherapy for Early-Stage Lung Squamous Cell Carcinoma.

PURPOSE/OBJECTIVE(S) We aimed to evaluate the safety of and maximum tolerated dose (MTD) for 5-fraction stereotactic ablative radiotherapy (SABR) for early-stage lung squamous cell carcinoma (SCC), for which local failure rates appears higher than that of early-stage adenocarcinomas. MATERIALS/METHODS A single institution phase I study was conducted in pathologically confirmed early-stage lung SCC. The 3+3 design involved 4 dose levels of 11, 12, 13, and 14 Gy x 5 fractions delivered every other day (dose levels DL1, DL2, DL3, and DL4, respectively). The primary objective was to determine the MTD of 5-fraction SABR, which was defined as the dose associated with a 33% rate of dose-limiting toxicities, defined as grade ≥3 respiratory/mediastinal or cardiac events (or grade ≥4 gastrointestinal or chest wall events), with toxicities reported until data lock in October 2020. Target volume delineation and organ-at-risk (OAR) constraints were based on RTOG 0813, using OAR constraints expressed in percent of prescription as fixed constraints from the 11 Gy per fraction prescription for all DL. Institutional constraints for the chest wall were used (V30 < 30cm3, up to 70 cm3 acceptable; maximum point dose 52.5 Gy). Target coverage objectives included ≥95% of the planning target volume (PTV) to be covered by 100% of the prescription dose, and 100% of the internal target volume (ITV) to receive ≥110% of the prescription dose. For technically challenging cases, it was permitted to prescribe as low as 50 Gy (BED of 100 Gy10) to a small portion of the GTV, ITV (if used) and corresponding PTV adjacent to a critical OAR (split target volume prescription). SABR was delivered using free breathing, deep inspiration breathhold, or expiratory gating. Thus, ITVs were not used for all patients. RESULTS From October 2017 to June 2020, 13 patients were enrolled: 3 in DL1, 3 in DL2, 4 in DL3, and 3 in DL4. Seven (IASLC definition) or 6 (RTOG) patients had central lesions, and the rest abutted the chest wall. All patients completed SABR per protocol without major deviations. At 18.4 months median follow-up (range, 2.4-25.6 months), no patient developed dose limiting toxicities. There were no grade 4-5 events and 1 grade 3 event (painful brachial plexopathy, which occurred in DL1 at 16 months post-SABR at the time of local progression). There were 7 instances of grade 2 toxicities. These included fatigue (n = 1, DL2), chest wall pain (n = 2, DL2 & DL4), cough (n = 1, DL3), dyspnea (n = 2, DL3 & DL4), nausea (n = 1, DL4), and wheezing (n = 1, DL4). Eight (62%) patients required utilization of "split target volume" prescriptions. The mean total dose to the GTV ranged from 60.9-62.2 Gy for DL1, 63.2-68.6 Gy for DL2, 67.3-78.8 Gy for DL3, and 75.5-82.2 Gy for DL4. The median GTV was 19.9 cc (range 5-57cc). CONCLUSION 5-fraction SABR for early-stage lung SCC up to 14 Gy x 5 fractions using a split target prescription for OAR adjacent tumor appears safe. Long term follow up will be needed to assess late toxicities and tumor control. (NCT03321747).