SABR for Ultra-Central Lung Tumors: Assessment of Trade-Offs between Target Coverage Versus Sparing of Critical Structures

Abstract

Although lung stereotactic ablative radiotherapy (SABR) is associated with low morbidity, there is an increased risk of treatment-related toxicity in tumors directly abutting or invading the proximal bronchial tree, termed ‘ultra-central’ tumors. For such tumors, there is no consensus regarding the most appropriate dose-fractionation scheme. The purpose of this planning study was thus to evaluate SABR treatment plans for ultra-central tumors using commonly utilized dose fractionation regimens. In this research ethics board approved study, 10 patients with ultra-central lung tumors (defined as abutting or invading the proximal bronchial tree) were identified from our institutional database. Clinically delivered doses were either 60 Gy in 8 fractions or 40-60 Gy in 15 fractions. New plans were generated for each of the 10 cases using 3 different hypofractionated schemes: 50 Gy in 5 fractions, 60 Gy in 8 fractions and 60 Gy in 15 fractions. For each of the three dose regimens, 2 plans were generated, one prioritizing tumor coverage (with dose to organs at risk as low as reasonably achievable) and the other plan compromising PTV coverage in order to respect the dose constraints for organs at risk. Descriptive statistics were generated to illustrate the dosimetric trade-offs of each scenario. The mean IGTV and PTV volumes were 30.1 cm3 (range 7.1-83.1 cm3) and 78.8cm3 (25.0-171.5 cm3), respectively. The mean volume of IGTV and PTV overlap with the proximal bronchial tree were 0.2 cm3 (0.01-0.5 cm3) and 1.1 cm3 (0.4-2.4 cm3), respectively. In the scenario where PTV coverage was prioritized, the mean maximum point dose to the proximal bronchial tree for 50 Gy in 5, 60 Gy in 8 and 60 Gy in 15 were 60.1 Gy (52.3-72.9), 70.7 Gy (63.8-78.1) and 60.8 Gy (57.0-62.8), respectively. When dose to the organs at risk was prioritized, PTV coverage for the 50 Gy in 5 fraction scheme was reduced from 96.0% (95.0-100) to 67.8% (19.6-89.2); for 60 Gy in 8, from 96.0% (95.0-99.9) to 68.5% (18.8-89.5); and for 60 Gy in 15, from 97.3% (95.5-100) to 91.6% (82.0-97.8). One patient with a large ultra-central tumor located close to the spinal cord required significant compromise of tumor coverage in order to respect the dose constraints for organs at risk. The range of PTV coverage for the other nine patients ranged from 56.8-89.2% for 50 Gy in 5, 59.4-89.5% for 60 Gy in 8 and 82.0-97.8% for 60 Gy in 15 fractions. With the use of hypofractionated radiotherapy or SABR for ultra-central lung tumors, the competing risks of tumor local control and acute treatment toxicities need to be considered. This planning study suggests that a conservative approach of 60 Gy in 15 fractions may allow for an acceptable dose to normal tissues with less compromise of the intended tumor dose. Further research on tumor-control probability (TCP) and normal tissue complication probability (NTCP) modelling to estimate the therapeutic ratio of these different fractionation schemes is planned.