Effect of Irradiation on Tumor Microenvironment and Bone Marrow Cell Migration in a Preclinical Tumor Model

To characterize the tumor microenvironment after standard radiation therapy (SRT) and pulsed radiation therapy (PRT) in Lewis lung carcinoma (LLC) allografts.

Subcutaneous LLC tumors were established in C57BL/6 mice. Standard RT or PRT was given at 2 Gy/d for a total dose of 20 Gy using a 5 days on, 2 days off schedule to mimic clinical delivery. Radiation-induced tumor microenvironment changes were examined after treatment using flow cytometry and antibody-specific histopathology. Normal tissue effects were measured using noninvasive (18)F-fluorodeoxyglucose positron emission tomography/computed tomography after naïve animals were given whole-lung irradiation to 40 Gy in 4 weeks using the same 2-Gy/d regimens.

Over the 2 weeks of therapy, PRT was more effective than SRT at reducing tumor growth rate (0.31 ± 0.02 mm(3)/d and 0.55 ± 0.04 mm(3)/d, respectively; P<.007). Histopathology showed a significant comparative reduction in the levels of Ki-67 (14.5% ± 3%), hypoxia (10% ± 3.5%), vascular endothelial growth factor (2.3% ± 1%), and stromal-derived factor-1α (2.5% ± 1.4%), as well as a concomitant decrease in CD45(+) bone marrow-derived cell (BMDC) migration (7.8% ± 2.2%) after PRT. The addition of AMD3100 also decreased CD45(+) BMDC migration in treated tumors (0.6% ± 0.1%). Higher vessel density was observed in treated tumors. No differences were observed in normal lung tissue after PRT or SRT.

Pulsed RT-treated tumors exhibited slower growth and reduced hypoxia. Pulsed RT eliminated initiation of supportive mechanisms utilized by tumors in low oxygen microenvironments, including angiogenesis and recruitment of BMDCs.