Effective Rat Lung Tumor Model for Stereotactic Body Radiation Therapy

Abstract

Stereotactic body radiation therapy (SBRT) has found an important role in the treatment of patients with non-small cell lung cancer, demonstrating improvements in dose distribution and even tumor cure rates, particularly for early-stage disease. Despite its emerging clinical efficacy, SBRT has primarily evolved due to advances in medical imaging and more accurate dose delivery, leaving a void in knowledge of the fundamental biological mechanisms underlying its activity. Thus, there is a critical need for the development of orthotropic animal models to further probe the biology associated with high-dose-per-fraction treatment typical of SBRT. We report here on an improved surgically based methodology for generating solitary intrapulmonary nodule tumors, which can be treated with simulated SBRT using the X-RAD 225Cx small animal irradiator and Small Animal RadioTherapy (SmART) Plan treatment system. Over 90% of rats developed solitary tumors in the right lung. Furthermore, the tumor response to radiation was monitored noninvasively via bioluminescence imaging (BLI), and complete ablation of tumor growth was achieved with 36 Gy (3 fractions of 12 Gy each). We report a reproducible, orthotopic, clinically relevant lung tumor model, which better mimics patient treatment regimens. This system can be utilized to further explore the underlying biological mechanisms relevant to SBRT and high-dose-per-fraction radiation exposure and to provide a useful model to explore the efficacy of radiation modifiers in the treatment of non-small cell lung cancer.

FIG. 1

CBCT images of lung tumor and treatment planning. Panel A: Lung tumor (blue line) and peripheral tissue (green line) contouring based on CBCT images. Panel B: Treatment planning of 12 Gy in 9 dynamic beams. Panel C: Dose map on contoured tissues.

FIG. 2

Panel A: Surgical implantation of lung tumor cells. Adult female athymic nude rats were anesthetized by 3% isoflurane. Rats were intubated through the trachea and supported by a small animal ventilator. Lungs were surgically exposed and injected with A549-Luc cells. Incisions were closed with suture and staples, and animals were monitored for recovery. Panel B: Tumor growth was observed three weeks after tumor implantation using multimodal imaging including bioluminescent imaging, CBCT and MRI. Tumor presence was confirmed after animals were euthanized. Panel C: Tumor development was monitored weekly with bioluminescent imaging after implantation. The pie chart shows percentages of lung tumor formation 3 weeks after surgical implantation.

FIG. 3

Dose response of A549-Luc orthotopic lung tumors to radiation. A549-Luc cells were implanted in female athymic 8-week-old nude rats. Tumor growth was monitored weekly by bioluminescence and the tumor was treated with 12 Gy × 1 and 12 Gy × 3 fractions at week 3. Panel A: Representative images from each group. Panel B: Data analysis among different groups. Radiation treatments were started at week 3 for both treatment groups (12 Gy × 1 fraction; 12 Gy × 3 fractions over 72 h).