A Novel Approach to Potentially Improving Soft-Tissue Sarcoma Survival: Prophylactic Lung Radiotherapy Inhibits Growth of Lung Metastases and Prolongs Survival in a Murine Soft-Tissue Sarcoma Model

Abstract

Background: Circulating tumor cells and clusters (CTC) from soft-tissue sarcoma (STS) that become entrapped in the lung can form micro-metastases and lead to pulmonary metastatic disease. Many patients with localized high-risk STS later develop metastases. Radiation is effective at reducing local recurrence by eradicating microscopic infiltration and satellites in the reactive zone surrounding the primary tumor. Prophylactic lung irradiation for patients with high-risk STS is a novel concept to potentially reduce the appearance of macroscopic metastases and improve survival. A proof-of-principle study was performed based on a novel approach: prophylactic lung radiation after resection of the primary tumor to address microscopic pulmonary deposits from CTC.

Methods: Immunocompromised mice and luciferase-expressing human fibrosarcoma (HT-1080-Luc) cell lines were used. In phase 1, HT-1080-Luc cells were injected into the tail vein to simulate CTC for the development of pulmonary metastases. Whole-lung irradiation (WLI) was then performed in the treated mice prior to the appearance of macroscopic metastases. In phase 2, a flank tumor was established to simulate a primary STS, followed by a tail-vein injection of HT-1080-Luc cells. Treatment groups included surgical removal of the primary STS and hemithoracic irradiation (HTI). Body weight and bioluminescence data were obtained and the mice were euthanized on Day 31 (phase 1) and Day 15 (phase 2) or when they reached 20% weight loss.

Results: In phase 1, prophylactic WLI increased survival and decreased pulmonary metastases. In phase 2, prophylactic HTI (left lung) decreased pulmonary metastases compared to controls. Lung histology showed dramatically decreased growth and number of established metastases with HTI. Resection of the primary tumor did not affect the growth of metastases.

Conclusion: Prophylactic WLI after resection of the primary tumor to inhibit the growth of pulmonary metastases from previously entrapped CTCs may be a promising approach to improve survival for patients with localized high-risk STS.

Keywords: lung metastasis; murine model; prophylactic; radiation; soft-tissue sarcoma.

Figure 1. Body weight percent change in radiation and control groups

The figure was created by the authors of this article.

Figure 2. Survival in treatment and control groups

The figure was created by the authors of this article.

Figure 3. Photon intensity of lungs vs. time

The figure was created by the authors of this article.

Figure 4. In-vivo bioluminescent imaging over time

The figure was created by the authors of this article.

Figure 5. Percent body weight change

The figure was created by the authors of this article.

Figure 6. Bioluminescent imaging demonstrating the development of pulmonary metastasis in all control mice that received IV injection or subcutaneous tumor plus IV injection and did not receive radiation

Figure 7. Bioluminescent imaging demonstrating minimal to no signal in the left lungs that received radiation

Yellow arrows indicate the lungs that received radiation.

Figure 8. Ratios of pulmonary metastases between left and right lung measured by photon intensity at euthanasia

ns: not significant *p<0.02, **p<0.01 The figure was created by the authors of this article.

Figure 9. Histologic images at 100× magnification of the right and left lungs of a mouse that had an establishment of a flank tumor and IV injection of HT-1080-Luc followed by resection of the flank tumor and HTI to the left lung

(A) Right lung demonstrates innumerable tumor nests, arranged around blood vessels, and of varying sizes from very small to large aggregates, diffusely throughout the lung parenchyma at 100× magnification. (B) The left lung demonstrates relatively few minute scattered tumor nests (red arrows) scattered throughout the lung parenchyma at 100× magnification. In contrast to the right lung, the left lung demonstrates a marked decrease in tumor nest frequency and size, and a lack of angiocentricity. (C) Ki67 immunohistochemical stain from the right lung, demonstrating a high proliferative index within tumor cells which form large aggregates and show angiocentricity at 400× magnification. (D) Ki67 immunohistochemical stain from the left lung, which shows a single minute cluster of tumor cells (red arrow) with a high proliferative index, and a marked decrease in tumor burden compared to the right lung at 200× magnification.