[Safety and feasibility of stereotactic radiation therapy on porcine ventricular septum: a preliminary study]

Objective: To explore the safety and feasibility of stereotactic radiation therapy (SBRT) strategy for irradiating porcine ventricular septum, see if can provide a preliminary experimental evidence for clinical SBRT in patients with hypertrophic obstructive cardiomyopathy (HOCM). Methods: Five male pigs (39-49 kg, 6 months old) were used in this study. Pigs were irradiated at doses of 25 Gy (n=2) or 40 Gy (n=3). Delineation of the target volume was achieved under the guidance of 3-dimensional CT image reconstruction, and SBRT was then performed on defined target volume of porcine ventricular septum. Blood biomarkers, electrocardiogram and echocardiography parameters were monitored before and after SBRT. Pathological examination (HE staining, Masson staining) was performed on the target and non-target myocardium at 6 months post SBRT. Results: SBRT was successful and all animals survived to the designed study endpoint (6 months) after SBRT. Serum cardiac troponin T (cTnT) level was significantly higher than the baseline level at 1 day post SBRT, and reduced at 1 week after SBRT, but was still higher than the baseline level(P<0.05). Serum N-terminal pro-B type natriuretic peptide (NT-proBNP) was also significantly increased at 1 day post SBRT (P<0.05) and returned to baseline level at 1 week post SBRT. The serum NT-proBNP level was (249±78), (594±37) and (234±46) pg/ml, respectively, and the cTnT was (14±7), (240±40) and (46±34) pg/ml, respectively at baseline, 1 day and 1 week after SBRT in the 40 Gy dose group. The serum NT-proBNP level was (184±20), (451±49) and (209±36) pg/ml, respectively, the cTnT values ​​were (9±1), (176±29) and (89±27) pg/ml, respectively at baseline, 1 day and 1 week after SBRT in the 25 Gy dose group. Both NT-proBNP and cTnT values tended to be higher post SBRT in the 40 Gy dose group as compared with the 25 Gy dose group, but the difference was not statistically significant (P>0.05). The left ventricular ejection fraction and the left ventricular end-diastolic diameter remained unchanged before and after SBRT (P>0.05). The interventricular septum thickness showed a decreasing trend at 6 months after SBRT, but the difference was not statistically significant ((9.54±0.24) mm vs. (9.82±8.00) mm, P>0.05). The flow velocity of the left ventricular outflow tract, and the valve function and morphology were not affected by SBRT. At 6 months after SBRT, HE staining revealed necrosis in the irradiated target area of ​​the myocardium in the 40 Gy dose group and the 25 Gy dose group, and the degree of necrosis in the irradiated interventricular septum was more obvious in the 40 Gy dose group as compared with the 25 Gy group. The combined histological analysis of the two groups showed that the necrotic area of ​​the irradiated target area accounted for (26±9)% of the entire interventricular septum area, which was higher than that of the non-irradiated area (0) (P<0.05). There was no damage or necrosis of myocardial tissue outside the target irradiation area in both groups. The results of Masson staining showed that the percentage area of myocardial fibrosis was significantly higher in the irradiated target area than non-irradiated area ((12.6±5.3)% vs. (2.5±0.8)%, P<0.05). Conclusion: SBRT is safe and feasible for irradiating porcine ventricular septum.