In recent reports, re-irradiation with SBRT for lung tumors previously treated with thoracic radiation therapy resulted in several serious toxicities [4, 6, 7, 10, 11, 17]. Most cases of serious non-lung toxicities were observed in patients with central tumors. Peulen et al. reported the following serious toxicities after re-irradiation with SBRT for central tumors: three cases of grade 5 hemoptysis and one case of grade 4 vena cava superior stenosis and grade 4 fistula between the trachea and gastric tube . Kilburn et al. reported a case of grade 5 aorta-esophageal fistula after re-irradiation with SBRT for a central tumor previously treated with chemoradiotherapy , while Trovo et al. reported a case of fatal hemoptysis after re-irradiation with SBRT for central disease .
In our case, the patient had a peripheral tumor that was treated twice with SBRT, and consequently developed serious gastric perforation. Referring to the endoscopic images, we suspected that this resulted from the high dose of the second SBRT delivered to the same gastric region as the first SBRT. Although we tried to extend the distance between the stomach and the tumor and to change the high-dose region in the stomach by inserting a spacer to reduce the stomach dose, we could not sufficiently extend the distance because of coalescent resulting from the first SBRT.
Bae et al. analyzed the predictors for severe gastroduodenal toxicity in patients treated with SBRT using 33 to 60 Gy (median, 45 Gy) in 3 fractions for abdominopelvic malignancies . Forty patients, including two and one patients with grade 4 gastric perforation and grade 3 gastric ulcer, respectively, were reviewed. The authors suggested that Dmax values of 35 and 38 Gy were respectively associated with 5% and 10% probabilities of severe gastroduodenal toxicity. In our case, the Dmax values in the first and second SBRT were 45.8 and 48.0 Gy, respectively. Considering that the maximum doses per fraction in our case and the predictive doses suggested by Bae et al.  were similar, the Dmax values in our first and second SBRT were higher than the above doses. We calculated the accumulated dose of the stomach in the first and second SBRT using MIM MaestroTM (version 6.5, MIM Software Inc., Cleveland, OH, USA). Rigid registration was used to create fusion CT images focusing on the fornix of the stomach. The Dmax of the stomach in the summed plan was 83.5 Gy, which was considerably higher than the predictive doses.
Finally, although a detailed case presentation on serious gastric perforation after SBRT for lung tumors has not yet been reported, based on the present case, we conclude that we should carefully evaluate the stomach doses in both the first and second SBRT. Moreover, we should also be cautious of the toxicity after the first SBRT, and should observe the stomach with endoscopy immediately before the second SBRT.
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