The project is intriguing demonstrating that targeted external irradiation of rabbits or dogs with energetic carbon ions can reduce fatal ventricular arrhythmia via upregulation of gap junction protein connexin 43 and that such antiarrhythmogenic potential persists at least one year after single irradiation.
Heart has long been considered as one of radioresistant tissues. For instance, TD5/5 for the heart (tolerance dose at normal tissue complication provability of 5% within 5 years after fractionated exposures) was considered to be 40-60 Gy depending on the volume irradiated within the heart, for which effect considered was perimyocarditis ( https://www.ncbi.nlm.nih.gov/pubmed/2032882 ).
However, the International Commission on Radiological Protection (ICRP) now classifies circulatory disease (cardio- and cerebrovascular disease) as tissue reactions (formerly called deterministic effects or non-stochastic effects) and has recently recommended the first ever threshold to the heart and brain. The threshold dose recommended was 0.5 Gy independent of dose rate, which was recommended as dose causing 1% circulatory disease mortality at >10 years after exposure.
https://www.researchgate.net/publication/262305338_Emerging_issues_in_radiogenic_cataracts_and_cardiovascular_disease?
https://www.researchgate.net/publication/262054513_Classification_of_radiation_effects_for_dose_limitation_purposes_History_current_situation_and_future_prospects
The dose used in the project is 15 Gy, which is significantly higher than 0.5 Gy (but of course with the caveat that the threshold for humans can not directly be compared with that for rabbits or dogs). In addition, the relative biological effectiveness (RBE) value of carbon ions for circulatory effects, which I do not think is available, may be high.
Therefore, in the short term the recovery from fatal ventricular arrhythmia will surely outweigh the risk of "second" life threatening circulatory effects, but in the long term the impact of such late onset circulatory effects may need to be considered.
Article Emerging issues in radiogenic cataracts and cardiovascular disease
Article Classification of radiation effects for dose limitation purp...
I hope these links can help you
http://www.nejm.org/doi/full/10.1056/nejmoa1209825#t=article
https://academic.oup.com/jnci/article/104/5/357/2517157/Second-Malignant-Neoplasms-and-Cardiovascular
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