Radiobiological rationale for stereotactic hypofractionated radiosurgery Part I. LQED2 or BED formalism

In conventional radiotherapy, 5R’s mechanisms influence tumour cell kill, but in SHRS they do not sufficiently explain the biology of large doses. Indirect cell death is also induced by endothelial damage, stem cell death and antitumour immunity are also activated by a single dose ≥ 12–15 Gy. These three processes defined as extra 3R’s are characterizers in details. Despite some controversies, LQED formalism seems not quite adequate for SHRS. Experimental and a few clinical studies suggest BED formalism as much more useful. Both formalisms are compared and discussed. Clinical reports show a monotonical increase in Tumour Cure Probability (TCP) with higher BED doses. The advantage of SHRS results in significant shortening overall treatment time and in delivery of the BED doses higher than 100 Gy, producing an increase in the TCP, likely unachievable by conventional dose fractionation