Response to 'Stereotactic ablative radiotherapy in the framework of classical radiobiology: response to Drs. Brown, Diehn, and Loo.' (Int J Radiat Oncol Biol Phys 2011;79:1599-1600) and 'Influence of tumor hypoxia on stereotactic ablative radiotherapy (SABR): response to Drs. Mayer and Timmerman.' (Int J Radiation Oncol Biol Phys 2011;78:1600).

摘要

To the Editor: In a recent issue of the International Journal of Radiation, Biology, and Physics, Drs. Meyer and Timmerman (1) and Brown et al. (2) discussed their differing views as to how to approach hypoxic protection in treating tumors with stereotactic body radiotherapy (SBRT) or stereotactic ablative radiotherapy (SABR). Brown et al. (3) previously suggested that hypoxic radiosensitizers will improve the response of tumors to SBRT or SABR because the currently used SBRT or, SABR protocols are unable to eradicate all the hypoxic cells in target tumors. Meyer and Timmerman (1), however, contended that fractionation of SBRT or SABR with a hypoxia transition dose per fraction would allow hypoxic cells to be reoxygenated during the fractionation intervals, thereby rendering tumor cells vulnerable to subsequent irradiations with a hypoxia transition dose. It is our view that, as Brown et al. stated (2), it would be difficult to identify precisely the hypoxic transition dose before each fractionated radiation, at least with presently available technology. In addition, we would like to point out an additional problem in the approach Meyer and Timmerman proposed. In the particular tumor model and survival curve that Meyer and Timmerman used for their discussion, as shown in their Fig. 1, the hypoxia transition dose was suggested to be 9 Gy. We believe that irradiation of tumors with 9-Gy ranges, especially when it is repeated, will cause considerable vascular damage throughout the tumors and deplete oxygen supply to tumor cells, leading to deoxygenation of oxic tumor cells rather than reoxygenation of hypoxic tumor cells. Consequently, expecting that "the hypoxic proportion remains constant (or less) between fractions," as Meyer and Timmerman stated, is quite unrealistic when tumors are irradiated with doses as high as 9 Gy per fraction.