In Regard to Dong et al

To the Editor: I would like to make a comment on the "4π radiotherapy" mentioned by Dong et al in 2013, in which 4π indicates the solid angle (1). Subsequently this concept was used by several other authors, and presently some institutions are recruiting patients in such studies (2-8). In geometry of 3-dimensional Euclidian space (solid geometry), angle, hence the solid angle at any point, is defined as , where ds is the surface area, and r is the radius vector.

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