General equations for the calculations of biologic effect ratios for parallel opposing fields.

The tolerance level of normal tissue, which is the concept of NSD, is the limiting factor in radiation therapy. It is well known that the two parallel opposing fields should be treated at each session instead of alternating one field per session. The biologic effect ratios between the normal tissue at the depth of maximum build-up and the midline for parallel opposing fields were published by ELLIS et coll. General formulae are now presented providing biologic effect ratio at any two locations in terms of per cent depth doses in the treatment volume for parallel opposing fields. Examples at the depth of maximum build-up and certain depth of the connective tissue at the tumor site are also given.