ASYMMETRIC COLLIMATION AND ITS CLINICAL APPLICATION

Asymmetric fields can be obtained by independent movement of radiation field defining collimators of certain linear accelerators. This technique eliminates the beam divergence which is an important problem for the treatment of breast cancer, head and neck cancers and other treatment fields with abutting borders. Both of our accelerators (Philips SL-25) have the ability to set both sets of collimators asymmetrically. Prior to clinical use of this technique, evaluation of the dosimetry of this nonstandard technique was made and it was started to use in the treatment of abutting fields since December 1994. The advantages obtained by this mono isocentric treatment technique are; eliminating the beam divergence, eliminating the use of heavy cerrobend blocks (beam splitters), ease and reproducibility of set-up's, obtaining the ideal match line dose distribution at the abutting fields as the patient does not have to move between fields. Dosimetry and treatment planning are more sophisticated than symmetric fields for the physicist, at least initially. Limitation of this technique is the field size of 20 cm at isocenter. In conclusion, this new treatment technique is an important part of the conformal radiotherapy and it must be used in centers having this facility.