Using asymmetric jaws to junction with previously treated areas

The feasibility of employing asymmetric jaws to match previously irradiated portals has been investigated. According to the isodose plot of one treatment planning system a near perfect match can be obtained by exactly matching the beam divergence of the initial port. However, another treatment planning system shows a cold junction. Films taken at 6 cm depth show a 6% cold spot when matching at the inferior border of the previously irradiated port and a 7% hot spot when matching at the superior border. As long as one is aware of the limitations of computer planning and collimator setting tolerance, junctioning with asymmetric jaws may still be preferred over the conventional method of gapping on the skin, because it avoids larger hot and cold spots below and above the matchpoint.