Adjacent photon fields abutting at depth of dose specification

In a treatment involving adjacent photon fields abutting at depth of dose specification, the overlap region (below the junction gets overdosed and the region above the junction gets overdosed).



In order to make the dose more uniform:

1) The gap on the surface can be increased or decreased in successive fractions

2) the two fields can be angled in opposite directions to make the beam edges parallel at the junction

3) The two fields can be treated by half fields (using half beam blocks or asymmetric jaws), the central axes becoming the matching edges of the two adjacent fields

4) A matching pair of POP fields can be used.



FOLLOWUP



Adjacent fields abut at the depth where the dose is required to be uniform (over the full length of the two abutting fields).



This however, creates high and low dose regions around the abutting junction. The high dose region is created by the adjacent fields diverging into each other. In order to accept this plan for treatment, the low end high dose regions must be free of tumor and organ at risk, respectively.



The "hot" and "cold" regions can be avoided by the various methods mentioned in this question, namely making the abutting field borders parallel to each other.