ABSTRACT
An additional problem with this type of virtual wedging is that the nomenclature used by treatment planning systems is not intuitive and not necessarily correlated to the nomenclature used by the treatment machines and/or R&V systems. erefore, an incorrect wedge direction could possibly go undetected by
physics checks and by therapists on the rst day of treatment. Unfortunately, in vivo dosimetry performed with a diode on the central axis will not catch this particular error. Although the facility had the therapists check the nal jaw position at the end of treatment (indicating the “toe” of the wedge), this policy was not carried out suciently to catch these potential errors. For the scenario described, the dosimetric error could be as high as 80% to a point 8 cm o-axis, for opposed 60° wedged elds. ey found this type of error occurred when the wedge orientation was not obvious in a 2D representation of the treatment, as is the case for central nervous system treatments requiring nonaxial elds. An example of a hypothetical intracranial treatment with incorrect wedge directions is seen in Figure 21.2. In this case, there are a total of seven beams and the wedge was reversed from the intended orientations for four of the seven beams. is would have resulted in an increased high-dose region (7800 cGy) outside of the tumor volume, as compared with 7000 cGy in the correct plan. e pathway for such an error is as follows:
1. Dosimetry sta prepares the intended plan for treatment based on MLC eld-shaping. Aer the planning is complete and the paper and electronic charts prepared, there is a decision to switch to cerrobend eld-shaping. To avoid having the weight of the block against the latch for some elds (a nonissue for MLC), the collimator is rotated 180° for these particular elds. Unfortunately, the original wedge direction is not corrected for the elds. (Mandated replanning would have avoided this error.)
2. A physicist checks individual R&V beam settings but does not detect the incorrect wedge direction.
