Quantifying the degree of conformity in radiosurgery treatment planning.

PURPOSE: To compare different parameters used to quantify the quality of a treatment plan and to evaluate the dose conformity and coverage clinically achieved using gamma knife radiosurgery.

METHODS AND MATERIALS: Various existing parameters for coverage and conformity are reviewed. Additionally, a modified conformity index (CI) has been defined as the ratio of the volume within the target irradiated to at least the prescription isodose over the total volume enclosed by the prescription isodose. These parameters are calculated for all the 551 evaluable patient treatment plans.

RESULTS

The median CI for all targets is 0.75, with a median target coverage of 94.6%. Regardless of the conformity parameter chosen, the conformity is seen to vary depending on the type of tumor and its location, reflecting the treatment planning philosophy. For tumors with volumes smaller than about 1 cm(3), the conformity parameter is also seen to be dependent on the target volume.

CONCLUSION

With gamma knife radiosurgery, it is possible to achieve highly conformal dose distributions. A single parameter for the quantification of a plan, though desirable, is not realistic, because of the competing components of high dose to the target and low dose to normal tissue. Thus, we propose the use of the CI, together with the target volume coverage.