Volumetric and dosimetric evaluation of radiation treatment plans: radiation conformity index.

PURPOSE: The use of conformal radiation therapy has grown substantially during the last years since three-dimensional (3D) treatment planning systems with beams-eye-view planning has become commercially available. We studied the degree of conformity reached in clinical routines for some common diagnoses treated at our department by calculating a radiation conformity index (RCI). METHODS AND MATERIALS: The radiation conformity index, determined as the ratio between the target volume (PTV) and the irradiated volume, has been evaluated for 57 patients treated with 3D treatment plans. RESULTS AND
CONCLUSION: The RCI was found to vary from 0.3 to 0.6 (average 0.4), a surprisingly low figure. The higher RCI is typical for pelvic treatments (e.g., prostate) and stereotactic treatments. The lower RCI is found for extended tumors, such as mammary carcinomas where the adjacent nodes are included. The latter is also valid for most lung cancer patients studied. The RCI gives a consistent method for quantifying the degree of conformity based on isodose surfaces and volumes. Care during interpretation of RCI must always be taken, since small changes in the minimum dose can dramatically change the treated volume.