Comparison of a simple dose-guided intervention technique for prostate radiotherapy with existing anatomical image guidance methods.

OBJECTIVES: A simple dose-guided intervention technique for prostate radiotherapy using an isodose overlay method combined with soft-tissue-based corrective couch shifts has been proposed previously. This planning study assesses the potential clinical impact of such a correction strategy.
METHODS: 10 patients, each with 8-11 on-treatment CT studies (n=97), were assessed using this technique and compared with no intervention, bony anatomy intervention and soft-tissue intervention methods. Each assessment technique used a 4-mm action level for intervention. Outcomes were evaluated using measures of sensitivity, specificity and dosimetric effect, and compared across intervention techniques. Dosimetric effect was defined as the change in dosimetric coverage by the 95% isodose from the no intervention case of an evaluation construct called the verification target volume.
RESULTS: Bony anatomy, soft tissue and dosimetric overlay-based interventions demonstrated sensitivity of 0.56, 0.73 and 1.00 and specificity of 0.64, 0.20 and 0.66, respectively. A detrimental dosimetric effect was shown in 7% of interventions for each technique, with benefit in 30%, 35% and 55% for bony anatomy, soft tissue and dosimetric overlay techniques, respectively.
CONCLUSION: Used in conjunction with soft-tissue-based corrective couch shifts, the dosimetric overlay technique allows effective filtering out of dosimetrically unnecessary interventions, making it more likely that any intervention made will result in improved target volume coverage.