A treatment planning study comparing HDR and AGIMRT for cervical cancer

The customization of brachytherapy dose distributions for gynecologic malignancies is limited by the spatial positioning of the applicators. We tested the hypothesis that applicator-guided intensity modulated radiation therapy (AGIMRT) has the potential to deliver highly conformal dose distributions to cervical tumors, representing improvement over distributions obtained with intracavitary brachytherapy. A commercial three-dimensional (3-D) treatment planning system was used to create plans for ten cervical cancer patients treated at our institution. Dose distributions of conventionally designed high dose rate (HDR) plans were compared against those of AGIMRT. Tumor delineation was based on a previously published binary threshold technique, using image intensity on positron emission tomography (PET) scans. AGIMRT treatment schedules were designed using two fraction sizes: 6.5 Gy, to directly reproduce the HDR fractionation, and 1.8 Gy, to simulate traditional external beam fractionation. The average minimum tumor dose was significantly greater for the AGIMRT dose distributions than for the HDR distributions (64.2 Gy vs 33.6 Gy; p = 0.005). The mean percent tumor volume at the prescription dose was higher for the AGIMRT plans (90.0% vs 58.2%; p = 0.005). Using AGIMRT, the mean percent volume at the tolerance limit was decreased for the bladder (6.1% vs 16.6%; p = 0.047) but increased for the rectum (4.1% vs 2.2%; p = 0.646). Our study suggests that there may be conceptual and dosimetric advantages to replacing HDR with AGIMRT for patients with large-volume cervical tumors. This investigation is being expanded using sequential PET images to model tumor regression and compare brachytherapy and AGIMRT throughout the course of therapy.