Po-topic IV-13: Dose optimization in hdr brachytherapy for cervical cancer using FDG-PET

Abstract

The purpose of this study was to investigate the possibility of dose optimization in high-dose rate (HDR) intracavitary brachytherapy for the treatment of cervical cancer with tumor volumes delineated using positron emission tomography (PET). 18F-fluorodeoxyglucose (FDG)-PET scans were acquired for 10 patients with implanted tandem and ovoid (T&O) applicators containing tubes of FDG. The FDG-PET images were transferred to a commercial 3D treatment planning system where the tumor volume was defined using a binary threshold technique. Tumor tissue was identified on FDG-PET images as any voxel in the 3D data set with counts greater than a fixed threshold fraction (40%) of the peak intensity of the tumor. This threshold level was selected from information gained by correlation of FDG-PET volumes with volume of cervical tumor clearly defined on CT scan images in a series of patients with cervical cancer. The bladder and rectum were contoured by the physician who treated the patients. The tandem and ovoid applicators, which contained catheters filled with FDG, had a characteristic appearance on the FDG-PET images and were uniquely identified by their expected positions. FDG-PET based 3D treatment plans were first simulated for conventional HDR delivery. The source distribution, based on our clinical protocols, was designed to deliver 6.5 Gy to Point A under ideal conditions. Point A was identified according to the classical definition relative to the reconstructed applicators. There were nine active dwell positions in the tandem and three active positions in each of the ovoids. Maintaining identical contours on the FDG-PET images, investigational treatment plans were then created my manipulating dwell positions and dwell times. The new plans used an integrated reference air kerma that was equivalent to the conventional plans. The source arrangement that best maximized tumor coverage was found to be a tandem hypothetically extended by 2 cm superiorly, continuing along its original trajectory. The extended tandem included 15 active dwell positions. Dwell times were customized to optimize dose distributions to the individual GTV versus dose distributions for the two sets of plans were compared by percent tumor coverage and by minimum dose within tumor. The optimized plans demonstrate improvement in both tumor coverage (mean 33.0%, std 15.1%, median 31.4%) and peripheral dose (mean 32.3%, std 43.9%, median 36.0%). The optimized plans demonstrate bladder reference point doses that are increased (mean 14.1%, std 32.5%, median 13.6%) and rectal dose that are decreased (mean −31.3%, std 22.2%, median −35.2%). This study demonstrates the potential for improving HDR brachytherapy dose distributions for FDG-PET-defined cervical cancer. Superior tumor coverage and peripheral dose can be achieved by utilizing a single extended tandem applicator compared with conventional T&O dose distributions, and by optimizing source weighting to conform to the geometry of the tumor. Acceptable levels of toxicity based on bladder and rectal dose volume histograms were observed in the investigational plans of most patients.