Megavoltage CT Image-based Intracavitary Low Dose Rate Brachytherapy for Cervical Carcinoma

Abstract

Purpose/Objective: To investigate the efficacy of megavoltage CT (MVCT) imaging for 3D CT-based intracavitary low-dose rate (LDR) brachytherapy for cervical carcinoma, and to compare computed tomography (CT)-based volumetric calculations and ICRU reference-point estimates of doses to bladder and rectum. Materials/Methods: Patients were treated with intracavitary LDR brachytherapy, using standard (non-CT compatible) Fletcher-Suit applicators and Cs-137 tube sources. Clinical treatment planning was performed via traditional methods using orthogonal films and reference dose points. Prior to implant loading, a helical tomotherapy unit was used to acquire MVCT images of the patient and implanted applicator with dummy sources inserted. Three-dimensional dose distributions were retrospectively computed using the same treatment planning system and the MVCT image sets, and were analyzed based upon rectal and bladder DVHs, and doses to the traditional system points. Results: MVCT imaging of standard Fletcher-Suit brachytherapy applicators provides images largely free of reconstruction errors characteristic of kVCT images. kVCT and MVCT images of a standard Fletcher-Suit applicator, in-phantom and in-vivo, are shown in the figure. The middle MVCT image (in-phantom) quality of the same applicator is notably better than the kVCT image at left. Reconstructed MVCT image quality in-vivo is similar (at right). MVCT images generally demonstrate poorer soft-tissue contrast than do kVCT images, due to the lesser differences in CT number as a function of electron density. However, the anatomy of interest in a typical implant (bladder, rectum) is still identifiable in MVCT images. At the time of this writing, we have treated a total of four patients (two patients with two insertions each and two patients with the first of two planned insertions complete). A summary of the completed patients bladder and rectal dosimetry (four insertions) is provided in the table below. As expected, the doses to the ICRU bladder and rectal points are less than the three-dimensional maximum point dose computed from dose-volume histograms. In three of the four insertions reported, the ICRU bladder point dose was also less than the minimum dose received by the most highly dosed 2cc of bladder volume (DBV2); in three of the four insertions reported, the ICRU rectal point dose was approximately equal to or less than the minimum dose received by the maximally dosed 2cc of rectal volume (DRV2). Conclusions: Helical tomotherapy MVCT imaging provides almost artifact-free images of metallic brachytherapy applicators, and thus allows CT-based brachytherapy dosimetry to be readily performed without the use of CT-compatible applicators. MVCT soft tissue contrast is poorer than with conventional kVCT images, but is still sufficient to allow delineation of pelvic structures for 3D brachytherapy dosimetry. Dose received at standard bladder and rectal reference points determined from two-dimensional radiographic images are not necessarily representative of the maximum 3D point dose or the dose received by the maximum-dosed 2cc of each structure. Tabled 1Bladder and rectal dosimetry summary (cGy) for two patients (four insertions). View Large Image Figure ViewerDownload (PPT)