Daily Dose Recalculation Using Megavoltage CT Images During Tomotherapy for Localized Prostate Cancer: Dosimetric Analysis for the Prostate Gland, Rectum and Bladder for the Full Treatment Course

Abstract

Purpose/Objective: To evaluate the dosimetric consequences of daily anatomical changes on the prostate gland, rectum and bladder. Materials/Methods: Five prostate cancer patients treated to 78 Gy at 2 Gy per fraction with helical tomotherapy constituted the study sample. Prior to each daily fraction, intraprostatic markers detected on a megavoltage CT (MVCT) are registered with the simulation kilovoltage CT (kVCT) for image guidance. The daily MVCTs can also be used to recalculate delivered doses. The recalculation technique uses the intended delivery and the MVCT image to calculate the dose distribution in the anatomy of the day, hence allows a full evaluation of the dosimetric impact of anatomical changes throughout the treatment course. This process was previously tested and verified in phantom studies to be accurate within 0.5%. The MVCT numbers were calibrated in terms of electron densities allowing the use of MVCTs for dose calculations. The entire treatment course (39 fractions per patient) was retrospectively analyzed for all 5 patients. A total of 195 MVCT sets were studied. The rectum and bladder were recontoured on each of the 195 daily MVCTs. The prostate contours were transferred from the original kVCT image set and altered only in position and shape if necessary. For each treatment fraction, DVHs were calculated based on the new contours and recalculated dose distributions. The volume of rectum and bladder that received the prescription dose (in ccs) and the dose to 95% of the prostate volume (D95) were scored. Results: Fig. 1, 2 and 3 show the average (±S.D) and range of daily rectal and bladder volumes receiving full prescription dose and the D95 for each patient. It is clear that the prostate D95 was remarkably constant throughout the course of treatment for all patients indicating adequate coverage of the target regardless of daily variations in the patients′ pelvic anatomy. However, there was remarkable variation in the rectal volumes receiving the prescription fraction dose, varying in some patients (e.g. patient #2) more than in others (e.g. patient #1). There was a trend for higher rectal doses with rectal distention. Distention pushed a larger rectal volume into the high dose region that was occupied by the prostate on the planning kVCT. The prostate D95 also tended to increase with rectal distention. This may be due to the presence of rectal gas and a subsequent reduction in attenuation for beams entering the prostate posteriorly. In the patient with the largest variation in rectal volumes, the degree of rectal distention was unpredictable. Bladder volumes changes were also noticeable, but of lesser magnitude compared to the rectum. Conclusions: Daily anatomical deformations have dosimetric consequences for the rectum, bladder and prostate. Rectal deformation is the most consequential. Daily recalculation of delivered doses using megavoltage images enables a complete dosimetric evaluation of an entire treatment course with external beam radiotherapy.