DAILY CT LOCALIZATION FOR CORRECTING PORTAL ERRORS IN THE TREATMENT OF PROSTATE-CANCER

Introduction: Improved prostate localization techniques should allow t he reduction of margins around the target to facilitate dose escalatio n in high-risk patients while minimizing the risk of normal tissue mor bidity. A daily CT simulation technique is presented to assess setup v ariations in portal placement and organ motion for the treatment of lo calized prostate cancer. Methods and Materials: Six patients who conse nted to this study underwent supine position CT simulation with an alp ha cradle cast, intravenous contrast, and urethrogram. Patients receiv ed 46 Gy to the initial Planning Treatment Volume (PTV,) in a four-fie ld conformal technique that included the prostate, seminal vesicles, a nd lymph nodes as the Gross Tumor Volume (GTV(1)). The prostate or pro state and seminal vesicles (GTV(2)) then received 56 Gy to PTV2. All d oses were delivered in 2-Gy fractions. After 5 weeks of treatment (50 Gy), a second CT simulation was performed. The alpha cradle was secure d to a specially designed rigid sliding board. The prostate was contou red and a new isocenter was generated with appropriate surface markers . Prostate-only treatment portals for the final conedown (GTV(3)) were created with a 0.25-cm margin from the GTV to PTV. On each subsequent treatment day, the patient was placed in his cast on the sliding boar d for a repeat CT simulation. The daily isocenter was recalculated in the anterior/posterior (AVP) and lateral dimension and compared to the 50-Gy CT simulation isocenter. Couch and surface marker shifts were c alculated to produce portal alignment. To maintain proper positioning, the patients were transferred to a stretcher while on the sliding boa rd in the cast and transported to the treatment room where they were t hen transferred to the treatment couch. The patients were then treated to the corrected isocenter. Portal films and electronic portal images were obtained for each field. Results: Utilizing CT-CT image registra tion (fusion) of the daily and 50-Gy baseline CT scans? the isocenter changes were quantified to reflect the contribution of positional (sur face marker shifts) error and absolute prostate motion relative to the bony pelvis. The maximum daily A/P shift was 7.3 mm. Motion was less than 5 mm in the remaining patients and the overall mean magnitude cha nge was 2.9 mm. The overall variability was quantified by a pooled sta ndard deviation of 1.7 mm. The maximum lateral shifts were less than 3 mm for all patients. With careful attention to patient positioning, m aximal portal placement error was reduced to 3 mm. Conclusion: In our experience, prostate motion after 50 Gy was significantly less than pr eviously reported. This may reflect early physiologic changes due to r adiation, which restrict prostate motion. This observation is being te sted in a separate study. Intrapatient;and overall population variance was minimal. With daily isocenter correction of setup and organ motio n errors by CT imaging, PTV margins can be significantly reduced or el iminated. We believe this will facilitate further dose escalation in h igh-risk patients with minimal risk of increased morbidity. This techn ique may also be beneficial in low-risk patients by sparing more norma l surrounding tissue. (C) 1998 Elsevier Science Inc.