TO EVALUATE THE SETUP DISCREPANCY OF CARCINOMA OF PROSTATE PATIENTS AND CALCULATE THE PLANNING TARGET VOLUME (PTV) MARGIN BY USING DIFFERENT IMAGING MODALITIES

Abstract

Introduction: Radical prostatectomy remains a primary treatment option for men with clinically localized prostate cancer. However, radiotherapy has demonstrated comparable outcomes in disease control. The prostate is a mobile organ located anterior to the rectum and inferior to the bladder. Therefore, managing both inter-fraction and intra-fraction prostate motion during radiotherapy is crucial for achieving optimal treatment outcomes. In addition, variations in bladder and rectal volumes can influence dose delivery and may increase the risk of treatment-related toxicity.

Materials and Methods:  A total of 25 patients with prostate cancer were enrolled in this study. Data were collected prospectively using the cone-beam computed tomography (CBCT), megavoltage (MV), and kilovoltage (kV) imaging systems integrated with the Varian TrueBeam linear accelerator. In total, 250 CBCT images and 500 MV and kV portal images were acquired. For each patient, kV-CBCT, planar kV, and MV portal images were obtained in accordance with the institutional imaging protocol. Data were entered and analyzed using SPSS software (Version 21). Setup margins (SM) were calculated according to the van Herk formula

Results: The systematic setup errors measured using CBCT were 0.165 cm, 0.140 cm, and 0.150 cm in the dorsoventral (DV), mediolateral (ML), and craniocaudal (CC) directions, respectively. These values were lower than those obtained using kV and MV portal imaging. Similarly, CBCT demonstrated the lowest random setup errors, with values of 0.23 cm (DV), 0.17 cm (ML), and 0.22 cm (CC). Random setup errors measured using kV imaging were 0.39 cm, 0.27 cm, and 0.32 cm in the DV, ML, and CC directions, respectively. The highest random setup errors were observed with MV portal imaging, measuring 0.37 cm (DV), 0.30 cm (ML), and 0.35 cm (CC). Consequently, the smallest setup margins were achieved with CBCT-based image guidance, with calculated margins of 0.53 cm, 0.44 cm, and 0.49 cm in the DV, ML, and CC directions, respectively.

Conclusion: CBCT is a valuable imaging modality for patient setup verification prior to the delivery of radiotherapy fractions. Compared with kV and MV imaging, CBCT resulted in smaller setup margins in all three directions. This reduction in treatment margins can help minimize radiation exposure to surrounding organs at risk, particularly the bladder and rectum, while maintaining adequate target coverage. The findings suggest that institutions without access to CBCT and relying on MV portal imaging for setup verification should establish and apply site-specific MV-based setup margins to ensure accurate treatment delivery.