Changes in the Pelvic Anatomy After an IMRT Treatment Fraction of Prostate Cancer

Purpose: To quantify the three-dimensional variations of pelvic anatomy after a single treatment fraction. Methods and Materials: Forty-six prostate cancer patients underwent computed tomography (CT) scanning with an in-room CT-on-rail system, before and immediately after one intensity-modulated radiotherapy (IMRT) session. To study the soft-tissue anatomy changes, the pre- and post-treatment CT images were registered using the bony structure with an in-house image registration software system. The center of volume for both the prostate and seminal vesicles was used to assess the relative displacement of the same structure after the treatment fraction. Results: During one treatment fraction (21 ± 4 min), both the prostate and seminal vesicles showed statistically significant systematic trends in the superior and anterior directions of the patient's anatomy. The net increase in bladder volume was huge (127 ± 79 cm³), yet this change did not translate into large target displacements. Although the population mean displacements in either direction were 1.3 ± 2.9 mm for the prostate and 1.2 ± 4.1 mm for the seminal vesicles in the anterior direction, a few patients had displacements as large as 8.4 mm and 15.6 mm, respectively. These large displacements correlated strongly (p < 0.001) with large rectal volume increases caused by gaseous build-up in the rectum. Conclusion: The observed intrafraction variations in anatomy during prostate IMRT sessions suggest that, for any given fraction, the organ motion and volume changes can potentially lead to compromised target coverage in about 15% of patients in whom the prostate position shifted >4 mm.