Respiratory-induced prostate motion: Quantification and characterization

Purpose: The precise localization of the prostate is critical for dose esca lated conformal radiotherapy. This study identifies and characterizes a pot ential cause of inaccurate prostatic localization-respiratory-induced movem ent. Methods and Materials: Prostate movement during respiration was measured fl uoroscopically using implanted gold fiducial markers. Twenty sequential pat ients with CT1-T-3 N-0 M-0 prostate carcinoma were evaluated prone, immobil ized in customized thermoplastic shells. A second 20 patients were evaluate d both prone (with and without their thermoplastic shells) and supine (with out their shells). Results: When the patients were immobilized prone in thermoplastic shells, the prostate moved synchronously with respiration. In the study the prostat e was displaced a mean distance of 3.3 +/- 1.8 (SD) mm (range, 1-10.2 mm), with 23% (9/40) of the displacements being 4 mm or greater. The respiratory -associated prostate movement decreased significantly when the thermoplasti c shells were removed. Conclusion: Significant prostate movement can be induced by respiration whe n patients are immobilized in thermoplastic shells. This movement presumabl y is related to transmitted intraabdominal pressure within the confined spa ce of the shells. Careful attention to the details of immobilization and to the possibility of respiratory-induced prostate movements is important whe n employing small field margins in prostatic radiotherapy. (C) 2000 Elsevie r Science Inc.