A phantom study of the immobilization and the indications for using virtual isocenter in stereoscopic X-ray image guidance system referring to position localizer in frameless radiosurgery.

A frameless radiosurgery system is using a set of thermoplastic mask for fixation and stereoscopic X-ray imaging for alignment. The accuracy depends on mask fixation and imaging. Under certain circumstances, the guidance images may contain insufficient bony structures, resulting in lesser accuracy. A virtual isocenter function is designed for such scenarios. In this study, we investigated the immobilization and the indications for using virtual isocenter. Twenty-four arbitrary imaginary treatment targets (ITTs) in phantom were evaluated. The external Localizer with positioner films was used as reference. The alignments by using actual and virtual isocenter in image guidance were compared. The deviation of the alignment after mask removing and then resetting was also checked. The results illustrated that the mean deviation between the alignment by image guidance using actual isocenter (Iso(img)) and the localizer(Iso(loc)) was 2.26 mm ± 1.16 mm (standard deviation, SD), 1.66 mm ± 0.83 mm for using virtual isocenter. The deviation of the alignment by the image guidance using actual isocenter to the localizer before and after mask resetting was 7.02 mm ± 5.8 mm. The deviations before and after mask resetting were insignificant for the target center from skull edge larger than 80 mm on craniocaudal direction. The deviations between the alignment using actual and virtual isocenter in image guidance were not significant if the minimum distance from target center to skull edge was larger or equal to 30 mm. Due to an unacceptable deviation after mask resetting, the image guidance is necessary to improve the accuracy of frameless immobilization. A treatment isocenter less than 30 mm from the skull bone should be an indication for using virtual isocenter to align in image guidance. The virtual isocenter should be set as caudally as possible, and the sella of skull should be the ideal point.

The different volume effects of small-bowel toxicity during pelvic irradiation between gynecologic patients with and without abdominal surgery: a prospective study with computed tomography-based dosimetry.

PURPOSE: To evaluate the effect of abdominal surgery on the volume effects of small-bowel toxicity during whole-pelvic irradiation in patients with gynecologic malignancies. METHODS AND MATERIALS: From May 2003 through November 2006, 80 gynecologic patients without (Group I) or with (Group II) prior abdominal surgery were analyzed. We used a computed tomography (CT) planning system to measure the small-bowel volume and dosimetry. We acquired the range of small-bowel volume in 10% (V10) to 100% (V100) of dose, at 10% intervals. The onset and grade of diarrhea during whole-pelvic irradiation were recorded as small-bowel toxicity up to 39.6 Gy in 22 fractions. RESULTS: The volume effect of Grade 2-3 diarrhea existed from V10 to V100 in Group I patients and from V60 to V100 in Group II patients on univariate analyses. The V40 of Group I and the V100 of Group II achieved most statistical significance. The mean V40 was 281 +/- 27 cm(3) and 489 +/- 34 cm(3) (p < 0.001) in Group I patients with Grade 0-1 and Grade 2-3 diarrhea, respectively. The corresponding mean V100 of Group II patients was 56 +/- 14 cm(3) and 132 +/- 19 cm(3) (p = 0.003). Multivariate analyses revealed that V40 (p = 0.001) and V100 (p = 0.027) were independent factors for the development of Grade 2-3 diarrhea in Groups I and II, respectively. CONCLUSIONS: Gynecologic patients without and with abdominal surgery have different volume effects on small-bowel toxicity during whole-pelvic irradiation. Low-dose volume can be used as a predictive index of Grade 2 or greater diarrhea in patients without abdominal surgery. Full-dose volume is more important than low-dose volume for Grade 2 or greater diarrhea in patients with abdominal surgery.