Intrafraction prostate motion during IMRT for prostate cancer

The purpose of this work is to prospectively assess the setup accuracy that can be achieved with a stereotactic body localizer (SBL) in immobilizing patients for stereotactic intensity‐modulated radiotherapy (IMRT) for prostate cancer. By quantifying this important factor and target mobility in the SBL, we expect to provide a guideline for selecting planning target volume margins for stereotactic treatment planning. We analyzed data from 40 computed tomography (CT) studies (with slice thickness of 3 mm) involving 10 patients with prostate cancer. Each patient had four sets of CT scans during the course of radiotherapy. For the purpose of this study, all four sets of CT scans were obtained with the patients immobilized in a customized body pillow formed by vacuum suction. Unlike other immobilization devices, this system consists not only of a customized body pillow, but also of a fixation sheet used to suppress patient respiratory motion, a stereotactic body frame to provide stereotaxy, and a carbon fiber base board to which both the body cushion and the frame are affixed. We identified four bony landmarks and measured their coordinates in the stereotactic body frame on each set of CT scans. The displacements of the bony landmarks from their corresponding positions on the simulation scan (first CT scan) were analyzed in three dimensions in terms of overall, systematic, and random categories. The initial planned isocenter was also marked on the patients' skin with fiducials for each CT study. The distance from each bony landmark to the fiducial‐based isocenter was measured and compared among the four sets of CT scans. The deviations in distances were also compared to those measured from the landmarks to the stereotactic frame center, in order to determine the effectiveness of the rigid body frame in positioning patients with prostate cancer. Target inter‐fraction motion in this system was also studied for five patients by measuring the deviations in distances from the target geometric center to the bony landmarks. Our results showed that the overall setup accuracy had standard deviations (SDs) of 2.58 mm, 2.41 mm, and 3.51 mm in lateral (LAT), anterior‐posterior (AP), and superior‐inferior (SI) directions, respectively. The random component had SDs of 1.72 mm, 2.06 mm, and 2.79 mm, and the systematic component showed SDs of 0.92 mm, –0.27 mm, and 0.90 mm in these three directions. In terms of three‐dimensional vector, the mean displacement over 116 measurements was 3.0 mm with an SD of 1.29 mm. Compared to the rigid reference, the skin‐mark‐based reference was less reliable for patient repositioning in terms of reproducing known bony landmark positions. The mean target mobility relative to the bony landmarks was 2.22±3.45 mm,0.17±1.11 mm, and 0.11±2.69 mm in the AP, LAT, and SI directions, respectively. In conclusion, the body immobilization system has the ability to immobilize prostate cancer patients with satisfactory setup accuracy for fractionated extracranial stereotactic radiotherapy. A rigid frame system ser...

Section: Resultssupporting

confidence: 87%

Stereotactic IMRT for prostate cancer: Setup accuracy of a new stereotactic body localization system

Wang

Jacob

Chen

et al. 2004

“…The average interfraction motions in the lateral, longitudinal, and AP directions were 3.0 mm, 3.2 mm, and 5.1 mm, respectively; these results are similar to the values in the treatment of definitive patients. ( 5 , 22 , 23 ) The CT scans were then used to evaluate the accuracy of BAT ultrasound localization in the postoperative setting. With the lack of a clearly defined target, such as the prostate, systematic errors in BAT ultrasound alignment were found.…”

Section: Discussionmentioning

confidence: 99%

Target localization for post‐prostatectomy patients using CT and ultrasound image guidance

Paskalev

Feigenberg

Jacob

et al. 2005

Self Cite

We conducted a study comparing B‐mode acquisition and targeting (BAT) ultrasound alignments based on CT data in the postoperative setting. CT scans were obtained with a Primatom CT‐on‐rails on nine patients. Two CT scans were obtained each week, while setup error was minimized by BAT ultrasounds. For the first three patients, a direct comparison was performed. For the next six patients, a template based on the shifts from the week 1 CT during treatment was used for subsequent setup. Comparison of isocenter shifts between the BAT ultrasound and CT was made by the difference, absolute difference, and improvement (using CT alignments as the reference technique). A total of 90 image comparisons were made. The average interfraction motion was 3.2 mm in the lateral, 3.0 mm in the longitudinal, and 5.1 mm in the AP direction. The results suggest that the CT‐based ultrasound templates can improve the localization of the prostate bed when the initial displacements are greater than 4 mm. For initial displacements smaller than 4 mm, the technique neither improved nor worsened target localization. However, ultrasound alignments performed without the use of a template deteriorated patient positioning for two out of three patients, demonstrating that the use of a CT template was beneficial even at small initial displacements.PACS numbers: 87.53.‐j, 87.53.Kn, 87.53.Xd

“…However, if a difference of a few minutes between scans contributed, at least in part, to our findings, one may wonder what the impact bladder filling may have during a radiotherapy fraction that lasts several minutes. Several recent reports have attempted to describe prostate motion during a radiotherapy fraction, 19 ‐ 22 however, the changes of bladder volume and/or filling during the same interval are not well known. Although changes in bladder volume may have only minimal impact on the position of the prostate as suggested by some, 17 this change, if it occurs in a significant manner, may impact the actual bladder dose and volume relationships which in turn could impact on toxicity.…”

Section: Discussionmentioning

confidence: 99%

“…, 6 ‐ 12 The imaging modality selected (CT, US, or MRI), may also influence the measurement of certain parameters such as organ volume 13 14 Finally and not the least, prostate motion itself, either during the whole course of treatment, 5 , 15 ‐ 18 or within a single fraction, 19 ‐ 22 may lead to variations in isocenter position and hence field placement or margin determination.…”

Section: Introductionmentioning

confidence: 99%

CT slice index and thickness: Impact on organ contouring in radiation treatment planning for prostate cancer

Berthelet¹,

Liu²,

Truong³

et al. 2003

Objective: To assess the impact of CT slice index and thickness (3 mm versus 5 mm) on (i) prostate volume, dimensions, and isocenter coordinates, (ii) bladder and rectal volumes, and (iii) DRR quality, in the treatment of prostate cancer. Methods: 16 patients with prostate cancer underwent two planning CT‐scans using 3 and 5 mm slice index/thickness. Prostate, bladder, and rectum were outlined on all scans. Prostate isocenter coordinates, maximum dimensions, and volumes were compared along with bladder and rectal volumes. Bladder volumes and maximum diameters were further investigated using a second observer. A comparative analysis of DRR quality was conducted as well as a dosimetric analysis using DVH. Results: The differences in measurements of prostate volume, isocenter coordinates and maximum dimensions between the 3 and 5 mm scans, were small and not statistically significant. Similar finding was seen for rectal volume. However, bladder volume was always larger on the 3 mm scan (mean difference=27.9 cc; SE=4.8 cc; 95% CI: 17.7−38.2 cc; p<0.001) and the findings were reproduced with the second observer (mean difference=31.9 cc; SE=4.7 cc; 95% CI: 21.9−41.9 cc; p<0.001). The differences in volume are caused by a slight increase in (1) the measurement of the longitudinal dimensions on the 3 mm scans, and (2) the slice by slice measured bladder area on the 3 mm scans. The latter is due to partial volume effect. The 3 mm DRR were slightly better than the 5 mm DRR. The bladder DVH differed significantly in some patients. Conclusion: Bladder volume is significantly larger on the 3 mm scans. Differences in contoured areas may be accounted for, in part, by the partial volume effect.PACS number(s): 87.57.–s, 87.53.–j