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Set-up verification using portal images of implanted prostate markers: an inter-observer study

2002; Elsevier BV; Volume: 54; Issue: 2 Linguagem: Inglês

10.1016/s0360-3016(02)03522-8

1879-355X

Michèle Aubin, Yifan Liu, K Langen, Katsuto Shinohara, C Anezino, M.I Osofsky, Vivian Weinberg, Jean Pouliot, Mack Roach,

Radiation Dose and Imaging

Purpose/Objective: Radioopaque markers that are implanted in the prostate are visible on portal images. Their expected location can be extracted from the radiographs that are digitally reconstructed from the treatment planning CT. Hardcopies of these latter images are available. Comparing the seed locations on the portal images with their expected location allows the determination of patient movement to realign the prostate in the radiation field. On a daily basis the radiation therapists need to decide in real time how to move the patient based on the two sets of images. Often this decision needs to be made quickly and in a stressful environment. The purpose of this study is to compare the actual patient moves performed by the therapists with those determined by two radiation oncologists in a retrospective analysis of the portal images. The true patient movement was calculated using image analysis software that allowed a precise measurement of the distance between the desired and actual seed location. These measurements were used as our reference values.Materials/Methods: Lateral portal images were acquired using an a-Si flat panel electronic portal imaging device (EPID). In the planning software, the seeds were contoured and hardcopies of the digitally reconstructed images were used for comparison with the portal images. The therapists were not given specific guidelines regarding the magnitude of the differences for which a move should be made. The moves performed by the therapists were recorded each day. Two radiation oncologists retrospectively and independently compared the portal images with the hardcopies of the reconstructed images. This process was done off-line and without any time pressure. Finally a careful measurement of the seed displacement was made for reference purposes by digitally overlaying the image sets. There are image sets available for 472 cases. To date 167 reviews of those cases from 18 patients have been completed.Results: The individual responses between every pair of evaluators were highly correlated (p < 0.004 for each comparison) for both, the ant/post and sup/inf direction.Compared with the measured reference ant/post seed movement, the mean of the absolute difference between the reference and the user determined values for the two radiation oncologists and the therapists were 2.4, 1.4, and 2.5 mm, respectively. In the sup/inf direction the mean of the absolute differences from the reference compared with the user determined values were 2.2 and 1.4 mm for the two radiation oncologists and 2.4 mm for the therapists. Even though, there is a statistically significant difference (p < 0.001) among the reviewers, this is solely due to one radiation oncologist's readings.The percent of cases requiring moves ≥ 3 mm according to the reference measurements were 54 and 47 % in the ant/post and sup/inf direction. Subtracting the moves determined by the radiation oncologists and therapists from these reference values allows us to calculate the remaining frequency of positioning errors ≥ 3 mm. These remaining frequencies were 23, 10, and 34% according to the assessment of the two radiation oncologists and therapists in the ant/post direction and 23, 11, and 27% respectively in the sup/inf direction. New positioning displacements of ≥ 3 mm were introduced by the radiation oncologists and therapists in 6, 1, and 2% of the case in the ant/post direction. In the sup/inf direction the respective values were 5, 1, and 2%.Conclusions: Implanted radioopaque markers can be used reliably for daily prostate repositioning by therapists. An off-line analysis of the portal images by two radiation oncologists does not necessarily improve upon the therapists' patient alignment. An on-line display of the expected seed location on the computer screen may be a useful tool to improve positioning accuracy. Purpose/Objective: Radioopaque markers that are implanted in the prostate are visible on portal images. Their expected location can be extracted from the radiographs that are digitally reconstructed from the treatment planning CT. Hardcopies of these latter images are available. Comparing the seed locations on the portal images with their expected location allows the determination of patient movement to realign the prostate in the radiation field. On a daily basis the radiation therapists need to decide in real time how to move the patient based on the two sets of images. Often this decision needs to be made quickly and in a stressful environment. The purpose of this study is to compare the actual patient moves performed by the therapists with those determined by two radiation oncologists in a retrospective analysis of the portal images. The true patient movement was calculated using image analysis software that allowed a precise measurement of the distance between the desired and actual seed location. These measurements were used as our reference values. Materials/Methods: Lateral portal images were acquired using an a-Si flat panel electronic portal imaging device (EPID). In the planning software, the seeds were contoured and hardcopies of the digitally reconstructed images were used for comparison with the portal images. The therapists were not given specific guidelines regarding the magnitude of the differences for which a move should be made. The moves performed by the therapists were recorded each day. Two radiation oncologists retrospectively and independently compared the portal images with the hardcopies of the reconstructed images. This process was done off-line and without any time pressure. Finally a careful measurement of the seed displacement was made for reference purposes by digitally overlaying the image sets. There are image sets available for 472 cases. To date 167 reviews of those cases from 18 patients have been completed. Results: The individual responses between every pair of evaluators were highly correlated (p < 0.004 for each comparison) for both, the ant/post and sup/inf direction. Compared with the measured reference ant/post seed movement, the mean of the absolute difference between the reference and the user determined values for the two radiation oncologists and the therapists were 2.4, 1.4, and 2.5 mm, respectively. In the sup/inf direction the mean of the absolute differences from the reference compared with the user determined values were 2.2 and 1.4 mm for the two radiation oncologists and 2.4 mm for the therapists. Even though, there is a statistically significant difference (p < 0.001) among the reviewers, this is solely due to one radiation oncologist's readings. The percent of cases requiring moves ≥ 3 mm according to the reference measurements were 54 and 47 % in the ant/post and sup/inf direction. Subtracting the moves determined by the radiation oncologists and therapists from these reference values allows us to calculate the remaining frequency of positioning errors ≥ 3 mm. These remaining frequencies were 23, 10, and 34% according to the assessment of the two radiation oncologists and therapists in the ant/post direction and 23, 11, and 27% respectively in the sup/inf direction. New positioning displacements of ≥ 3 mm were introduced by the radiation oncologists and therapists in 6, 1, and 2% of the case in the ant/post direction. In the sup/inf direction the respective values were 5, 1, and 2%. Conclusions: Implanted radioopaque markers can be used reliably for daily prostate repositioning by therapists. An off-line analysis of the portal images by two radiation oncologists does not necessarily improve upon the therapists' patient alignment. An on-line display of the expected seed location on the computer screen may be a useful tool to improve positioning accuracy.