TU‐D‐BRC‐06: Towards Online Image Guided Radiotherapy for Cervical Cancer: Accurate Cervix‐Uterus Prediction Based On Measured Bladder Volumes

M. Bondar, M. Hoogeman, G. Dhawtal, J. Mens, E. Vasquez Osorio, I. de Pree, S. Quint, R. Ahmad, B. Heijmen

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Purpose: To investigate whether variable bladder filling CT‐scans can be used to predict the cervix‐uterus shape and position based on measured bladder volumes and to determine the number of CT‐scans required for an accurate prediction. Methods and Materials: Two series of CT‐scans were acquired for eleven patients in prone position, the first before EBRT and the second after 40 Gy. Each series consisted of a full bladder CT‐scan and four subsequent CT‐scans with a naturally filling bladder (empty to full). The cervix‐uterus and bladder were manually contoured and 3D cervix‐uterus surfaces were generated. For each patient non‐rigid registration was used to generate corresponding points on all ten surfaces. Patient‐specific models were built by fitting the coordinates of the corresponding points of a variable number of first series surfaces to linear functions of the bladder volume. Each model was used to predict, based on bladder volume the cervix‐uterus surfaces excluded from the model generation. The prediction error was quantified by the margin required around the predicted to accommodate 95% of the observed surface. Results: The maximum cervix‐uterus displacement range was 14–49 mm at planning and 16–72 mm after 40 Gy. The prediction error moderately increased with the decrease of the number of input surfaces (from 5 to 7 mm at planning and from 8 to 9 mm after 40 Gy for 4 to 2 input surfaces). For 9/11 patients the bladder vs. cervix‐uterus relationship was hardly influenced by radiotherapy (error range 6–7 mm). Conclusion: This work demonstrates the potential for accurate cervix‐uterus localization by using a prediction model based on measured bladder volumes. For most patients the prediction error was well below the extent of motion of the cervix‐uterus, even if only two CT‐scans were included in the model. The model could be used to facilitate the adaptation of treatment plans.

Original languageEnglish
Pages (from-to)2735
Number of pages1
JournalMedical Physics
Issue number6
Publication statusPublished - Jun 2009

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