G. Dhawtal

Individualized nonadaptive and online-adaptive intensity-modulated radiotherapy treatment strategies for cervical cancer patients based on pretreatment acquired variable bladder filling computed tomography scans

PURPOSE To design and evaluate individualized nonadaptive and online-adaptive strategies based on a pretreatment established motion model for the highly deformable target volume in cervical cancer patients. METHODS AND MATERIALS For 14 patients, nine to ten variable bladder filling computed tomography (CT) scans were acquired at pretreatment and after 40 Gy. Individualized model-based internal target volumes (mbITVs) accounting for the cervix and uterus motion due to bladder volume changes were generated by using a motion-model constructed from two pretreatment CT scans (full and empty bladder). Two individualized strategies were designed: a nonadaptive strategy, using an mbITV accounting for the full-range of bladder volume changes throughout the treatment; and an online-adaptive strategy, using mbITVs of bladder volume subranges to construct a library of plans. The latter adapts the treatment online by selecting the plan-of-the-day from the library based on the measured bladder volume. The individualized strategies were evaluated by the seven to eight CT scans not used for mbITVs construction, and compared with a population-based approach. Geometric uniform margins around planning cervix-uterus and mbITVs were determined to ensure adequate coverage. For each strategy, the percentage of the cervix-uterus, bladder, and rectum volumes inside the planning target volume (PTV), and the clinical target volume (CTV)-to-PTV volume (volume difference between PTV and CTV) were calculated. RESULTS The margin for the population-based approach was 38 mm and for the individualized strategies was 7 to 10 mm. Compared with the population-based approach, the individualized nonadaptive strategy decreased the CTV-to-PTV volume by 48% ± 6% and the percentage of bladder and rectum inside the PTV by 5% to 45% and 26% to 74% (p < 0.001), respectively. Replacing the individualized nonadaptive strategy by an online-adaptive, two-plan library further decreased the percentage of bladder and rectum inside the PTV (0% to 10% and -1% to 9%; p < 0.004) and the CTV-to-PTV volume (4-96 ml). CONCLUSIONS Compared with population-based margins, an individualized PTV results in better organ-at-risk sparing. Online-adaptive radiotherapy further improves organ-at-risk sparing.

Toward an individualized target motion management for IMRT of cervical cancer based on model-predicted cervix–uterus shape and position

BACKGROUND AND PURPOSE To design and evaluate a 3D patient-specific model to predict the cervix-uterus shape and position. METHODS AND MATERIALS For 13 patients lying in prone position, 10 variable bladder filling CT-scans were acquired, 5 at planning and 5 after 40Gy. The delineated cervix-uterus volumes in 2-5 pre-treatment CT-scans were used to generate patient-specific models that predict the cervix-uterus geometry by bladder volume. Model predictions were compared to delineations, excluding those used for model construction. The prediction error was quantified by the margin required around the predicted volumes to accommodate 95% of the delineated volume and by the predicted-to-delineated surface distance. RESULTS The prediction margin was significantly smaller (average 50%) than the margin encompassing the cervix-uterus motion. The prediction margin could be decreased (from 7 to 5mm at planning and from 10 to 8mm after 40Gy) by increasing (from 2 to 5) the number of CT-scans used for the model construction. CONCLUSION For most patients, even with a model based on only two CT-scans, the prediction error was well below the margin encompassing the cervix-uterus motion. The described approach could be used to create prior to treatment, an individualized treatment strategy.

A margin-of-the-day online adaptive intensity-modulated radiotherapy strategy for cervical cancer provides superior treatment accuracy compared to clinically recommended margins: A dosimetric evaluation

Abstract Purpose. To dosimetrically evaluate a margin-of-the-day (MoD) online adaptive intensity-modulated radiotherapy (IMRT) strategy for cervical cancer patients. The strategy is based on a single planning computed tomography (CT) scan and a pretreatment constructed IMRT plan library with incremental clinical target volumes (CTV)-to-planning target volumes (PTV) margins. Material and methods. For 14 patients, 9–10 variable bladder filling CT scans acquired at pretreatment and after 40 Gy were available. Bladder volume variability during the treatment course was recorded by twice-weekly US bladder-volume measurements. A MoD strategy that selects the best IMRT plan of the day from a library of plans with incremental margins in steps of 5 mm was compared with a clinically recommended population-based margin (15 mm). To compare the strategies, for each fraction that had a recorded US bladder-volume measurement, the CT scan with the nearest bladder volume was selected from the pretreatment CT series and from the CT series acquired after 40 Gy. A frequency-weighted average of the dose-volume histograms (DVH) parameters calculated for the two selected CT scans was used to estimate the DVH parameters of the fraction of interest. Results. The 15-mm recommended margin resulted in cervix-uterus underdosage in six of 14 patients. Compared with the 15-mm margin, the MoD strategy resulted in significantly better cervix-uterus coverage (p = 0.008) without a significant difference in the sparing of rectum, bladder, and small bowel. For each patient, 3–8 (median 5) plans were needed in the library of plans for the MoD strategy. The required range of the MoD was 5–45 mm (median 15 mm). Twenty-five percent of all fractions could be treated with a MoD of 5 mm and 81% of all fractions could be treated with a MoD up to 25 mm. Conclusions. Compared with a clinically recommended margin, a simple online adaptive strategy resulted in better cervix-uterus coverage without compromising organs at risk sparing.

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

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.

TH‐D‐213A‐07: A Novel Inverse‐Consistent Feature‐Based Non‐Rigid Registration Method That Improves the Mapping of Organs with Large‐Scale Deformations

Purpose: Unidirectional feature‐based registration methods can result in inconsistent correspondence between the forward and the backward transformation and in incoherent anatomical mapping. The aim of this work is to develop, test and validate an inverse‐consistent feature‐based non‐rigid registration method in order to improve the registration of organs that exhibit large deformations. Methods and Materials: Thin Plate Splines Robust Point Matching (TPS‐RPM) is a unidirectional algorithm that iteratively calculates the correspondence and the transformation between two point sets (e.g., anatomical structures,organs). An inverse‐consistent version of TPS‐RMP (IC‐TPS‐RPM) was developed that jointly estimates the forward and the backward transformations and that uses both transformations to determine the correspondence. IC‐TPS‐RPM was compared with TPS‐RPM by registering organs with large deformations in five patients. For each patient the contoured cervix‐uteri and bladders on a series of three variable bladder filling CT‐scans (empty to full) were employed. The mean ratio between the volume of full bladder and the volume of empty bladder was 5.7. The registration accuracy error, the inverse‐consistency error, the residual distances after transforming anatomical landmarks and the registration time were calculated using both algorithms. Results: The registrations performed with IC‐TPS‐RPM have on average 10% and 70% better accuracy and inverse‐consistency, respectively when compared with the non‐symmetric TPS‐RPM. By using IC‐TPR‐RPM the residual distances after transforming anatomical landmarks for the registration of full to empty bladder reduced by 47% and by 11% for all landmarks. Moreover, the registration time for computing the forward and the backward transformations decreased by 29%. Conclusions: Compared with TPS‐RPM the new IC‐TPS‐RPM method improves the registration accuracy, the inverse‐consistency and the anatomical correspondence. For cases with large deformations accurate transformations were obtained with IC‐TPS‐RPM, while TPS‐RPM failed. Furthermore, IC‐TPS‐RPM requires less time to compute the forward and the backward transformations.