PO-0876: Quantitative evaluation of manual and automatic plan-of-the-day procedures for bladder cancer radiotherapy

ESTRO 33, 2014

S93 algorithm employed by a commercial software(OnQ rts v2.0, OSL, UK) were then applied: (i) Dose_Deform: Directly apply the deformable translation (as required to register the anatomy from CT1 to CT2) to the dose distribution (ii) Dose_Recalc: Deform CT1 anatomy to match CT2. Export the resulting image (dCT1) and structure set to the treatment planning system (TPS) (Monaco, Elekta, Sweden) and recalculate the dose distribution. Validation of these methods against the dose recalculated in the TPS on CT2, here assumed to be our 'Ground_Truth' dose, was done by comparison of dose-volume histograms (DVH) and clinically important parameters, for various anatomical structures and the PTV. Results: Comparing Dose_Deform against the Ground_Truth, significant differences (Wilcoxon test, p≤0.05) were observed in several clinically important parameters, such as maximum dose received by the spinal cord, brainstem and chiasm, mean dose to contralateral parotid, and minimum dose to the PTV. Conversely, Dose_Recalc revealed good agreement with small observed differences and no statistical significance.

3-beam Plan: Bone and air introduced for 10 of 30 fx. Notable differences are highlighted.

Conclusions: A framework for the validation of strategies to calculate delivered dose on daily anatomy using deformable registration is presented. Direct deformation of dose according to observed anatomical deformations can generate significant errors unrelated to the accuracy of the DIR algorithm. The strategy of recalculating dose on the original CT scan, deformed to match the daily anatomy, is found to be more accurate and represents a promising strategy for actual delivered dose estimation and deformable dose accumulation throughout treatment. Conclusions: We found the beam system to have negligible uncertainty. The mechanical testing revealed extensive table rotations (mutli-fields daily) could introduce a mm of uncertainty. Using an incorrect CT#/PSP ratio table would impact SF/day treatments more so than multiple fields. Immobilization was found rigid enough not to impact uncertainties for long treatment times. If 3 fields are used, it is best to pair orthogonal. Our study confirmed that the aggregate of uncertainties favors multiple fields per day. PO-0875 A framework for the validation of actual delivered dose estimation strategies Y.G. Roussakis1, H. Dehghani2, S. Green3, G.J. Webster3 1 University of Birmingham, PSIBS Doctoral Training Centre, Birmingham, United Kingdom 2 University of Birmingham, School of Computer Science, Birmingham, United Kingdom 3 University Hospitals Birmingham NHS Foundation Trust, Hall Edwards Radiotherapy Research Group Radiotherapy Physics, Birmingham, United Kingdom Purpose/Objective: This simulation study uses Deformable Image Registration (DIR) to compare two strategies intended to quantify the actual delivered dose at a radiotherapy treatment fraction. The accuracy of this process is integral to the safe implementation of deformable dose accumulation in Adaptive Radiotherapy (ART). Materials and Methods: Twelve H&N patient datasets, consisting of a DICOM CT dataset, structure set, 6MV IMRT plan and corresponding dose distribution, were used in this study. For each patient, the CT dataset (CT1) and structures were transferred to a commercial simulation software (ImSimQA v3.0.77, OSL, UK), where four clinically-realistic artificial deformations were manually introduced (to create four 'CT2' datasets). These deformations simulated forward and backward neck flexion, weight loss by shrinking the neck region and upward shoulder movement. Two strategies for deformable dose accumulation using the Demon's

PO-0876 Quantitative evaluation of manual and automatic plan-of-the-day procedures for bladder cancer radiotherapy S. Wognum1, L. Bondar2, M.C.C.M. Hulshof1, M.S. Hoogeman2, A. Bel1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands 2 Erasmus Medical Center, Radiation Oncology, Rotterdam, The Netherlands Purpose/Objective: To deal with the large variation in bladder volumes during the course of bladder cancer radiotherapy, we implemented a plan selection procedure. For each fraction the best fitting plan-of-theday was selected from a library of plans generated pretreatment for different bladder volumes. Tumor fiducial markers provided a means to assess the accuracy of our manual selection method as well as to develop a method for automatic plan selection. The aim of this study was to retrospectively evaluate and compare the accuracy of manual and automatic plan selection strategies. Materials and Methods: Daily CBCTs (N=70) of 4 patients treated with a plan selection procedure (40Gy on bladder, boost of 55Gy on Gross Target Volume (GTV)) were retrospectively analyzed. For each patient, a library of contour sets (bladder, GTV, markers) was generated from two pretreatment CT scans with a full (100%) and empty (0%) bladder, using a point-based nonrigid registration technique. The resulting deformation field was scaled (33, 67, 133%) to create additional contours for different bladder volumes. On each treatment day, the bladder contour that best fitted the bladder on CBCT was selected while ensuring good tumor coverage with the boost (Strategy A). A manual adjustment of the bone match was performed to improve bladder and GTV coverage if needed (clinical procedure, Strategy B). In addition, an automated selection procedure was designed (Strategy C), in which the best plan was selected based on the shortest marker residual distance error (RDE) between segmented markers on CBCT and the library markers. Marker RDE was also calculated for strategies A and B. GTV coverage was assessed by quantifying the volume overlap between the selected boost-

S94 PTV and the markers on CBCT, relative to the ideal volume overlap of the PTV100% with the markers on the full bladder CT (RelV). RelV ≥ 1 was optimal. Plan selection accuracy (RDE) and RelV were compared between strategies with the related-samples Wilcoxon Signed Rank test. Results: With clinical planselection (B), manual adjustment of the match was performed in 32/70 cases,resulting in a significantly better accuracy as compared to A (RDEA =3.3 ± 1.5 mm (±1SD), RDEB = 2.9 ± 1.5 mm, p<.0001). Automatic selection (C) significantly improved the accuracy as compared to B(RDEC = 2.6 ± 1.3 mm, p=.007) (Figure: results per patient). Plan selection was different between Strategy B and C for 43/70 cases. For all strategies, RelV was not different from the optimal value of 1 for 3/4 patients.

ESTRO 33, 2014 -1.20-1.85 cm and marker pairs with larger initial distances were prone to larger variations (2 examples, Fig.1). Given no time trend, these distance variations were likely caused by target deformation rather than marker migration. The interfractional displacements of each marker (in total 14 markers) relative to the bony anatomy were large, especially in the superior-inferior direction (-1.37-1.41 cm, Fig.1), signifying the importance of using the target volume for setup verification. However, due to the large target deformation and limited number of markers, marker-based registrations were not feasible: all automatic registrations failed and manual registrations were challenging. Conclusions: The preliminary results imply that gold markers ≥ 5 mm in length are required for visibility on CBCT. Further, because markerbased registrations were not feasible, our preliminary conclusion is that for esophageal tumors, markers can be used to visually check whether the actual target will be covered adequately during dose delivery after bony anatomy registration.

Conclusions: Quantitative evaluation of our plan-of-the-day procedure revealed good accuracy based on tumor markers and showed that a manual adjustment of the match was needed (Strategy B). In addition, we developed an automatic plan selection procedure based on tumor marker distance with improved accuracy (Strategy C). Tumor coverage, as assessed via PTV-marker overlap was good for both strategy B and C. RDE-based plan selection will reduce operator-dependency of plan selection in daily clinical practice. PO-0877 First experience with the use of gold markers for image guidance in esophageal cancer radiotherapy P. Jin1, M.C.C.M. Hulshof1, M.A.J. De Jong1, J.E. Van Hooft2, A. Van der Horst1, N. Van Wieringen1, M. Machiels1, A. Bel1, T. Alderliesten1 1 Academic Medical Center, Radiation Oncology, Amsterdam, The Netherlands 2 Academic Medical Center, Gastroenterology & Hepatology, Amsterdam, The Netherlands Purpose/Objective: In image-guided radiotherapy of esophageal cancer, bony anatomy registration rather than esophageal tumor registration is currently used for setup verification. For a variety of treatment sites, fiducial markers have been successfully applied to provide guidance during setup verification. In this study, we present preliminary results on the use of gold markers in esophageal cancer radiotherapy. We investigated the 1) marker visibility on CT and cone beam CT (CBCT), 2) potential marker migration, 3) interfractional position variation of the markers relative to bony anatomy, and 4) feasibility of using the markers for registration in setup verification. Materials and Methods: Eight esophageal cancer patients underwent endoscopic ultrasound (EUS) guided implantation of 3-5 gold markers. For each patient, a planning CT and 7-8 CBCTs (extended No Action Level correction protocol: CBCTs in the first 4 successive days + weekly followups) were obtained in 23 or 28 fractions (Table 1). Retrospectively, the markers were identified and their positions on (CB)CTs were obtained to calculate the pairwise distances of markers over time for examining marker migration. In addition, bony anatomy registrations were performed for CBCT to CT to calculate the interfractional displacement of each marker relative to bony anatomy. Further, marker-based registrations (considering all markers) were performed automatically or manually (if the automatic registration failed). Results: The numbers of markers implanted and visible on CT and CBCT are summarized in Table 1. We observed marker detachment for 6 markers (4 patients). 2 patients were excluded for further analysis since no markers could be identified on CBCTs due to the combined effect of internal motion and the small size of the markers (≤ 4 mm). The distance variations of marker pairs (in total 15 pairs) fluctuated within a range of

PO-0878 MRI-based adaptive treatment planning for stereotactic radiotherapy of spinal bone metastases S.J. Hoogcarspel1, J.M. Van der Velden1, M.E. Philippens1, E. Seravalli1, M. Van Vulpen1, J.J.W. Lagendijk1, B.W. Raaymakers1 1 UMC Utrecht, Radiotherapy, Utrecht, The Netherlands Purpose/Objective: The MR-linac, a novel treatment modality, has the unique ability to visualize the tumor and the surrounding organs before, or during treatment using MR images with a superior soft-tissue contrast. This novel radiotherapy treatment modality is therefore ideal for on-line MRI based adaptive radiotherapy. Potentially, a major issue with MRI based treatment plan adaptation is the lack of electron density information needed for dose calculations since there is no direct relationship between MR signal and electron density. To overcome this limitation, a pseudo-CT (pCT) image can be derived from MRI by assuming electron density values for segmented structures in the MR image. Additionally, pre-treatment CT information can be registered in the pCT for online treatment plan adaption if a structure is rigid by nature such as a vertebral body. The purpose of this study was to develop an accurate on-line MRI-based plan adaptation strategy for the stereotactic irradiation of spinal bone metastases. Materials and Methods: The CT data of 20 patients who were treated for spinal bone metastasis at our department were included. For each patient, a total number of six increasingly complex pCT's were created