S. Paloor

Generation of synthetic CT using multi-scale and dual-contrast patches for brain MRI-only external beam radiotherapy

PURPOSE To create a synthetic CT (sCT) from conventional brain MRI using a patch-based method for MRI-only radiotherapy planning and verification. METHODS Conventional T1 and T2-weighted MRI and CT datasets from 13 patients who underwent brain radiotherapy were included in a retrospective study whereas 6 patients were tested prospectively. A new contribution to the Non-local Means Patch-Based Method (NMPBM) framework was done with the use of novel multi-scale and dual-contrast patches. Furthermore, the training dataset was improved by pre-selecting the closest database patients to the target patient for computation time/accuracy balance. sCT and derived DRRs were assessed visually and quantitatively. VMAT planning was performed on CT and sCT for hypothetical PTVs in homogeneous and heterogeneous regions. Dosimetric analysis was done by comparing Dose Volume Histogram (DVH) parameters of PTVs and organs at risk (OARs). Positional accuracy of MRI-only image-guided radiation therapy based on CBCT or kV images was evaluated. RESULTS The retrospective (respectively prospective) evaluation of the proposed Multi-scale and Dual-contrast Patch-Based Method (MDPBM) gave a mean absolute error MAE=99.69±11.07HU (98.95±8.35HU), and a Dice in bones DIbone=83±0.03 (0.82±0.03). Good agreement with conventional planning techniques was obtained; the highest percentage of DVH metric deviations was 0.43% (0.53%) for PTVs and 0.59% (0.75%) for OARs. The accuracy of sCT/CBCT or DRRsCT/kV images registration parameters was <2mm and <2°. Improvements with MDPBM, compared to NMPBM, were significant. CONCLUSION We presented a novel method for sCT generation from T1 and T2-weighted MRI potentially suitable for MRI-only external beam radiotherapy in brain sites.

MRI reduces variation of contouring for boost clinical target volume in breast cancer patients without surgical clips in the tumour bed

Abstract Background Omitting the placement of clips inside tumour bed during breast cancer surgery poses a challenge for delineation of lumpectomy cavity clinical target volume (CTVLC). We aimed to quantify inter-observer variation and accuracy for CT- and MRI-based segmentation of CTVLC in patients without clips. Patients and methods CT- and MRI-simulator images of 12 breast cancer patients, treated by breast conserving surgery and radiotherapy, were included in this study. Five radiation oncologists recorded the cavity visualization score (CVS) and delineated CTVLC on both modalities. Expert-consensus (EC) contours were delineated by a senior radiation oncologist, respecting opinions of all observers. Inter-observer volumetric variation and generalized conformity index (CIgen) were calculated. Deviations from EC contour were quantified by the accuracy index (AI) and inter-delineation distances (IDD). Results Mean CVS was 3.88 +/− 0.99 and 3.05 +/− 1.07 for MRI and CT, respectively (p = 0.001). Mean volumes of CTVLC were similar: 154 +/− 26 cm3 on CT and 152 +/− 19 cm3 on MRI. Mean CIgen and AI were superior for MRI when compared with CT (CIgen: 0.74 +/− 0.07 vs. 0.67 +/− 0.12, p = 0.007; AI: 0.81 +/− 0.04 vs. 0.76 +/− 0.07; p = 0.004). CIgen and AI increased with increasing CVS. Mean IDD was 3 mm +/− 1.5 mm and 3.6 mm +/− 2.3 mm for MRI and CT, respectively (p = 0.017). Conclusions When compared with CT, MRI improved visualization of post-lumpectomy changes, reduced interobserver variation and improved the accuracy of CTVLC contouring in patients without clips in the tumour bed. Further studies with bigger sample sizes are needed to confirm our findings.

SU‐E‐T‐169: Initial Investigation into the Use of Optically Stimulated Luminescent Dosimeters (OSLDs) for In‐Vivo Dosimetry of TBI Patients

PURPOSE To report on an initial investigation into the use of optically stimulated luminescent dosimeters (OSLDs) for in-vivo dosimetry for total body irradiation (TBI) treatments. Specifically, we report on the determination of angular dependence, sensitivity correction factors and the dose calibration factors. METHODS The OSLD investigated in our work was InLight/OSL nanoDot dosimeters (Landauer Inc.). Nanodots are 5 mm diameter, 0.2 mm thick disk-shaped Carbon-doped Al2O3, and were read using a Landauer InLight microstar reader and associated software.OSLDs were irradiated under two setup conditions: a) typical clinical reference conditions (95cm SSD, 5cm depth in solid water, 10×10 cm field size), and b) TBI conditions (520cm SSD, 5cm depth in solid water, 40×40 cm field size,). The angular dependence was checked for angles ranging ±60 degree from normal incidence. In order to directly compare the sensitivity correction factors, a common dose was delivered to the OSLDs for the two setups. Pre- and post-irradiation readings were acquired. OSLDs were optically annealed under various techniques (1) by keeping over a film view box, (2) Using multiple scan on a flat bed optical scanner and (3) Using natural room light. RESULTS Under reference conditions, the calculated sensitivity correction factors of the OSLDs had a SD of 2.2% and a range of 5%. Under TBI conditions, the SD increased to 3.4% and the range to 6.0%. The variation in sensitivity correction factors between individual OSLDs across the two measurement conditions was up to 10.3%. Angular dependence of less than 1% is observed. The best bleaching method we found is to keep OSLDs for more than 3 hours on a film viewer which will reduce normalized response to less than 1%. CONCLUSIONS In order to obtain the most accurate results when using OSLDs for in-vivo dosimetry for TBI treatments, sensitivity correction factors and dose calibration factors should all be determined under clinical TBI conditions.

SU‐E‐J‐177: Comparison Between VMAT CT Planning and Segmented MRI Images with Assigned Bulk Density: A Dosimetric Study for Intact Prostate Patients

Purpose: Variability in CT delineation and visualization of target volume has long been a problem in radiotherapy. USe of MR based radiotherapy(MR‐RT) is currently being extensively investigated due to the superior soft tissue contrast which can be enhanced by using appropriate pulse sequences. Current MR Systems ensure that there is no loss of geometric accuracy and distortion however lack of electron density information remains a problem for MR‐RT. This is a retrospective study using VMAT planning on segmented MR images with bulk density assigned for intact prostate patients. Methods: MR images for 10 patients were acquired on GE Optima MRI scanner and segmented, the bulk density was assigned as per ICRU 46 to the contours. VMAT plans were created and optimized on Varian Eclipse TPS using the AAA algorithm on the Pseudo CT/ MR study data. The resulting dose distributions were assessed for PTV coverage and OAR constraints to obtain clinically valid plans.The resulting VMAT plan dose distributions were assessed by re calculating the MR optimized plans on the original CT data sets keeping the plan parameters the same. Using plan analysis features available in Eclipse the resulting differences in the dose distributions were analyzed using DVH data as well as slice by slice dose distribution. The TCP and NTCP were calculated for both sets of plans. The optimized results for TCP and NTCP data will be presented for PTVs and OARs together with DVH comparisons. Quantitative analysis of the differences in dose distribution using Gamma Index Analysis will be performed using SunNuclear ArccheckTM and all results will be presented. Results: Detailed data analysis of the 10 Patients results will be presented. Conclusion: Initial assessment of the data indicates VMAT planning on MR only images with appropriately assigned bulk density information is clinically acceptable for intact prostate cases.

SU-E-T-97: A Methodology for Using Gafchromic EBT2-Films for Accurate Relative 2D-Dosimetry Without the Need of An Accurate Calibration Curve

PURPOSE Calibration curve (CC) for EBT2 film dosimeters is mainly dependend on film batch, film scanning and analysis conditions. CC errors are translated to 2D dosimetry errors. Aiim of study is to present a methodology for using EBT2-films for accurate 2D relative dose measurements without need of an accurate CC. METHODS A batch of EBT2 films has been used for calibration irradiations using doses up to 2500 cGy. An arbitrary parameter (AP) (inverse pixel intensity using ImageJ software) was used as the dependent variable in a film dose response relationship. Linear dose-response region was evaluated. Within this dose region, the relative changes of dose (D%) equals the relative changes of the APnet = [(AP-APbackground)%]. A film from same batch was irradiated using a Stereotactic Radiosurgery treatment plan (Maximum dose lied within the linear film dose range). The film derived relative dose map was compared against corresponding TPS calculations using Gamma Index (2%, 2mm)Results: Linear Dose AP response was observed for doses up to ∼400 cGy. Therefore, within this dose range, (D%) = (APnet%). 2D map of measured SRS irradiated EBT2 film (Apnet%) values have been measured and compared to corresponding TPS calculation (D%) values. Satisfying agreement between the two data sets was observed (gamma <1 for ∼96% of the pixels). CONCLUSION EBT2 films can be used for accurate 2D relative dose measurements as long as they are irradiated within the dose range of 0 to 400 cGy and their batch linear dose response is ensured. There is no need to ensure a certain slope of the linear response, but just the linearity of response itself. Since films are mainly useful for relative rather than absolute dose measurements, the proposed methodology could offer a simple solution for 2D dosimetry in a large number of radiotherapy QA and/or plan verification purposes.

SU‐E‐T‐649: Evaluation of RapidArc‐ Based Stereotactic Cranial Radiotherapy Plans with MU Objective Using Multiple Non Coplanar Arcs in Comparison with Conventional Dynamic Conformal Arc Technique

PURPOSE Previous researches reported that RapidArc plans for stereotactic cranial radiotherapy have two to three times more MUs as compared to Conventional Dynamic Conformal Arc (DCA) Technique. This study aims to evaluate RapidArc plans using multiple non- coplanar arcs, developed with MU objective constraint in the optimization stage. METHODS Five single brain metastasis and three multiple metastases cases previously planned using DCA techniques in BrainLab iPlan Version 4.1 were investigated in this study. For each case, the target was defined on CT-MR fused images in iPlan. The CT images and contours of these patients were exported from iPlan to Varian Eclipse TPS Version 8.6. For each case, a DCA plan and a RapidArc plan with multiple non-coplanar arcs with and without using MU objective in the optimization stage were generated using Varian Trilogy machine with Millennium 120 MLC keeping the same prescription and critical structure dose limits. All plans were evaluated according to Conformity Index (CI-modified Paddick) Homogeneity Index (HI), and the normal tissue volume receiving various dose levels (V80%, V50%, V25% and V10%). RESULTS In all the plans, the target objectives were met and dose to OARs was within tolerance dose constraints. RapidArc plans with and without MU objective showed better CI and HI as supposed to DCA plans. V80%, V50%, V25% and V10% of normal tissue for RapidArc plans are equal or lesser than DCA plans. Single isocentre RapidArc plan for closely spaced multiple metastases cases showed better dose fall off between the lesions as supposed to DCA plans. RapidArc plans with MU objective resulted in comparable MUs as that of DCA plans. CONCLUSIONS Our study showed RapidArc plans done with and without MU objective have no significant dosimetric difference in plan objectives. Therefore, multiple non-coplanar RapidArc plans with MU objective is clinically feasible and can provide better treatment plans than conventional DCA plans, especially for complicated cases.