Ling Zhuang

Evaluation of image guided motion management methods in lung cancer radiotherapy

PURPOSE To evaluate the accuracy and reliability of three target localization methods for image guided motion management in lung cancer radiotherapy. METHODS Three online image localization methods, including (1) 2D method based on 2D cone beam (CB) projection images, (2) 3D method using 3D cone beam CT (CBCT) imaging, and (3) 4D method using 4D CBCT imaging, have been evaluated using a moving phantom controlled by (a) 1D theoretical breathing motion curves and (b) 3D target motion patterns obtained from daily treatment of 3 lung cancer patients. While all methods are able to provide target mean position (MP), the 2D and 4D methods can also provide target motion standard deviation (SD) and excursion (EX). For each method, the detected MP/SD/EX values are compared to the analytically calculated actual values to calculate the errors. The MP errors are compared among three methods and the SD/EX errors are compared between the 2D and 4D methods. In the theoretical motion study (a), the dependency of MP/SD/EX error on EX is investigated with EX varying from 2.0 cm to 3.0 cm with an increment step of 0.2 cm. In the patient motion study (b), the dependency of MP error on target sizes (2.0 cm and 3.0 cm), motion patterns (four motions per patient) and EX variations is investigated using multivariant linear regression analysis. RESULTS In the theoretical motion study (a), the MP detection errors are -0.2 ± 0.2, -1.5 ± 1.1, and -0.2 ± 0.2 mm for 2D, 3D, and 4D methods, respectively. Both the 2D and 4D methods could accurately detect motion pattern EX (error < 1.2 mm) and SD (error < 1.0 mm). In the patient motion study (b), MP detection error vector (mm) with the 2D method (0.7 ± 0.4) is found to be significantly less than with the 3D method (1.7 ± 0.8,p < 0.001) and the 4D method (1.4 ± 1.0, p < 0.001) using paired t-test. However, no significant difference is found between the 4D method and the 3D method. Based on multivariant linear regression analysis, the variances of MP error in SI direction explained by target sizes, motion patterns, and EX variations are 9% with the 2D method, 74.4% with the 3D method, and 27% with the 4D method. The EX/SD detection errors are both < 1.0 mm for the 2D method and < 2.0 mm for the 4D method. CONCLUSIONS The 2D method provides the most accurate MP detection regardless of the motion pattern variations, while its performance is limited by the accuracy of target identification in the projection images. The 3D method causes the largest error in MP determination, and its accuracy significantly depends on target sizes, motion patterns, and EX variations. The 4D method provides moderate MP detection results, while its accuracy relies on a regular motion pattern. In addition, the 2D and 4D methods both provide accurate measurement of the motion SD/EX, providing extra information for motion management.

Investigating ion recombination effects in a liquid‐filled ionization chamber array used for IMRT QA measurements

PURPOSE PTWs Octavius 1000 SRS array performs IMRT quality assurance (QA) measurements with liquid-filled ionization chambers (LICs) to allow closer detector spacing and higher resolution, compared to air-filled QA devices. However, reduced ion mobility in LICs relative to air leads to increased ion recombination effects and reduced collection efficiencies that are dependent on Linac pulse frequency and pulse dose. These pulse parameters are variable during an IMRT delivery, which affects QA results. In this study, (1) 1000 SRS collection efficiencies were measured as a function of pulse frequency and pulse dose, (2) two methods were developed to correct changes in collection efficiencies during IMRT QA measurements, and the effects of these corrections on QA pass rates were compared. METHODS To obtain collection efficiencies, the OCTAVIUS 1000 SRS was used to measure open fields of varying pulse frequency, pulse dose, and beam energy with results normalized to air-filled chamber measurements. Changes in ratios of 1000 SRS to chamber measured dose were attributed to changing collection efficiencies, which were then correlated to pulse parameters using regression analysis. The usefulness of the derived corrections was then evaluated using 6 MV and 10FFF SBRT RapidArc plans delivered to the OCTAVIUS 4D system using a TrueBeam (Varian Medical Systems) linear accelerator equipped with a high definition multileaf collimator. For the first correction, matlab software was developed that calculates pulse frequency and pulse dose for each detector, using measurement and DICOM RT Plan files. Pulse information is converted to collection efficiency, and measurements are corrected by multiplying detector dose by ratios of calibration to measured collection efficiencies. For the second correction the MU/min in the daily 1000 SRS calibration was chosen to match the average MU/min of the volumetric modulated arc therapy plan. Effects of the two corrections on QA results were examined by performing 3D gamma analysis comparing predicted to measured dose, with and without corrections. RESULTS Collection efficiencies correlated linearly to pulse dose, while correlations with pulse frequency were less defined, generally increasing as pulse frequency decreased. After complex matlab corrections, average 3D gamma pass rates improved by [0.07%,0.40%,1.17%] for 6 MV and [0.29%,1.40%,4.57%] for 10FFF using [3%/3 mm,2%/2 mm,1%/1 mm] criteria. Maximum changes in gamma pass rates were [0.43%,1.63%,3.05%] for 6 MV and [1.00%,4.80%,11.2%] for 10FFF using [3%/3 mm,2%/2 mm,1%/1 mm] criteria. On average, pass rates of simple daily calibration corrections were within 1% of complex matlab corrections. CONCLUSIONS OCTAVIUS 1000 SRS ion recombination effects have little effect on 6 MV measurements. However, the effect could potentially be clinically significant for higher pulse dose unflattened beams when using tighter gamma tolerances, especially when small aperture sizes are used, as is common for SRS/SBRT. In addition, ion recombination effects are strongly correlated to changing MU/min, therefore MU/min used in daily 1000 SRS calibrations should be matched to the expected average MU/min of the IMRT plan.

A comprehensive evaluation of adaptive daily planning for cervical cancer HDR brachytherapy

The purpose of this study was to evaluate adaptive daily planning for cervical cancer patients who underwent high‐dose‐rate intracavitary brachytherapy (HDR‐BT) using comprehensive interfractional organ motion measurements. This study included 22 cervical cancer patients who underwent 5 fractions of HDR‐BT. Regions of interest (ROIs) including high‐risk clinical tumor volume (HR‐CTV) and organs at risk (OARs) were manually contoured on daily CT images. All patients were clinically treated with adaptive daily plans (ADP), which involved ROI delineation and dose optimization at each treatment fraction. Single treatment plans (SP) were retrospectively generated by applying the first treatment fractions dwell times adjusted for decay and dwell positions of the applicator to subsequent treatment fractions. Various existing similarity metrics were calculated for the ROIs to quantify interfractional organ variations. A novel similarity (JRARM) score was established, which combined both volumetric overlap metrics (DSC, JSC, and RVD) and distance metrics (ASD, MSD, and RMSD). Linear regression was performed to determine a relationship between interfractional organ variations of various similarity metrics and D2cc variations from both plans. Wilcoxon signed‐rank tests were used to assess ADP and SP by comparing EQD2D2cc(α/β=3) for OARs. For interfractional organ variations, the sigmoid demonstrated the greatest variations based on the JRARM, DSC, and RMSD metrics. Comparisons between paired ROIs showed differences in metrics at each treatment fraction. RVD, MSD, and RMSD were found to be significantly correlated to D2cc variations for bladder and sigmoid. The comparison between plans found ADP provided lower EQD2 D2cc of OARs than SP. Specifically, the sigmoid demonstrated statistically significant dose variations (p=0.015). Substantial interfractional organ motion occurs during HDR‐BT based on comprehensive measurements and may significantly affect D2cc of OARs. Adaptive daily planning provides improved dose sparing for OARs compared to single planning with the extent of sparing being different among OARs. PACS number(s): 87.55.D, 87.55.de, 87.55.kh, 87.57.njThe purpose of this study was to evaluate adaptive daily planning for cervical cancer patients who underwent high-dose-rate intracavitary brachytherapy (HDR-BT) using comprehensive interfractional organ motion measurements. This study included 22 cervical cancer patients who underwent 5 fractions of HDR-BT. Regions of interest (ROIs) including high-risk clinical tumor volume (HR-CTV) and organs at risk (OARs) were manually contoured on daily CT images. All patients were clinically treated with adaptive daily plans (ADP), which involved ROI delineation and dose optimization at each treatment fraction. Single treatment plans (SP) were retrospectively generated by applying the first treatment fractions dwell times adjusted for decay and dwell positions of the applicator to subsequent treatment fractions. Various existing similarity metrics were calculated for the ROIs to quantify interfractional organ variations. A novel similarity (JRARM) score was established, which combined both volumetric overlap metrics (DSC, JSC, and RVD) and distance metrics (ASD, MSD, and RMSD). Linear regression was performed to determine a relationship between interfractional organ variations of various similarity metrics and D2cc variations from both plans. Wilcoxon signed-rank tests were used to assess ADP and SP by comparing EQD2D2cc(α/β=3) for OARs. For interfractional organ variations, the sigmoid demonstrated the greatest variations based on the JRARM, DSC, and RMSD metrics. Comparisons between paired ROIs showed differences in metrics at each treatment fraction. RVD, MSD, and RMSD were found to be significantly correlated to D2cc variations for bladder and sigmoid. The comparison between plans found ADP provided lower EQD2 D2cc of OARs than SP. Specifically, the sigmoid demonstrated statistically significant dose variations (p=0.015). Substantial interfractional organ motion occurs during HDR-BT based on comprehensive measurements and may significantly affect D2cc of OARs. Adaptive daily planning provides improved dose sparing for OARs compared to single planning with the extent of sparing being different among OARs. PACS number(s): 87.55.D, 87.55.de, 87.55.kh, 87.57.nj.

Dosimetric evaluation of Point A and volume-based high-dose-rate plans: a single institution study on adaptive brachytherapy planning for cervical cancer

Purpose External beam radiation therapy (EBRT) and brachytherapy (BT) with concurrent cisplatin is the standard of care for locally advanced cervical cancer. The applicability of image-guided adaptive volume-based high-dose-rate (HDR) intracavitary brachytherapy planning is an active area of investigation. In this study, we examined whether volume-based HDR-BT (HDRVOL) plans leads to more conformal plans compared to Point A (HDRPointA)-based plans. Material and methods Two hundred and forty HDRPointA plans from 48 cervical cancer patients treated with chemoradiotherapy were retrospectively collected. Point A plans were renormalized with respect to the high-risk clinical target volume (HR-CTV) for the HDRVOL plans. The doses to organs at risk (OAR; rectum, sigmoid, and bladder), and HR-CTV and the conformal index were compared between HDRPointA and HDRVOL plans. Results HDRVOL plans resulted in a 6-12% reduction in the total dose (EBRT + HDR-BT) to 0.1 cc, 1.0 cc, and 2.0 cc of the OAR as well as an 8-37% reduction in the dose to 2 cc of OAR per HDR-BT fraction compared to HDRPointA plans. Differences in the conformal indexes between the two groups of plans showed an 18-31% relative increase per HDR-BT fraction for HDRVOL plans. The D90 of the HR-CTV was reduced by 11% by HDRVOL planning and had a median dose of 86 Gy. Conclusions Our study reports the relative improvement in OAR doses per HDR-BT fraction by HDRVOL planning compared to HDRPointA planning and demonstrates the dosimetric advantages of volume-based HDR-BT planning in creating more conformal plans.

WE‐AB‐207B‐04: A Preliminary Investigation of Indicators for Treatment Outcomes of CT Guided Cervical Cancer Brachytherapy

PURPOSE To identify indicators for recurrence in cervical cancer patients treated with high-dose-rate brachytherapy (HDR-BT). METHODS A total of 37 biopsy proven uterine cervical cancer patients of stage IB-IVA treated between 2011 and 2015 were included in this study. All patients were treated with 37.8-52.2Gy of external beam radiotherapy (EBRT) plus 5 × 5.0-6.5Gy of HDR-BT. Patient age, standard deviation of high-risk clinical tumor volume (HR-CTVSD) collected throughout HDR-BT, and D90 (EQD2, α/β=10Gy) of the HR-CTV were investigated as potential indicators for local/distant recurrence using ROC analysis. The optimal cutoff value was identified through the Youden index and was subsequently used to obtain a group assignment for all patients. Another two comparing groups were defined per evidence of post-EBRT tumor shrinkage based on the pre-HDR MR scan. Kaplan-Meier curves were generated for recurrence-free proportions for comparing groups where time was measured from the final HDR treatment date to the date of the final follow-up exam and compared through Cox regression. RESULTS Patients had a median follow-up of 12 months, where 16% had local pelvic recurrence and 16% had distant recurrence. HR-CTVSD was identified as a statistically significant indicator for recurrence (AUC=0.802, p=0.007) compared to D90 (AUC=0.655, p=0.167) and patient age (AUC=0.683, p=0.103). HR-CTVSD of 6.26cc was chosen as the optimal cutoff value, which was used as the basis for patient group assignment. Patients with HR-CTVSD>6.26cc had a hazard ratio of 3.92 (95% CI-1.37, 11.24; p=0.011) for recurrence compared to HRCTVSD≤ 6.26cc. Patients without evidence of post-EBRT tumor shrinkage compared to those with had a hazard ratio of 4.28 (95% CI-1.14, 16.15; p=0.032) for recurrence. CONCLUSION HR-CTVSD was identified as an indicator for recurrence and an optimal value of 6.26cc was established in our study. In addition, patients without evidence of post-EBRT tumor shrinkage demonstrated an increased recurrence rate.

SU-C-202-02: A Comprehensive Evaluation of Adaptive Daily Planning for Cervical Cancer HDR Brachytherapy

PURPOSE To evaluate adaptive daily planning for cervical cancer patients who underwent high-dose-rate intra-cavitary brachytherapy (HDR-ICBT). METHODS This study included 22 cervical cancer patients who underwent 5 fractions of HDR ICBT. Regions of interest (ROIs) including high-risk clinical tumor volume (HR-CTV) and organs-at-risk (OARs) were manually contoured on daily CT images. All patients were treated with adaptive daily plans, which involved ROI delineation and dose optimization at each treatment fraction. Single treatment plans were retrospectively generated by applying the first treatment fractions dwell times adjusted for decay and dwell positions of the applicator to subsequent treatment fractions. Various existing similarity metrics were calculated for the ROIs to quantify interfractional organ variations. A novel similarity score (JRARM) was established, which combined both volumetric overlap metrics (DSC, JSC, and RVD) and distance metrics (ASD, MSD, and RMSD). Linear regression was performed to determine a relationship between inter-fractional organ variations of various similarity metrics and D2cc variations from both plans. Wilcoxon Signed Rank Tests were used to assess adaptive daily plans and single plans by comparing EQD2 D2cc (α/β=3) for OARs. RESULTS For inter-fractional organ variations, the sigmoid demonstrated the greatest variations based on the JRARM and DSC similarity metrics. Comparisons between paired ROIs showed differences in JRARM scores and DSCs at each treatment fraction. RVD, MSD, and RMSD were found to be significantly correlated to D2cc variations for bladder and sigmoid. The comparison between plans found that adaptive daily planning provided lower EQD2 D2cc of OARs than single planning, specifically for the sigmoid (p=0.015). CONCLUSION Substantial inter-fractional organ motion can occur during HDR-BT, which may significantly affect D2cc of OARs. Adaptive daily planning provides improved dose sparing for OARs compared to single planning.

SU‐F‐T‐584: Investigating Correction Methods for Ion Recombination Effects in OCTAVIUS 1000 SRS Measurements

PURPOSE PTWs Octavius 1000 SRS array performs IMRT QA measurements with liquid filled ionization chambers (LICs). Collection efficiencies of LICs have been shown to change during IMRT delivery as a function of LINAC pulse frequency and pulse dose, which affects QA results. In this study, two methods were developed to correct changes in collection efficiencies during IMRT QA measurements, and the effects of these corrections on QA pass rates were compared. METHODS For the first correction, Matlab software was developed that calculates pulse frequency and pulse dose for each detector, using measurement and DICOM RT Plan files. Pulse information is converted to collection efficiency and measurements are corrected by multiplying detector dose by ratios of calibration to measured collection efficiencies. For the second correction, MU/min in daily 1000 SRS calibration was chosen to match average MU/min of the VMAT plan. Usefulness of derived corrections were evaluated using 6MV and 10FFF SBRT RapidArc plans delivered to the OCTAVIUS 4D system using a TrueBeam equipped with an HD- MLC. Effects of the two corrections on QA results were examined by performing 3D gamma analysis comparing predicted to measured dose, with and without corrections. RESULTS After complex Matlab corrections, average 3D gamma pass rates improved by [0.07%,0.40%,1.17%] for 6MV and [0.29%,1.40%,4.57%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. Maximum changes in gamma pass rates were [0.43%,1.63%,3.05%] for 6MV and [1.00%,4.80%,11.2%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. On average, pass rates of simple daily calibration corrections were within 1% of complex Matlab corrections. CONCLUSION Ion recombination effects can potentially be clinically significant for OCTAVIUS 1000 SRS measurements, especially for higher pulse dose unflattened beams when using tighter gamma tolerances. Matching daily 1000 SRS calibration MU/min to average planned MU/min is a simple correction that greatly reduces ion recombination effects, improving measurements accuracy and gamma pass rates. This work was supported by PTW.

A large-scale measurement of dielectric properties of normal and malignant colorectal tissues obtained from cancer surgeries at Larmor frequencies

PURPOSE Knowledge of dielectric properties of malignant human tissues is necessary for the recently developed magnetic resonance (MR) technique called MR electrical property tomography. This technique may be used in early tumor detection based on the obvious differentiation of the dielectric properties between normal and malignant tissues. However, the dielectric properties of malignant human tissues in the scale of the Larmor frequencies are not completely available in the literature. In this study, the authors focused only on the dielectric properties of colorectal tumor tissue. METHODS The dielectric properties of 504 colorectal malignant samples excised from 85 patients in the scale of the Larmor frequencies were measured using the precision open-ended coaxial probe method. The obtained complex-permittivity data were fitted to the single-pole Cole-Cole model. RESULTS The median permittivity and conductivity for the malignant tissue sample were 79.3 and 0.881 S/m at 128 MHz, which were 14.6% and 17.0% higher, respectively, than those of normal tissue samples. Significant differences between normal and malignant tissues were found for the dielectric properties (p < 0.05). CONCLUSIONS Experimental results indicated that the dielectric properties were significantly different between normal and malignant tissues for colorectal tissue. This large-scale clinical measurement provides more subtle base data to validate the technique of MR electrical property tomography.

SU‐E‐T‐485: Investigation of a Synthetic Diamond Detector for Tomotherapy Dosimetry

PURPOSE Tomotherapy treatments are characterized by rotational deliveries of flattening-filter free fields resulting in high-gradient dose distributions. Small volume, rotationally independent detectors are needed for accurate dosimetry. PTWs microDiamond detector, with its small sensitive volume (0.004mm3 ), could potentially be an ideal detector for Tomotherapy. The microDiamond detector was tested against a small volume Exradin A1SL ion chamber for Tomotherapy open-field and IMRT commissioning measurements. METHODS Custom detector holders were fabricated to allow A1SL and microDiamond measurements in the Tomotherapy Cheese phantom and a square solid water phantom. The microDiamond rotational dependence within the Tomotherapy phantom was tested by incrementally rotating the detector in between static-gantry angle Tomotherapy irradiations. Longitudinal Tomotherapy profiles, for all field sizes, were measured with the microDiamond and A1SL detectors at 1.5cm depth in the square phantom, and compared to film. Detector axes were aligned parallel to table motion. Per TG-119 recommendations, both detectors were calibrated to known doses in phantoms and used to measure high-dose points in TG-119 H&N and Prostate plans. The measurements were compared to the treatment planning system and subsequently compared to published TG-119 confidence limits. RESULTS The microDiamond angular dependence was less than 0.5%. The average difference between the detectors and film-measured longitudinal profile 80-20% penumbras were 0.03+/-0.04mm and 1.36+/-0.22mm for the microDiamond and A1SL, respectively. The average difference between the detector and filmmeasured field sizes were 0.07+/-0.01mm and 0.09+/-0.02mm for the microDiamond and A1SL, respectively. The measured confidence limits were 0.023 and 0.015 for microDiamond and A1SL, respectively. TG-119 reported a confidence limit of 0.034. CONCLUSION The microDiamond measured open-field longitudinal Tomotherapy profiles more closely resembled film measurements, compared with the A1Sl chamber. Furthermore, the microDiamond was shown to have low angular dependence and both detectors were able to accurately measure the highdose points in TG-119 H&N and prostate plans. Our institution has research grants with PTW.