D Thongphiew

A planning quality evaluation tool for prostate adaptive IMRT based on machine learning

PURPOSE To ensure plan quality for adaptive IMRT of the prostate, we developed a quantitative evaluation tool using a machine learning approach. This tool generates dose volume histograms (DVHs) of organs-at-risk (OARs) based on prior plans as a reference, to be compared with the adaptive plan derived from fluence map deformation. METHODS Under the same configuration using seven-field 15 MV photon beams, DVHs of OARs (bladder and rectum) were estimated based on anatomical information of the patient and a model learned from a database of high quality prior plans. In this study, the anatomical information was characterized by the organ volumes and distance-to-target histogram (DTH). The database consists of 198 high quality prostate plans and was validated with 14 cases outside the training pool. Principal component analysis (PCA) was applied to DVHs and DTHs to quantify their salient features. Then, support vector regression (SVR) was implemented to establish the correlation between the features of the DVH and the anatomical information. RESULTS DVH/DTH curves could be characterized sufficiently just using only two or three truncated principal components, thus, patient anatomical information was quantified with reduced numbers of variables. The evaluation of the model using the test data set demonstrated its accuracy approximately 80% in prediction and effectiveness in improving ART planning quality. CONCLUSIONS An adaptive IMRT plan quality evaluation tool based on machine learning has been developed, which estimates OAR sparing and provides reference in evaluating ART.

Volumetric Arc Intensity–Modulated Therapy for Spine Body Radiotherapy: Comparison With Static Intensity-Modulated Treatment

PURPOSE This clinical study evaluates the feasibility of using volumetric arc-modulated treatment (VMAT) for spine stereotactic body radiotherapy (SBRT) to achieve highly conformal dose distributions that spare adjacent organs at risk (OAR) with reduced treatment time. METHODS AND MATERIALS Ten spine SBRT patients were studied retrospectively. The intensity-modulated radiotherapy (IMRT) and VMAT plans were generated using either one or two arcs. Planning target volume (PTV) dose coverage, OAR dose sparing, and normal tissue integral dose were measured and compared. Differences in treatment delivery were also analyzed. RESULTS The PTV DVHs were comparable between VMAT and IMRT plans in the shoulder (D(99%)-D(90%)), slope (D(90%)-D(10%)), and tail (D(10%)-D(1%)) regions. Only VMAT(2arc) had a better conformity index than IMRT (1.09 vs. 1.15, p = 0.007). For cord sparing, IMRT was the best, and VMAT(1arc) was the worst. Use of IMRT achieved greater than 10% more D(1%) sparing for six of 10 cases and 7% to 15% more D(10%) sparing over the VAMT(1arc). The differences between IMRT and VAMT(2arc) were smaller and statistically nonsignificant at all dose levels. The differences were also small and statistically nonsignificant for other OAR sparing. The mean monitor units (MUs) were 8711, 7730, and 6317 for IMRT, VMAT(1arc), and VMAT(2arc) plans, respectively, with a 26% reduction from IMRT to VMAT(2arc). The mean treatment time was 15.86, 8.56, and 7.88 min for IMRT, VMAT(1arc,) and VMAT(2arc). The difference in integral dose was statistically nonsignificant. CONCLUSIONS Although VMAT provided comparable PTV coverage for spine SBRT, 1arc showed significantly worse spinal cord sparing compared with IMRT, whereas 2arc was comparable to IMRT. Treatment efficiency is substantially improved with the VMAT.

Tradeoffs of integrating real-time tracking into IGRT for prostate cancer treatment

This study investigated the integration of the Calypso real-time tracking system, based on implanted ferromagnetic transponders and a detector array, into the current process for image-guided radiation treatment (IGRT) of prostate cancer at our institution. The current IGRT process includes magnetic resonance imaging (MRI) for prostate delineation, CT simulation for treatment planning, daily on-board kV and CBCT imaging for target alignment, and MRI/MRS for post-treatment assessment. This study assesses (1) magnetic-field-induced displacement and radio-frequency (RF)-induced heating of transponders during MRI at 1.5 T and 3 T, and (2) image artifacts caused by transponders and the detector array in phantom and patient cases with the different imaging systems. A tissue-equivalent phantom mimicking prostate tissue stiffness was constructed and implanted with three operational transponders prior to phantom solidification. The measurements show that the Calypso system is safe with all the imaging systems. Transponder position displacements due to the MR field are minimal (<1.0 mm) for both 1.5 T and 3 T MRI scanners, and the temperature variation due to MRI RF heating is <0.2 degrees C. The visibility of transponders and bony anatomy was not affected on the OBI kV and CT images. Image quality degradation caused by the detector antenna array is observed in the CBCT image. Image artifacts are most significant with the gradient echo sequence in the MR images, producing null signals surrounding the transponders with radii approximately 1.5 cm and length approximately 4 cm. Thus, Calypso transponders can preclude the use of MRI/MRS in post-treatment assessment. Modifications of the clinical flow are required to accommodate and minimize the substantial MRI artifacts induced by the Calypso transponders.

The impact of respiratory motion and treatment technique on stereotactic body radiation therapy for liver cancer

Stereotactic body radiation therapy (SBRT), which delivers a much higher fractional dose than conventional treatment in only a few fractions, is an effective treatment for liver metastases. For patients who are treated under free-breathing conditions, however, respiration-induced tumor motion in the liver is a concern. Limited clinical information is available related to the impact of tumor motion and treatment technique on the dosimetric consequences. This study evaluated the dosimetric deviations between planned and delivered SBRT dose in the presence of tumor motion for three delivery techniques: three-dimensional conformal static beams (3DCRT), dynamic conformal arc (DARC), and intensity-modulated radiation therapy (IMRT). Five cases treated with SBRT for liver metastases were included in the study, with tumor motions ranging from 0.5 to 1.75 cm. For each case, three different treatment plans were developed using 3DCRT, DARC, and IMRT. The gantry/multileaf collimator (MLC) motion in the DARC plans and the MLC motion in the IMRT plans were synchronized to the patients respiratory motion. Retrospectively sorted four-dimensional computed tomography image sets were used to determine patient-organ motion and to calculate the dose delivered during each respiratory phase. Deformable registration, using thin-plate-spline models, was performed to encode the tumor motion and deformation and to register the dose-per-phase to the reference phase images. The different dose distributions resulting from the different delivery techniques and motion ranges were compared to assess the effect of organ motion on dose delivery. Voxel dose variations occurred mostly in the high gradient regions, typically between the target volume and normal tissues, with a maximum variation up to 20%. The greatest CTV variation of all the plans was seen in the IMRT technique with the largest motion range (D99: -8.9%, D95: -8.3%, and D90: -6.3%). The greatest variation for all 3DCRT plans was less than 2% for D95. Dose variations for DARC fell between the 3DCRT and IMRT techniques. The dose volume histogram variations for normal organs were negligible. Therefore, the IMRT technique may be a preferable treatment choice in cases where the target volume and critical organs are in close proximity, or when normal organ protection is a high priority, provided that motion effect for the target volume can be managed.

Adaptive prostate IGRT combining online re-optimization and re-positioning: a feasibility study.

In prostate radiation therapy, inter-fractional organ motion/deformation has posed significant challenges on reliable daily dose delivery. To correct for this issue, off-line re-optimization and online re-positioning have been used clinically. In this paper, we propose an adaptive images guided radiation therapy (AIGRT) scheme that combines these two correction methods in an anatomy-driven fashion. The AIGRT process first tries to find a best plan for the daily target from a plan pool, which consists of the original CT plan and all previous re-optimized plans. If successful, the selected plan is used for daily treatment with translational shifts. Otherwise, the AIGRT invokes the re-optimization process of the CT plan for the anatomy of the day, which is afterward added to the plan pool as a candidate for future fractions. The AIGRT scheme is evaluated by comparisons with daily re-optimization and online re-positioning techniques based on daily target coverage, organs at risk (OAR) sparing and implementation efficiency. Simulated treatment courses for 18 patients with re-optimization alone, re-positioning alone and AIGRT shows that AIGRT offers reliable daily target coverage that is highly comparable to daily re-optimization and significantly improves from re-positioning. AIGRT is also seen to provide improved OAR sparing compared to re-positioning. Apart from dosimetric benefits, AIGRT in addition offers an efficient scheme to integrate re-optimization to current re-positioning-based IGRT workflow.

On-Line Adaptive Radiation Therapy: Feasibility and Clinical Study

The purpose of this paper is to evaluate the feasibility and clinical dosimetric benefit of an on-line, that is, with the patient in the treatment position, Adaptive Radiation Therapy (ART) system for prostate cancer treatment based on daily cone-beam CT imaging and fast volumetric reoptimization of treatment plans. A fast intensity-modulated radiotherapy (IMRT) plan reoptimization algorithm is implemented and evaluated with clinical cases. The quality of these adapted plans is compared to the corresponding new plans generated by an experienced planner using a commercial treatment planning system and also evaluated by an in-house developed tool estimating achievable dose-volume histograms (DVHs) based on a database of existing treatment plans. In addition, a clinical implementation scheme for ART is designed and evaluated using clinical cases for its dosimetric qualities and efficiency.

Arc-modulated radiation therapy based on linear models

This paper reports an inverse arc-modulated radiation therapy planning technique based on linear models. It is implemented with a two-step procedure. First, fluence maps for 36 fixed-gantry beams are generated using a linear model-based intensity-modulated radiation therapy (IMRT) optimization algorithm. The 2D fluence maps are decomposed into 1D fluence profiles according to each leaf pair position. Second, a mixed integer linear model is used to construct the leaf motions of an arc delivery that reproduce the 1D fluence profile previously derived from the static gantry IMRT optimization. The multi-leaf collimator (MLC) sequence takes into account the starting and ending leaf positions in between the neighbouring apertures, such that the MLC segments of the entire treatment plan are deliverable in a continuous arc. Since both steps in the algorithm use linear models, implementation is simple and straightforward. Details of the algorithm are presented, and its conceptual correctness is verified with clinical cases representing prostate and head-and-neck treatments.

Development of a 4D dosimetry simulation system in radiotherapy

4D CT image sets are used to encode patient-organ motion and to calculate the dose delivered during each respiratory phase. The dose-per-phase is summarised to the total dose via deformable registration. The system incorporates the actual beam parameters and synchronises the beam motion (e.g., gantry/MLC motion of the DARC and MLC motion of the IMRT) to the patient’s respiratory motion. Furthermore, this system incorporates different treatment techniques such as 3D conformal (3DCRT), dynamic arc (DARC), and Intensity-Modulated Radiation Therapy (IMRT), thus allows better understanding of organ motion effect on dose delivery, treatment techniques and corresponding optimal margins.

TU-E-BRB-03: A Planning Quality Evaluation Tool for Adaptive IMRT Treatment Based on Machine Learning

Purpose: To monitor the quality of adaptive IMRT plans, especially dose sparing for the organs‐at‐risk (OARs), a plan evaluation tool is developed to predict the dose volume histogram (DVH) based on patients anatomical information and a database of high quality prior treatment plans. The predicted DVH provides a guideline for judging the “goodness” of a new treatment plan. Materials and Method: First, using machine learning to establish a relationship between patients anatomical information and the DVH curves in a database of high quality treatment plans. Anatomical information and DVHs of the PTV (encapsulates prostate and seminal vesicles) and OARs (rectum and bladder) were extracted from the CT/CBCT images and dose distributions. Principal Component Analysis (PCA) is used to characterize the DVH and the anatomical information. And a statistical analysis tool is used to seek the correlation between the DVH characteristics and anatomical features. The second is validation, in which treatment plans outside the database are used to test the performance of the tool. Result: A total of 198 treatment plans were included in the database for machine learning. DVHs of the OARs were characterized by two PCA components that cover 90% variances. Patient anatomical information is reduced to a set of variables, including the two PCA components of the distance volume histogram and organ volumes. Validation test used 14 treatment plans outside the database. The prediction is successful if the actual DVH falls in the 95% confidence band of the predicted DVH curve. Overall, 13 of 14 bladder DVH predications and 12 of 14 rectum DVH predications were successful. Conclusion: An IMRT plan quality evaluation tool based on machine learning is developed to assure the quality of treatment plans. The input is patients anatomical information, and the output is the predicted DVHs for the OARs. (Research sponsored by Varian Corporation)

WE-B-BRA-03: Adaptive Image-Guided Radiation Therapy (AIGRT) for Hypo-Fractionated Prostate Cancer Treatment

Purpose: The large fractional dose of hypo‐fractionated prostate treatment requires high accuracy of daily treatmentdelivery. This study analyzes the performance of an adaptive IGRT (AIGRT) technique, which combines image guidance and adaptive re‐planning to maximize the efficiency and accuracy of such treatment.Method and Materials: 18 patients were studied retrospectively with the hypo‐fractionated treatment regime of 4.4 Gy × 10 fractions. The CTV (prostate + seminal vesicles) and OARs (bladder and rectum) of the planning CT and daily CBCT were contoured by attending physician to maintain consistency in contouring. For each fraction, the AIGRT initiates an automated “soft‐tissue matching” process to find the best fit for the “anatomy‐of‐the‐day” in the patient‐specific plan‐pool (all delivered plans). The matching is successful if the previous smallest PTV can cover current CTV (i.e. minimum deformation and OAR overexposure). If the matching fails, a re‐planning process will re‐optimizes the fluence maps to generate an adaptive plan, which will be used to treat the patient for that fraction. Furthermore, this adapted plan is added to the plan‐pool. Daily DVHs and key dosimetric parameters for the CTVs and OARs were analyzed. The AIGRT technique is compared to soft‐tissue matching (“Soft‐Plan”, current standard), and the daily re‐planning treatments (“Re‐Plan”, the most conformai treatment).Result: (1) Daily D99% to CTVs for all patients ranged from 99.3%–105.1% (AIGRT), 99.9%–105.1% (Re‐Plan), 72.6%–104.7% (Soft‐Plan), respectively, indicating Soft‐Plan is the only technique fails to deliver consistent daily dose to CTV (p=0.0194). (2) V100% and V65% volumes (representing high and medium dose regions) of the bladder and rectum using AIGRT and Re‐Plan techniques were 2.4%–8.3% smaller than those using Soft‐Plans (p<0.0001). (3) AIGRT reduced re‐planning frequency by 43% averaged for all patients. Conclusion: AIGRT improves the efficiency compared to Re‐Plan and the accuracy compared to Soft‐Plan. (Research sponsored by Varian Medical Systems)