Dose-volume Histogram Changes Research Articles

Dose Prescription to Isodose Lines in Static Multi-Beam Stereotactic Body Radiotherapy for Lung Tumors: Which Line Is Optimal?

This study investigated the appropriate dose prescription method in static multi-beam stereotactic body radiotherapy for lung tumors. Static multi-beam stereotactic body radiotherapy is a mainstream treatment in Japan. Based on the hypothesis that dose prescription to lower isodose lines may improve planning target volume dose coverage and decrease doses to organs at risk, we investigated changes in dose-volume histograms with prescription to various isodose lines for planning target volume in static multi-beam stereotactic body radiotherapy. In all treatment plans, 45 Gy in 4 fractions were prescribed to 95% of the planning target volume. By adjusting the leaf margins of each beam, various prescription isodose lines encompassing 95% volume of the planning target volume were generated. The prescription isodose lines investigated were 40, 50, 60, 70, 80 and 90% lines relative to the maximum dose of each planning target volume. The conformity index, homogeneity index, mean lung dose, and V5-V40 of the lung were evaluated. The dose was calculated by the adaptive convolve algorithm. The conformity index was lowest in the 70% or 80% isodose plan. The mean lung doses and V10-V40 of the lung decreased steeply from the 90% to the 70% isodose plan, and was lowest in the 60% and 70% isodose plans. These indices increased in the 40% and 50% isodose plans. The optimal stereotactic body radiotherapy plans appeared to be dose prescription to the 60% or 70% isodose line. Further investigation is warranted to clarify the advantage of using this method clinically.

Dosimetry advantages of intraoperatively built custom-linked seeds compared with loose seeds in permanent prostate brachytherapy.

PurposeThe aim of this study was to compare the implant quality between intraoperatively built custom-linked seeds (IBCL) and loose seeds (LS) retrospectively.Material and methodsThis study included 74 prostate cancer patients who were treated with permanent prostate brachytherapy (PPB) using IBCL (n = 37) or LS (n = 37) between July 2014 and June 2016. Dose-volume histogram (DVH) parameters, seed migration, and operation time were compared between the IBCL and LS groups. In addition to the standard target volume of the whole prostate gland, DVH parameters for prostate plus a 3 mm margin (CTV) were evaluated.ResultsIn intraoperative planning, prostate V150 was lower (54.8% vs. 59.6%, p = 0.027), and CTV V100 (88.1% vs. 85.6%, p = 0.019) and D90 (98.5% vs. 92.6%, p = 0.0033) were higher in the IBCL group compared with in the LS group. In post-implant dosimetry, prostate V100 (96.9% vs. 95.2%, p = 0.020), CTV V100 (85.6% vs. 81.7%, p = 0.046), and CTV D90 (94.2% vs. 86.5%, p < 0.001) were higher, and prostate V150 (57.1% vs. 64.5%, p = 0.0051) and CTV V150 (31.5% vs. 35.7%, p = 0.046) were lower in the IBCL group compared with in the LS group. Regarding DVH changes between intraoperative planning and post-implant dosimetry, the decrease in prostate D90 was significantly lower in the IBCL group than in the LS group (–1.16% vs. –4.17%, p < 0.001). For the IBCL group, the operation time was slightly but significantly longer than that for the LS group (50.5 minutes vs. 43.7 minutes, p = 0.011). However, the seed migration rate was significantly lower in the IBCL group than in the LS group (5% vs. 41%, p < 0.001).ConclusionsIntraoperatively built custom-linked is more advantageous than LS in terms of dosimetric parameters and migration.

Clinical Implementation of Dual-energy CT for Proton Treatment Planning on Pseudo-monoenergetic CT scans

To determine whether a standardized clinical application of dual-energy computed tomography (DECT) for proton treatment planning based on pseudomonoenergetic CT scans (MonoCTs) is feasible and increases the precision of proton therapy in comparison with single-energy CT (SECT). To define an optimized DECT protocol, CT scan settings were analyzed experimentally concerning beam hardening, image quality, and influence on the heuristic conversion of CT numbers into stopping-power ratios (SPRs) and were compared with SECT scans with identical CT dose. Differences in range prediction and dose distribution between SECT and MonoCT were quantified for phantoms and a patient. Dose distributions planned on SECT and MonoCT datasets revealed mean range deviations of 0.3mm, γ passing rates (1%, 1mm) greater than 99.9%, and no clinically relevant changes in dose-volume histograms. However, image noise and CT-related uncertainties could be reduced by MonoCT compared with SECT, which resulted in a slightly decreased dependence of SPR prediction on beam hardening. Consequently, DECT was clinically implemented at the University Proton Therapy Dresden in 2015. Until October 2016, 150 patients were treated based on MonoCTs, and morethan 950 DECT scans of 351 patients were acquired during radiationtherapy. A standardized clinical use of MonoCT for treatment planning is feasible, leads to improved image quality and SPR prediction, extends diagnostic variety, and enables a stepwise clinical implementation of DECT toward a physics-based, patient-specific, nonheuristic SPR determination. Further reductions of CT-related uncertainties, as expected from such SPR approaches, can be evaluated on the resulting DECT patient database.

Validation of moving target doses using ArcCHECK Planned Dose Perturbation Algorithm

 The aim of the study is to use Planned Dose Perturbation (PDP™) measurement-guided reconstruction method to estimate dynamic 4D dose and evaluate both 3D dose and DVH changes caused by target motion resulting from respiration. Five patients of Ca lung were selected for the study. Target and Organ at risks were (OARs) delineated on 4DCT data set of each patient. Dose of moving target vs. stationary target were simulated and compared for OARs and target by analyzing 3D dose and DVH(Dose Volume Histogram).There was almost 3.5% higher target maximum doses measured with moving target after applying the target motion trajectory data as compared to the stationary target whereas OARs doses were comparable. The results, clinically signifies the importance of motion management in lung tumors.

SU-E-T-213: Initial Experience with VMAT Plan and Delivery Verification Using a DICOM-RT Framework and Linac Delivery Log Files

Purpose: Mobius3D/MobiusFX (M3D/MFX), a commercial DICOM-RT based plan and delivery verification system, was used to compare calculated and delivered volumetric modulated arc therapy (VMAT) dose distributions using TrueBeam delivery log files (TrajectoryLogs). Methods: M3D/MFX utilizes measured linac commissioning data to generate institution specific beam models for evaluating planned and delivered dose distributions. 30 RapidArc prostate plans and 30 head and neck SmartArc plans were used in this study. For every plan, CT images, contoured structure sets, RT-plan, and RT-dose files were exported to M3D, which recalculated the patients’ planning CT dose distributions using a collapsed-cone-convolutionsuperposition algorithm. MFX utilized the acquired TrajectoryLogs to compute patients’ delivered dose distributions based on actual treatment delivery parameters. The agreement between computed and delivered dose distributions was evaluated utilizing a (3%, 3mm) global 3D-gamma analysis and dose-volume histogram changes for targets and organs at risk. Results: Excellent 3D-gamma agreements were observed for all VMAT plans. On average, for computed and delivered RapidArc and SmartArc plans the gamma passing rates were (99.0%±1.4%) and (96.8%±1.8%), respectively. The average difference for primary target prescription dose percent-coverage between calculated and delivered plans was (− 0.09%±2.52%) for RapidArc and (−2.71%±4.62%) for SmartArc cases. Similarly, the planning target mean dose differences were (1.38%±0.96%) for RapidArc and (1.17%±0.72%) for SmartArc plans. For the prostate plans, the calculated and delivered variations of the maximum dose for a 2cc volume for bladder and rectum were (1.32%±1.26%) and (0.65%±1.44%), respectively. The spinal-cord 2cc maximum dose differences of (3.26%±1.68%) were observed for the SmartArc plans. Conclusions: Clinical quality assurance practice based on linac treatment log files for verification of delivered 3D dose distributions in the patients’ geometries represents a paradigm shift from dose measurements in a phantom. This approach captures treatment planning beam modeling differences as well as the linac uncertainties during treatment delivery of the plan.

Dosimetric Analysis of Inter fractional Dose and Volume Changes in Bladder and Rectum during Intensity Modulated Radiation Therapy for Gynecological Cancers with Daily Cone Beam CT guidance

To assess the changes in dose volume histograms from inter fractional changes in bladder and rectum as evaluated by daily KV cone beam CT (CBCT) during Intensity Modulated Radiation Therapy (IMRT) for post operative endometrial cancers.

Influence of Daily Set-Up Errors on Dose Distribution During Pelvis Radiotherapy

An external beam radiotherapy (EBRT) using megavoltage beam of linear accelerator is usually the treatment of choice in cancer patients. The goal of EBRT is to deliver the prescribed dose to the target volume, with as low as possible dose to the surrounding healthy tissue. A large number of procedures and different professions involved in radiotherapy, uncertainty of equipment and daily patient set-up errors can cause a difference between the planned and delivered dose.We investigated a part of this difference caused by measuring daily patient set-up errors for 35 patients. These set-up errors were simulated on five patients, using 3D treatment planning software XiO. The simulation investigated differences in dose distributions between the planned and shifted geometry. Additionally, we investigated the influence of the error on treatment plan selection by analysing changes in dose volume histograms, planning target volume conformity index (CIPTV), and homogeneity index (HI).Simulations showed that patient daily set-up errors can cause significant differences between the planned and actual dose distributions. Moreover, for some patients, those errors could affect the choice of treatment plan since CIPTV fell under 97%. Surprisingly, HI was not as sensitive to set-up errors as CIPTV. Our results have confirmed the need to minimise daily set-up errors through quality assurance programmes.

Prediction of DVH parameter changes due to setup errors for breast cancer treatment based on 2D portal dosimetry

Electronic portal imaging devices (EPIDs) are increasingly used for portal dosimetry applications. In our department, EPIDs are clinically used for two-dimensional (2D) transit dosimetry. Predicted and measured portal dose images are compared to detect dose delivery errors caused for instance by setup errors or organ motion. The aim of this work is to develop a model to predict dose-volume histogram (DVH) changes due to setup errors during breast cancer treatment using 2D transit dosimetry. First, correlations between DVH parameter changes and 2D gamma parameters are investigated for different simulated setup errors, which are described by a binomial logistic regression model. The model calculates the probability that a DVH parameter changes more than a specific tolerance level and uses several gamma evaluation parameters for the planning target volume (PTV) projection in the EPID plane as input. Second, the predictive model is applied to clinically measured portal images. Predicted DVH parameter changes are compared to calculated DVH parameter changes using the measured setup error resulting from a dosimetric registration procedure. Statistical accuracy is investigated by using receiver operating characteristic (ROC) curves and values for the area under the curve (AUC), sensitivity, specificity, positive and negative predictive values. Changes in the mean PTV dose larger than 5%, and changes in V90 and V95 larger than 10% are accurately predicted based on a set of 2D gamma parameters. Most pronounced changes in the three DVH parameters are found for setup errors in the lateral-medial direction. AUC, sensitivity, specificity, and negative predictive values were between 85% and 100% while the positive predictive values were lower but still higher than 54%. Clinical predictive value is decreased due to the occurrence of patient rotations or breast deformations during treatment, but the overall reliability of the predictive model remains high. Based on our predictive model, 2D transit dosimetry measurements can now directly be translated in clinically more relevant DVH parameter changes for the PTV during conventional breast treatment. In this way, the possibility to design decision protocols based on extracted DVH changes is created instead of undertaking elaborate actions such as repeated treatment planning or 3D dose reconstruction for a large group of patients.

Quantifying Variations in Delivered Doses Secondary to Intrafraction Prostate Motion in Patients Treated With Radiotherapy (RT) for Localized Prostate Cancer

The aim of this work was to develop a method to study the variations in delivered doses to the prostate during a full course of image-guided external beam radiotherapy and real time tracking using the Calypso® 4D Localization System and Beacon® transponders. A patient with localized prostate cancer was treated to 78 Gy at 2 Gy per fraction in 39 fractions. PTV margins were 8 mm all around except 5 mm posteriorly (Plan A). A second plan (Plan B) was performed to study the impact of reducing PTV margins to 6 mm all around except 4 mm posteriorly. Daily target localization and tracking was performed using the Calypso System. A new Matlab subroutine was developed that works within CERR (University of Washington), which reads the patient plan in RTOG format. This subroutine uses the Calypso System real time prostate tracking data to generate dose volume histograms (DVHs). The study endpoints were D95 (dose to 95% of the prostate–the dose in Gy that covers 95% of the target volume) and how it varies during a full course of radiation therapy. Table 1 shows the D95 for Plans A and B with median and maximum shifts. Figure 1a shows changes in dose volume histogram (DVH) when maximum shifts during individual session were applied. Figure 1b shows the DVH when the maximum shift over the entire course and the cumulative average of the maximum shifts were applied, along with planned DVH. Figure 2a and 2b show the same with Plan B, respectively. Fig 3a shows separation of the DVHs for average of maximum shifts of the individual sessions for Plans A and B. We have developed a new method to apply and study changes in DVH during an entire course of RT using continuous, real time tracking data from the Calypso System. This method empowers the clinician to both study the impact of changing margins while taking appropriate action levels and implement adaptive radiation protocols as needed. Future work will look at the dose to the peripheral zone of the prostate and normal structures.

Interfractional dose variation during intensity-modulated radiation therapy for cervical cancer assessed by weekly CT evaluation

To investigate the interfractional dose variation of a small-bowel displacement system (SBDS)-assisted intensity-modulated radiation therapy (IMRT) for the treatment of cervical cancer. Four computed tomography (CT) scans were carried out in 10 patients who received radiotherapy for uterine cervical cancer. The initial CT was taken by use of the SBDS, before the beginning of radiotherapy, and 3 additional CT scans with the SBDS were done in subsequent weeks. IMRT was planned by use of the initial CT, and the subsequent images were fused with the initial CT set. Dose-volume histogram (DVH) changes of the targets (planning target volume [PTV] = clinical target volume [CTV] + 1.5 cm) and of the critical organs were evaluated after obtaining the volumes of each organ on 4 CT sets. No significant differences were found in PTV volumes. Changes on the DVH of the CTVs were not significant, whereas DVH changes of the PTVs at 40% to 100% of the prescription dose level were significant (V(90%); 2nd week: p = 0.0091, 3rd week: p = 0.0029, 4th week: p = 0.0050). The changes in the small-bowel volume included in the treatment field were significant. These were 119.5 cm3 (range, 26.9-251.0 cm3), 126 cm3 (range, 38.3-336 cm3), 161.9 cm3 (range, 37.7-294.6 cm3), and 149.1 cm3 (range, 38.6-277.8 cm3) at the 1st, 2nd, 3rd, and 4th weeks, respectively, and were significantly correlated with the DVH change in the small bowel, which were significant at the 3rd (V80%; p = 0.0230) and 4th (V80%; p = 0.0263) weeks. The bladder-volume change correlated to the large volume change (>20%) of the small-bowel volume. Significant DVH differences for the small bowel can result because of interfractional position variations, whereas the DVH differences of the CTV were not significant. Strict bladder-filling control and an accurate margin for the PTV, as well as image-guided position verification, are important to achieve the goal of IMRT.

Investigation of using a power function as a cost function in inverse planning optimization

The purpose of this paper is to investigate the use of a power function as a cost function in inverse planning optimization. The cost function for each structure is implemented as an exponential power function of the deviation between the resultant dose and prescribed or constrained dose. The total cost function for all structures is a summation of the cost function of every structure. When the exponents of all terms in the cost function are set to 2, the cost function becomes a classical quadratic cost function. An independent optimization module was developed and interfaced with a research treatment planning system from the University of North Carolina for dose calculation and display of results. Three clinical cases were tested for this study with various exponents set for tumor targets and sensitive structures. Treatment plans with these exponent settings were compared, using dose volume histograms. The results of our study demonstrated that using an exponent higher than 2 in the cost function for the target achieved better dose homogeneity than using an exponent of 2. An exponent higher than 2 for serial sensitive structures can effectively reduce the maximum dose. Varying the exponent from 2 to 4 resulted in the most effective changes in dose volume histograms while the change from 4 to 8 is less drastic, indicating a situation of saturation. In conclusion, using a power function with exponent greater than 2 as a cost function can effectively achieve homogeneous dose inside the target and/or minimize maximum dose to the critical structures.

238 oral Influence of set-up displacements in conformal radiotherapy of prostate cancer on changes in dose-volume histograms (DVH), tumour control probabilities (TCP) and normal tissue complication probabilities (NTCP) of the bladder and rectum

238 oral Influence of set-up displacements in conformal radiotherapy of prostate cancer on changes in dose-volume histograms (DVH), tumour control probabilities (TCP) and normal tissue complication probabilities (NTCP) of the bladder and rectum