Sliding Window IMRT Research Articles

Clinical Implementation and Initial Validation of Respiratory-Gated Stereotactic Body Radiotherapy for Thoracoabdominal Tumors Under Abdominal Compression Using an Anzai Laser-Based Gating Device With Visual Guidance on an Elekta Linear Accelerator.

We have clinically implemented gated stereotactic body radiotherapy under abdominal compression using an Anzai laser-based gating device with visual guidance in combination with an Elekta linear accelerator. To ensure accuracy, we configured the gating window for each patient by correlating the respiratory curve from the laser sensor and the tumor positions from the 4D computed tomography (CT) images reconstructed with the aid of the respiratory curve. This allowed us to define a patient-specific gating window to keep the tumor displacement below 5 mm from the end-expiration, assuming the reproducibility of the tumor trajectories and the laser-based body surface measurements. Results are summarized as follows: 1) A patient-specific gating window internal target volume (ITV) with a prespecified maximum tumor displacement relative to the end-expiration was obtained by acquiring a 4D CT consisting of 20 phase CT sets and a respiratory curve from the Anzai system. 2) Respiratory hysteresis was managed by setting two different thresholds on the respiratory curve based on the predetermined maximum tumor displacement relative to end-expiration. 3) Abdominal compression increased gating window width, thereby presumably leading to faster gated-beam delivery. 4) Gamma index pass rates in sliding-window gated intensity-modulated radiotherapy (IMRT) were superior to those in gated volumetric modulated arc therapy (VMAT). 5) Intrafraction gated cone-beam computed tomography (CBCT) demonstrated that the tumor appeared to remain within the gating window ITV during the stereotactic gated sliding-window IMRT. In conclusion, we have successfully implemented gated stereotactic body radiotherapy at our clinic and achieved a favorable clinical validation result. More cases need to be evaluated to increase the validity.

Post-mastectomy radiotherapy: Impact of bolus thickness and irradiation technique on skin dose

PurposeTo determine 10 MV IMRT and VMAT based protocols with a daily bolus targeting a skin dose of 45 Gy in order to replace the 6 MV tangential fields with a 5 mm thick bolus on alternate days method for post-mastectomy radiotherapy. MethodWe measured the mean surface dose along the chest wall PTV as a function of different bolus thicknesses for sliding window IMRT and VMAT plans. We analyzed surface dose profiles and dose homogeneities and compared them to our standard 6 MV strategy. All measurements were performed on a thorax phantom with Gafchromic films while dosimetric plans were computed using the Acuros XB algorithm (Varian). ResultsWe obtained the best compromise between measured surface dose (mean dose and homogeneity) and skin toxicity threshold obtained from the literature using a daily 3 mm thick bolus. Mean surface doses were 91.4 ± 2.8% [85.7% – 95.4%] and 92.2 ± 2.3% [85.6% – 95.2%] of the prescribed dose with IMRT and VMAT techniques, respectively. Our standard 6 MV alternate days 5 mm thick bolus leads to 89.0 ± 3.7% [83.6% – 95.5%]. Mean dose differences between measured and TPS results were < 3.2% for depths as low as 2 mm depth. Conclusion10 MV IMRT-based protocols with a daily 3 mm thick bolus produce a surface dose comparable to the standard 6 MV 5 mm thick bolus on alternate days method but with an improved surface dose homogeneity. This allows for a better control of skin toxicity and target volume coverage.

Markerless motion tracking with simultaneous MV and kV imaging in spine SBRT treatment—a feasibility study

Objective. Motion tracking with simultaneous MV-kV imaging has distinct advantages over single kV systems. This research is a feasibility study of utilizing this technique for spine stereotactic body radiotherapy (SBRT) through phantom and patient studies. Approach. A clinical spine SBRT plan was developed using 6xFFF beams and nine sliding-window IMRT fields. The plan was delivered to a chest phantom on a linear accelerator. Simultaneous MV-kV image pairs were acquired during beam delivery. KV images were triggered at predefined intervals, and synthetic MV images showing enlarged MLC apertures were created by combining multiple raw MV frames with corrections for scattering and intensity variation. Digitally reconstructed radiograph (DRR) templates were generated using high-resolution CBCT reconstructions (isotropic voxel size (0.243 mm)3) as the reference for 2D–2D matching. 3D shifts were calculated from triangulation of kV-to-DRR and MV-to-DRR registrations. To evaluate tracking accuracy, detected shifts were compared to known phantom shifts as introduced before treatment. The patient study included a T-spine patient and an L-spine patient. Patient datasets were retrospectively analyzed to demonstrate the performance in clinical settings. Main results. The treatment plan was delivered to the phantom in five scenarios: no shift, 2 mm shift in one of the longitudinal, lateral and vertical directions, and 2 mm shift in all the three directions. The calculated 3D shifts agreed well with the actual couch shifts, and overall, the uncertainty of 3D detection is estimated to be 0.3 mm. The patient study revealed that with clinical patient image quality, the calculated 3D motion agreed with the post-treatment cone beam CT. It is feasible to automate both kV-to-DRR and MV-to-DRR registrations using a mutual information-based method, and the difference from manual registration is generally less than 0.3 mm. Significance. The MV-kV imaging-based markerless motion tracking technique was validated through a feasibility study. It is a step forward toward effective motion tracking and accurate delivery for spinal SBRT.

Dosimetric Comparison of Supine vs. Prone Position Utilizing Traditional Linac-Based Stereotactic Body Radiation Therapy for Accelerated Partial Breast Irradiation

<h3>Purpose/Objective(s)</h3> Our institution recently completed a pilot trial using stereotactic body radiation therapy (SBRT) for accelerated partial breast irradiation (APBI) on a traditional Linac delivering 30 Gy in 5 fractions. In this study, we compared dosimetry of patients treated in supine versus prone positions. <h3>Materials/Methods</h3> Patients enrolled on our recent pilot trial and additional post-accrual patients using the same SBRT technique were included. All patients had early-stage breast cancer, met the APBI ASTRO suitability criteria, and were treated on a traditional Linac. Treatment position, breast laterality and volume, BMI, and organs at risk (OAR) dosimetry were recorded. Prone treatments were delivered on a prone breast board using sliding window IMRT, and supine plans used either IMRT or partial arcs. All prone and 15 of 18 supine plans used free breathing technique (3 utilized deep inspiration breath-hold). CTV was 1 cm expansion from GTV. PTV was a 3 mm expansion from CTV and excluded skin surface+3 mm. Laterality was controlled for heart dosimetry. A small subset of patients was simulated in both supine and prone positions with associated radiation plans available for comparison. For unpaired data, unadjusted comparisons of means were performed using two-group t-test, and analysis of covariance was used for adjusted comparisons. For paired data, unadjusted comparisons were performed using the paired t-test, and adjusted comparisons used mixed models repeated measures analysis. Statistical tests were two-sided. <h3>Results</h3> This study included 35 patients with 36 treated plans. One patient had bilateral breast cancer. Of the 36 treated plans, 18 were supine (11 right-sided breast cancer, 7 left-sided) and 18 prone (8 right-sided, 10 left-sided). Eight patients had comparison plans in both positions. For patients with left-sided breast cancer, heart mean dose supine vs prone was 0.49 and 1.14 Gy, respectively (p < 0.001), and continued to be significant when controlling for BMI and treated breast volume. Mean heart dose was not different for right-sided patients supine vs prone, 0.36 vs 0.47 Gy, respectively (p = 0.870). Ipsilateral lung V1.5 Gy was higher for supine vs prone, 39.44 vs 29.89% (p=0.017). Max rib dose was higher for supine compared to prone, 28.29 vs 16.12 Gy (p<0.001). For the subset of patients with plans in both positions, mean heart dose continued to be lower for left-sided patients treated supine vs prone (p = 0.001). Ipsilateral lung V1.5 Gy and max dose to ribs were lower for prone with p values of 0.003 and <0.001, respectively. <h3>Conclusion</h3> Supine treatment provided heart dosimetry benefit for left-sided breast cancer patients compared to prone. Benefit was independent of BMI and breast volume. Prone position offered lower dose to ipsilateral lung and ribs. The results from this study can help determine the best treatment position based on patient-specific OAR concerns.

Verification of stereotactic radiosurgery plans for multiple brain metastases using a virtual phantom-based procedure.

BackgroundThe purpose of this study was to describe the use of the VIPER software for patient-specific quality assurance (PSQA) of single-isocenter multitarget (SIMT) stereotactic radiosurgery (SRS) plans.Materials and methodsTwenty clinical of intensity-modulated (IMRT) SIMT SRS plans were reviewed. A total of 88 brain metastases were included. Number of lesions per plan and their individual volumes ranged from 2 to 35 and from 0.03 to 32.8 cm3, respectively. Plans were designed with the Eclipse system, and delivered using a Varian CLINAC linac. SRS technique consisted of non-coplanar static-field sliding-window IMRT. Each plan was mapped onto a virtual cylindrical water phantom (VCP) in the Eclipse to calculate a 3D dose distribution (verification plan). The VIPER software reconstructed the 3D dose distribution inside the VCP from the acquired in-air electronic portal image device (EPID) images of the treatment fields. A 3D gamma analysis was used to compare the reconstructed doses to the Eclipse planned doses on the VCP. Gamma passing rates (GPRs) were calculated using 3% global/2 mm criteria and dose thresholds ranged from 10% to 90% of the maximum dose.ResultsThe averages (± 1 SD) of the 3D GPRs over the 20 SRS plans were: 99.9 ± 0.2%, 99.7 ± 0.3%, 99.6 ± 0.5%, 99.3 ± 0.9%,99.1 ± 1.6%, 99.0 ± 1.6%, and 98.5 ± 3.3%, for dose thresholds of 10%, 20%, 30%, 50%, 70%, 80% and 90% respectively.ConclusionsThis work shows the feasibility of the VIPER software for PSQA of SIMT SRS plans, being a reliable alternative to commercially available 2D detector arrays.

Dosimetric Comparison of Volumetric Modulated Arc Therapy and Intensity Modulated Radiation Therapy for Soft Tissue Sarcoma of the Extremities

PurposeRadiation therapy is a standard part of limb conserving therapy for extremity soft tissue sarcoma (STS) at high risk of recurrence. Toxic effects increase with radiation dose and volume of normal tissue irradiated. This study sought to compare dosimetry of volumetric modulated arc therapy (VMAT) with intensity modulated radiation therapy (IMRT) and to investigate the optimal planning technique. Methods and MaterialsTwenty patients with extremity STS who underwent preoperative radiation therapy (50 Gy in 25 fractions) between 2016 and 2020 at a specialised sarcoma center were included. The original treatment techniques were sliding window IMRT or 3-dimensional conformal. VMAT plans were retrospectively generated according to the original tumor and organ-at-risk constraints. Quality assurance was performed as per departmental protocol. Wilcoxon signed-rank test was used to compare dosimetric parameters (for planning target volume [PTV], in-field bone, and soft tissue structures), monitor units (MUs), and treatment time. ResultsMedian patient age was 65 years and the majority were male (n = 14, 70%). The most common subtype was undifferentiated pleomorphic sarcoma (n = 14, 70%), and most tumors were located on the thigh (n = 12, 60%). Median PTV was 1110 cm3 and median volume of in-field bone 236 cm3. VMAT plans had significantly lower average MU (480 vs 862 MU, P < .001) and overall treatment time (300 vs 153 seconds, P < .001). PTV coverage favored VMAT, with marginally higher mean, minimum, and maximum doses and higher conformity index. However, differences were not statistically significant. Dose to infield bone and soft tissue structures were similar or slightly lower with VMAT. ConclusionsIn extremity STS, VMAT plans demonstrated a favorable trend toward tumor coverage and dose conformity compared with IMRT along with significantly lower MUs and half the overall treatment time.

Individually selected teletherapy technique for accelerated partial breast irradiation.

BackgroundThe aim of the study was to individualize accelerated partial breast irradiation based on optimal dose distribution, protect risk organ and predict most advantageous technique.Materials and methods138 breast cancer patients receiving postoperative APBI were enrolled. APBI plans were generated using 3D-conformal (3D-CRT), sliding window intensity-modulated radiotherapy (IMRT) and volumetric-modulated arc therapy (VMAT). In the case of superficial tumours, additional plans were developed by adding electron beam. To planning target volume (PTV) 37.5 Gy/10 fractions, 1 fraction/day was prescribed. A novel plan quality index (PQI) served as the basis for comparisons.ResultsIMRT was the most advantageous technique regarding homogeneity. VMAT provided best conformity, 3D-CR T — the lowest lung and heart exposure. PQI was the best in 45 (32.61%) VMAT, 13 (9.42%) IMRT, 9 (6.52%) 3D-CRT plans. In 71 cases (51.45%) no difference was detected. In patients with large PTV, 3D-CRT was the most favourable. Additional electron beam improved PQI of 3D-CRT plans but had no meaningful effect on IMRT or VMAT. IMRT was superior to VMAT if the tumour was superficial (p < 0.001), situated in the medial (p = 0.032) or upper quadrant (p = 0.046).ConclusionsIn half of all cases, individually selected teletherapy techniques provide superior results over others; relevance of a certain technique may be predicted by volume and PTV localization.

Evaluation of target autocrop function in nasopharyngeal carcinoma SIB IMRT plan.

A new target autocrop function was introduced in the Varian Eclipse™ treatment planning software (version 15.5 above) (Lohynská in Klin Onkol 33(4):288-294, 2020). The study aimed to evaluate this new target autocrop impact on nasopharyngeal carcinoma (NPC) plan quality and delivery efficiency. Randomly 66 approved NPC simultaneous integrated boost (SIB) intensity-modulated radiation therapy (IMRT) treatment plans were retrospectively studied. The manual cropping-based plans served as reference and were designed using sliding-window IMRT. Reference plans were re-optimized with identical plan parameters following the institutional clinical protocol, except for the redundant optimization objective of the manual cropping targets deleted. Additionally, each target within 5mm of another had one minimum objective at 100% volume and one maximum objective at 0% volume for the autocrop plans. Plan quality was assessed based on selected parameters, including TCP (tumor control probability), NTCP (normal tissue complication probability), conformality index (CI), homogeneity index (HI), and dose-volume characteristics. Additionally, the delivery efficiency, the total plan treatment time defined as a sum of monitor units (MUs) for each treated field, and delivery accuracy, γ passing rate of treatment plan quality assurance (QA) also were compared. Both the manual cropping plans and the autocrop plans could be approved by an experienced oncologist. Overall, the autocrop plans could provide approximately a 13% reduction in linac MU while maintaining comparable plan quality, radiobiological ranking, and accuracy to the manual cropping plans. The new target autocrop tip facilitated the SIB IMRT plans for nasopharyngeal cancer patients. The autocrop could guarantee the quality and delivery accuracy of the radiotherapy plan and improved the planning efficiency, treatment efficiency, and reduced machine wear and tear. It was a promising tool for optimal plan selection for NPC SIB IMRT.

Correlation between the \u03b3 passing rates of IMRT plans and the volumes of air cavities and bony structures in head and neck cancer

BackgroundBoth patient-specific dose recalculation and γ passing rate analysis are important for the quality assurance (QA) of intensity modulated radiotherapy (IMRT) plans. The aim of this study was to analyse the correlation between the γ passing rates and the volumes of air cavities (Vair) and bony structures (Vbone) in target volume of head and neck cancer.MethodsTwenty nasopharyngeal carcinoma and twenty nasal natural killer T-cell lymphoma patients were enrolled in this study. Nine-field sliding window IMRT plans were produced and the dose distributions were calculated by anisotropic analytical algorithm (AAA), Acuros XB algorithm (AXB) and SciMoCa based on the Monte Carlo (MC) technique. The dose distributions and γ passing rates of the targets, organs at risk, air cavities and bony structures were compared among the different algorithms.ResultsThe γ values obtained with AAA and AXB were 95.6 ± 1.9% and 96.2 ± 1.7%, respectively, with 3%/2 mm criteria (p > 0.05). There were significant differences (p < 0.05) in the γ values between AAA and AXB in the air cavities (86.6 ± 9.4% vs. 98.0 ± 1.7%) and bony structures (82.7 ± 13.5% vs. 99.0 ± 1.7%). Using AAA, the γ values were proportional to the natural logarithm of Vair (R2 = 0.674) and inversely proportional to the natural logarithm of Vbone (R2 = 0.816). When the Vair in the targets was smaller than approximately 80 cc or the Vbone in the targets was larger than approximately 6 cc, the γ values of AAA were below 95%. Using AXB, no significant relationship was found between the γ values and Vair or Vbone.ConclusionIn clinical head and neck IMRT QA, greater attention should be paid to the effect of Vair and Vbone in the targets on the γ passing rates when using different dose calculation algorithms.

A Dosimetric Study of Volumetric Arc Modulation with RapidArc Versus Intensity-modulated Radiotherapy inCervical Cancer Patients

Introduction: The method of radiotherapy has moved away from two-dimensional and three-dimensional conformal radiotherapy towards Volumetric-Modulated Arc Therapy (VMAT) for advanced carcinomas. VMAT treatments often result in significant clinical advantage, particularly when concave dose distributions are required as is often the situation since these tumours are in close proximity to several critical structures. Aim:To investigate the potential clinical role of volumetric arc therapy on cervical cancer patients and its comparison with fixed-field Intensity-Modulated Radiotherapy (IMRT) was used as a benchmark. Materials and Methods: Retrospectively, radiotherapy treatment plans of fifteen cervical cancer patients were selected for this study. These patients were previously treated with sliding window IMRT techniques during January 2020 to November 2020. For dosimetric comparison of sliding window IMRT techniques with RapidArc, a new set of plans were created using VMAT/RapidArc technique. For each patient two plans were generated and in this way total 30 plans were analysed. The prescription dose to Planning Target Volume (PTV) was 50.4 Gy in 28 fractions (1.8Gy/fraction) for the 6 MV photon beam. Comparison of each plan done on the basis of Organs At Risk (OARs) sparing, coverage index (C), Conformity Index (CI), Homogeneity Index (HI), dose Gradient Index (GI), and Unified Dosimetry Index (UDI). This study utilised UDI scoring for evaluation and comparison of RapidArc and IMRT plans. Treatment Time (TT) for patient comfort and the number of Monitor Units (MUs) for long-term side-effects was also taken into consideration. A paired two-tailed t-test was executed for the dosimetric study of volumetric arc modulation with RapidArc and its comparison with the IMRT technique in the radiotherapy treatment of cervical cancer patients. All the collected data was analysed using Statistical Package for Social Sciences (SPSS) version 20.0. The (p-value&lt;0.05) was contemplated for the level of statistical significance. Results: Comparable target coverage and better sparing of OARs were achieved with the RapidArc technique in comparison to IMRT. As was evident with results of present study, the values of CI (1.55±0.07), HI (1.07±0.07), GI (0.98±0.01) and UDI (1.25±0.11) of RapidArc technique showed significant difference from respective values of IMRT Technique (1.67±0.06, 1.10±0.06, 0.96±0.01 and 1.38±0.13). Values of MUs (1560.47±52.16) and treatment time (3.71±0.73 mins) were significantlly high in IMRT technique as compared to RapidArc technique (542.33±51.09 and 2.39±0.35 mins respectively). Conclusion: From this study, it is clear that a similar planning goal can be achieved by RapidArc in comparison to fixed-field IMRT with less normal organ toxicity. RapidArc is a faster and precise treatment technique. The most significant change comes to see in the number of MUs and TT, which is much lesser in RapidArc.

Technical Note: Synthetic treatment beam imaging for motion monitoring during spine SBRT treatments - a phantom study.

One of the biggest challenges in applying megavoltage (MV) treatment beam imaging for monitoring spine motion in stereotactic body radiotherapy (SBRT) is the small beam apertures in the images due to strong beam modulations in IMRT planning. The purpose of this study is to investigate the feasibility of a markerless motion tracking method in spine SBRT delivery using a novel enhanced synthetic treatment beam (ESTB) imaging technique. Three clinical spine SBRT plans using 6XFFF beams and sliding window IMRT technique were transferred to a thorax phantom and delivered by a TrueBeam machine. Before delivery, the phantom was aligned to the plan isocenter using CBCT setup and verified with a second CBCT, and then, 2mm shifts were introduced in both the craniocaudal (CC) and the left-right (LR) directions with the couch. During beam delivery, MV images were continuously taken with an electronic portal imaging device (EPID) and automatically grabbed by Varian iTools Capture software with a frame rate of 11.6Hz. After preprocessing for scatter correction and beam intensity compensation, every 50 frames of MV images were combined to generate a series of ESTB images for each beam. The ESTB images were then registered to the projections of the verification CBCT at the matched beam angles to detect the 2mm shifts. Compared to snapshot MV images, the ESTB images had significantly enlarged fields of view (FOVs) and improved image quality. Based on two-dimensional (2D) rigid registration, the ESTB image to CBCT projection matching showed submillimeter accuracy in detecting motion. Specifically, the root mean square errors in detecting the LR/CC shifts were 0.35/0.28, 0.32/0.35, 0.63/0.44, 0.55/0.51, and 0.69/0.42mm at gantry angles 180, 160, 140, 120, and 100, respectively. Our results in the phantom study suggest that ESTB images from a sliding window IMRT plan can be used to detect spine motion, with submillimeter precision in the 2D plane perpendicular to the beam.

The structural similarity index for IMRT quality assurance: radiomics‐based error classification

The implementation of radiomics and machine learning (ML) techniques on analyzing two-dimensional gamma maps has been demonstrated superior to the conventional gamma analysis for error identification in intensity modulated radiotherapy (IMRT) quality assurance (QA). Recently, the Structural SIMilarity (SSIM) sub-index maps were shown to be able to reveal the error types of the dose distributions. In this study, we aimed to apply radiomics analysis on SSIM sub-index maps and develop ML models to classify delivery errors in patient-specific dynamic IMRT QA. Twenty-one sliding-window IMRT plans of 180 beams for three treatment sites were involved in this study. Four types of machine-related errors of various magnitudes were simulated for each beam at each control point, including the monitor unit (MU) variations, same-directional and opposite-directional shifts of the multileaf collimators (MLCs) and random mispositioning of the MLCs. In the QA process, a total of 1620 portal dose (PD) images were acquired for the beams with and without errors. The predicted PD images of the original beams were set as references. To quantify the agreement between a measured PD image and the corresponding predicted PD image, four difference maps including three SSIM sub-index maps, and one dose difference-derived map were calculated. Then, radiomic features were extracted from the four difference maps of each measured PD image. We tested four typical classifiers including linear discriminant classifier (LDC), two supporting vector machine (SVM) classifiers, and random forest (RF) for this multiclass classification task. A nested cross-validation scheme was used for model evaluations, where the SVM recursive feature elimination method was applied for feature selection. Finally, the performance of the ML model on identifying the error-free and the erroneous cases was compared to that of the conventional gamma analysis. The statistics of the selected features showed that all of the difference maps and the feature categories made balanced contributions to solve this classification task. Best performance was achieved by the Linear-SVM model with average overall classification accuracy of 0.86. Specifically, the average classification accuracies of the shift, opening, and the random errors were around 0.9. Moreover, ~80% of error-free and MU errors were correctly classified. Using gamma analysis, the 3mm/3% criterion was found insensitive to errors (sensitivity was only 0.33). Although the sensitivity to errors with the 2mm/2% criterion increased to 0.79, still 8% worse than that of the ML model. We proposed an ML-based method for machine-related error identification in patient-specific dynamic IMRT QA, where radiomic analysis on SSIM sub-index maps were used for feature extraction. With extensive validation to select the best features and classifiers, high accuracies in error classification were achieved. Compared with the conventional gamma threshold method, this approach has great potential in error identification for the patient-specific IMRT QA process.

Assessing the out-of-field dose calculation accuracy by eclipse treatment planning system in sliding window IMRT of prostate cancer patients

AimThe objective of this study was to evaluate out-of-field dose distribution calculation accuracy by the Anisotropic Analytical Algorithm (AAA), version 13.0.26, in Eclipse TPS, (Varian Medical Systems, Palo Alto, Ca, USA) for sliding window IMRT delivery technique in prostate cancer patients. Materials and methodsProstate IMRT plans with nine coplanar were calculated with the AAA Eclipse treatment planning system. To assess the accuracy of dose calculation predicted by the Eclipse in normal tissue and OARs located out of radiation field areas, including the rectum, bladder, right and left head of the femur, absolute organ dose value, and dose distribution were measured using the Delta4+ IMRT phantom. ResultsIn the out-of-field areas, underestimation of −0.66% in organs near the field edge to −39.63% in organs far from the field edge (2.5 and 7.3 cm respectively) occurred in the TPS calculations. The percentage of dose deviation for the femoral heads was 95.7 on average while for the organ closer to the target (rectum) it was 79.81. ConclusionsAAA dosimetry algorithm (used in Eclipse TPS) showed poor dose calculation in areas beyond the treatment fields border where underestimation varies with the distance from the field edges. A significant underestimation was found for the AAA algorithm in the sliding window IMRT technique (P-value > 0.05).

Dosimetric evaluation of field-in-field and sliding-window IMRT in endometrium cancer patients with a new approach for the conformity index

Dosimetric evaluation of field-in-field and sliding-window IMRT in endometrium cancer patients with a new approach for the conformity index

A sliding-window approach for improved VMAT dose calculation accuracy

The continuous delivery of volumetric modulated arc therapy (VMAT) plans is usually approximated by discrete apertures at evenly-spaced gantry angles for dose calculation purposes. This approximation can potentially lead to large dose calculation errors if the gantry angle spacings are large and/or there are large changes in the MLC apertures from one control point (CP) to the next. In this work, we developed a sliding-window (SW) method to improve VMAT dose calculation accuracy. For any 2 adjacent VMAT CPs ni and ni + 1, the dose distribution was approximated by a 2-CP SW IMRT beam with the starting MLC positions at CP ni and ending MLC positions at CP ni + 1, with the gantry angle fixed in the middle of the 2 VMAT CPs. Therefore, a VMAT beam with N CPs was approximated by a SW plan with N-1 SW beams. To validate the method, VMAT plans were generated for 10 patients in Pinnacle using 4° gantry spacing. Each plan was converted to a SW plan and dose was recalculated. Another VMAT plan, with 1° gantry spacing, was created by interpolating the original VMAT beam. The original plans were delivered on an Elekta Versa HD and measured with ArcCHECK. For both the isodose distribution and DVH, there were significant differences between the original VMAT plan and either the SW or the interpolated plan. However, they were indistinguishable between the SW and the interpolated plans. When compared with measurement, the average passing rates of the original VMAT plans were 87.3 ± 2.8% and 93.1 ± 1.0% for the 5 HN and 5 spine SBRT cases, respectively. On the other hand, the passing rates for both the VMAT1 and SW plans were above 95% for all the 10 cases studied. The dose calculation times of the original VMAT plans and the SW plans were very similar. We conclude that the proposed SW approach improves VMAT dose calculation accuracy without increase in dose calculation time.

A sliding-window approach for improved VMAT dose calculation accuracy

A sliding-window (SW) methodology for VMAT dose calculation was developed. For any two adjacent VMAT control points (CPs) n and n+1, the dose distribution was approximated by a 2-CP SW IMRT beam with the starting MLC positions at CP n and ending MLC positions at CP n+1, with the gantry angle fixed in the middle of the two VMAT CPs. Therefore, for any VMAT beam with N CPs, the dose is calculated with N-1 SW beams. VMAT plans were generated for ten patients in Pinnacle using 4° gantry spacing. For each patient, the VMAT plan was converted to a SW IMRT plan and dose was re-calculated. Another VMAT plan, with 1° gantry spacing, was created by interpolating the original VMAT beam. The original plans were delivered on an Elekta Versa HD and measured with Mapcheck2 using an in-house developed subarc method. For both the isodose distribution and DVH, there were significant differences between the original VMAT plan and either the SW or the interpolated plan. However, they were indistinguishable between the SW and interpolated plans. The average passing rate between the original VMAT plan and measurements was 84%. For both the interpolated and SW plans, the average passing rate was 96%. We conclude that the proposed SW approach improves VMAT dose calculation accuracy without increase in dose calculation time.

Dosimetric comparison of volumetric‐arc therapy versus sliding window intensity‐modulated radiotherapy in postoperative treatment for primary soft tissue sarcoma of the thigh

Intensity-modulated radiotherapy (IMRT) has demonstrated improved local control in extremity soft tissue sarcoma (STS) after limb-sparing surgery compared with three-dimensional conformal radiation therapy. Our purpose was to evaluate sliding-window IMRT (SW-IMRT) and volumetric arc therapy (VMAT) in planning target volume (PTV) coverage and dose to organs-at-risk (OAR). Sixteen patients undergoing postoperative RT for lower extremity STS were included. For each patient, one VMAT plan and one SW-IMRT plan were proposed. Both were evaluated using cumulative dose-volume histogram data for OAR and PTVs. Prescribed dose was 66Gy (2Gy/fraction) to PTV1 and 56Gy (1.69Gy/fraction) to PTV2. OARs contoured were femur, neurovascular bundle, minimum tissue corridor, normal tissue outside PTV2, joint and genitalia. T-Student test was performed. Eleven male (69%) and five female patients (31%) were analyzed. Mean age was 60 years. Both techniques showed optimal target coverage, conformity index (CI) and homogeneity index (HI). VMAT PTV2 CI was 1.13 (mean)±0.08 (standard deviation) versus 1.19±0.10 SW-IMRT PTV2 CI (P<0.05). VMAT PTV1 HI was 0.09±0.01 versus 0.08±0.01 SW-IMRT PTV1 HI (P<0.05). Regarding OARs, VMAT delivered lower dose to femur, genitalia, normal tissue outside PTV2 and joints. SW-IMRT spared tissue corridor mean dose (10.4Gy±6.8Gy) versus (14.7±6.5Gy) VMAT (P<0.05). Both techniques achieved great conformity, homogeneity and coverage of PTV. VMAT produced lower dose to OARS and SW-IMRT was superior in sparing dose to normal-tissue-corridor, which could reduce risk of lymphedema.

204. In-silico implementation of Linac MRI based radiotherapy: A dosimetrical comparison with rectal cancer

Purpose MR-guided radiotherapy is a new promising technique. Our department is equipped with a MRIdian 60Co system (Viewray) and we are implementing the TPS of the first 6 MV MR-Linac available for clinical practice (ViewRay). The aim of this study is to assess the impact of the MRIdian Linac through a planning analysis for rectal cancer treatments. Methods Four plans (MRIdian Linac, MRIdian 60Co, RapidArc and sliding windows IMRT) were calculated for 10 patients affected by rectal cancer. ROIs were contoured on Eclipse TPS (version 11.0, Varian). The body, the bowel bag and the bladder were the OaR considered. The prescribed dose for PTV2 was 45 Gy and 55 Gy for PTV1 through SIB. RapidArc (6–15 MV) and 5 beams (6–15 MV) sliding windows IMRT treatment plans were calculated on Eclipse using the Analytical Anisotropic Algorithm. 60Co step and shoot IMRT and 7 beams 6 MV step and shoot IMRT were perfomed with the MRIdian systems, equipped with a GPU-accelerated Monte Carlo Algorithm. The PTV V95 and OaRs QUANTEC dose constraints on the DVHs were considered for the QA of the plans. Results Both MRIdian 60Co and MRIdian Linac showed a better PTV1 coverage. Comparable mean doses for the bladder were registered, while a higher bowel V45 was observed for MRIdian 60Co. Low dose body V5 was higher for the MRIdian 60Co plans whilst the MRIdian Linac low doses resulted to be comparable to the conventional Linac. Conclusions MRIdian allowed reaching dosimetrical goals comparable to RapidArc and IMRT gold standards. The MRIdian Linac resulted in smaller low doses distribution, comparable to the conventional IMRT technique. The availability of an on-board MR imaging system might lead to the reduction of the target margin. This aspect will be investigated when the system will become operative.

Comparison between two different methods used for IMRT plans QA of Prostate cancer

Introduction: The aim of this study is evaluation of two Quality Assurance methods in sliding window IMRT technique and Determination of gamma index in both methods Materials and Methods: In this study two tools named phantom and software has been used to perform QA on the treatment plan before the actual treatment by Varian linear accelerator Clinac 600C. For this purpose, 20 patient’s treatment plan were assessed with the 3% and 3mm criteria. Results: Each treatment field demonstrated that the gamma index in the delta4 phantom was more than the Epiqa software for all patients. The most uncertainty for a treatment plan in delta4 phantom was 0.88% and for Epiqa software was 1.53%. (p- value <0.05) Conclusion: In spite of these variations in results and also studying similar researches, these tools can be used as a reliable QA method in radiotherapy centers. Delta4 phantom is preferred to Epiqa software because it is a dosimeter. However, Epiqa software is economical to install. therefore can be used for those centers which cannot afford delta4 phantom due to economic consideration.

P236] Rapid Arc versus a new sectorial sliding window IMRT template in radiotherapy brain treatments: Lens sparing

The ICRP Publication 118 introduced new threshold doses for tissue reactions and other non cancer effects on lens:the new threshold doses resulted significantly lower than previous scientific evidences. Lenses are often involved in brain radiotherapy treatment volumes and the risk of cataratta becomes relevant. Our study compares the results obtained from treatment plans calculated with both Rapid Arc technique and a new SSW IMRT template, for each patient recruited in the study, in terms of lens maximum absorbed doses. 10 patients with brain tumours diagnosis (PTV medium size af about 90 cc) localized in different areas were treated with the new SSW IMRT template, that consists in five fields spreading over a The dose prescription for all patients was 60 Gy in 2 Gy daily fractions. On the same patients, Rapid Arc plans were conformed with equal optimization objectives. The target coverage was at least 97% of prescription dose to 95 % of volume in both calculated plans. All the patiens treated with the SSW IMRT technique received a lower maximum omolateral lens dose (range from 0.4 to 2.4 Gy; mean value 1.7 Gy) compared to doses obtained with Rapid Arc plans (range from 0.5 to 4.4 Gy; mean value 2.4 Gy). Relatively to contralateral dose lens, we obtained a mean value of max dose of 1.5 Gy (range 0.4–2.2 Gy) for SSW IMRT plans and 2.2 Gy (range 0.5–4.6 Gy) for Rapid Arc plans. Patients treated with the new SSW IMRT template showed a lens dose sparing of about the 28% compared to doses they would have received with Rapid Arc plans.The other OAR involved as chiasm, optical nerve and brainstem are widely lower than the corresponding constraints. The new SSW IMRT template could be applied to treat target with different size, shape and anatomical position. Furthermore, the lens maximum dose reduction could potentially reduce the risk of cataract.