IMRT Dose Distributions Research Articles

Dosimetric impact of MLC positional errors on dose distribution in IMRT.

Optimizing the positional accuracy of multileaf collimators (MLC) for radiotherapy is important for dose accuracy and for reducing doses delivered to normal tissues. This study investigates dose sensitivity variations and complexity metrics of MLC positional error in volumetric modulated arc therapy and determines the acceptable ranges of MLC positional accuracy in several clinical situations. Treatment plans were generated for four treatment sites (prostate cancer, lung cancer, spinal, and brain metastases) using different treatment planning systems (TPSs) and fraction sizes. Each treatment plan introduced 0.25-2.0mm systematic or random MLC leaf bank errors. The generalized equivalent uniform dose (gEUD) sensitivity and complexity metrics (MU/Gy and plan irregularity) were calculated, and the correlation coefficients were assessed. Furthermore, the required tolerances for MLC positional accuracy control were calculated. The gEUD sensitivity showed the highest dependence of systematic positional error on the treatment site, followed by TPS and fraction size. The gEUD sensitivities were 6.7, 4.5, 2.5, and 1.7%/mm for Monaco and 8.9, 6.2, 3.4, and 2.3%/mm (spinal metastasis, lung cancer, prostate cancer, and brain metastasis, respectively) for RayStation. The gEUD sensitivity was strongly correlated with the complexity metrics (r=0.88-0.93). The minimum allowable positional error for MLC was 0.63, 0.34, 1.02, and 0.28mm (prostate, lung, brain, and spinal metastasis, respectively). The acceptable range of MLC positional accuracy depends on the treatment site, and an appropriate tolerance should be set for each treatment site with reference to the complexity metric. It is expected to enable easier and more detailed MLC positional accuracy control than before by reducing dose errors to patients at the treatment planning stage and by controlling MLC quality based on complexity metrics, such as MU/Gy.

Relapse patterns and radiation dose exposure in IDH wild-type glioblastoma at first radiographic recurrence following chemoradiation

PurposeTo quantify the radiation dose distribution and lesion morphometry (shape) at baseline, prior to chemoradiation, and at the time of radiographic recurrence in patients with glioblastoma (GBM).MethodsThe IMRT dose distribution, location of the center of mass, sphericity, and solidity of the contrast enhancing tumor at baseline and the time of tumor recurrence was quantified in 48 IDH wild-type GBM who underwent postoperative IMRT (2 Gy daily for total of 60 Gy) with concomitant and adjuvant temozolomide.ResultsAverage radiation dose within enhancing tumor at baseline and recurrence was ≥ 60 Gy. Centroid location of the enhancing tumor shifted an average of 11.3 mm at the time of recurrence with respect to pre-IMRT location. A positive correlation was observed between change in centroid location and PFS in MGMT methylated patients (P = 0.0007) and Cox multivariate regression confirmed centroid distance from baseline was associated with PFS when accounting for clinical factors (P = 0.0189). Lesion solidity was higher at recurrence compared to baseline (P = 0.0118). Tumors that progressed > 12 weeks after IMRT were significantly more spherical (P = 0.0094).ConclusionMost GBMs recur local within therapeutic IMRT doses; however, tumors with longer PFS occurred further from the original tumor location and were more solid and/or nodular.

Calculation of Jaws-only IMRT (JO-IMRT) dose distributions based on the AAPM TG-119 test cases using Monte Carlo simulation and Prowess Panther treatment planning system

The aim of this study is to calculate the JO-IMRT dose distributions based on the AAPM TG-119 using Monte Carlo (MC) simulation and Prowess Panther treatment planning system (TPS) (Panther, Prowess Inc., Chico, CA). JO-IMRT dose distributions of AAPM TG-119 were calculated by the TPS and were recalculated by MC simulation. The DVHs and 3D gamma index using global methods implemented in the PTW-VeriSoft with 3%/3 mm were used for evaluation. JO-IMRT dose distributions calculated by TPS and MC were matched the TG-119 goals. The gamma index passing rates with 3%/3 mm were 98.7% for multi-target, 96.0% for mock prostate, 95.4% for mock head-and-neck, and 96.6% for C-shape. The dose in the planning target volumes (PTV) for TPS was larger than that for the MC. The relative dose differences in D99 between TPS and MC for multi-target are 1.52%, 0.17% and 1.40%, for the center, superior and inferior, respectively. The differences in D95 are 0.16% for C-shape; and 0.06% for mock prostate. Mock head-and-neck difference is 0.40% in D99. In contrast, the organ curve for TPS tended to be smaller than MC values. JO-IMRT dose distributions for the AAPM TG-119 calculated by the TPS agreed well with the MC.

The Effect of Total Fields' Area and Dose Distribution in Step and Shoot IMRT on Gamma Passing Rate Using OCTAVIUS 4D-1500 Detector Phantom

Introduction: Quality assurance is necessary for every IMRT plan.‎Octavius 4D-1500 detector phantom is ‎one of the new phantoms for ‎determining the treatment plan quality‎.‎ This study aimed to examine the IMRT plans using the ‎Octavius 4D-1500‎‏ ‏‎to determine if ‎it is a reliable, ‎dependable, and durable. Material and Methods: IMRT QA conducted for 30 cases: HN and pelvis. The Monaco TPS used for treatment planning. The treatment plans ‎were then applied ‎to the Octavius 4D-1500 phantom (virtually and actually), ‎‎the γ-index was calculated in VeriSoft program to evaluate the IMRT plans. Results: Significant differences were observed between the ‎‎‎measured and calculated dose distributions for HN and pelvic plans, while the treatment ‎sites did not affect the GP rate. The results of the global Gp ‎ were higher than the local GP, ‎regardless of the study criteria. The ‎HN plans ‎showed a more significant difference ‎than the pelvic plans. The HN plans, ‎a ‎strong significant correlation was found between the total ‎fields’area and %GP in both global ‎‎and local analyses, while in the pelvic plans, there was only a significant association with the ‎local %GP. Conclusion: The measured dose distributions significantly different from calculated distributions. The relationship between the ‎‎fields’ ‏‎ area ‎and ‎‎‎%GP was inverse. In the HN plans, a ‎significant correlation found between the total ‎‎fields’ ‎area ‎and %GP in both global and ‎local, while only local %GP in the pelvic plans was significant correlation. Overall, the Octavius 4D-1500 detector phantom ‎might be applicable for ‎assessing the QA of IMRT plans.

Prediction of Three-Dimensional Radiotherapy Optimal Dose Distributions for Lung Cancer Patients With Asymmetric Network.

The iterative design of radiotherapy treatment plans is time-consuming and labor-intensive. In order to provide a guidance to treatment planning, Asymmetric network (A-Net) is proposed to predict the optimal 3D dose distribution for lung cancer patients. A-Net was trained and tested in 392 lung cancer cases with the prescription doses of 50Gy and 60Gy. In A-Net, the encoder and decoder are asymmetric, able to preserve input information and to adapt the limitation of GPU memory. Squeeze and excitation (SE) units are used to improve the data-fitting ability. A loss function involving both the dose distribution and prescription dose as ground truth are designed. In the experiment, A-Net is separately trained and tested in the 50Gy and 60Gy dataset and most of the metrics A-Net achieve similar performance as HD-Unet and 3D-Unet, and some metrics slightly better. In the 50Gy-and-60Gy-combined dataset, most of the A-Net's metrics perform better than the other two. In conclusion, A-Net can accurately predict the IMRT dose distribution in the three datasets of 50Gy and 50Gy-and-60Gy-combined dataset.

Effects of variable-width jaw motion on beam characteristics for Radixact Synchrony®.

PurposeRadixact Synchrony corrects for target motion during treatment by adjusting the jaw and MLC positions in real time. As the jaws move off axis, Synchrony attempts to adjust for a loss in output due to the un‐flattened 6 MV beam by increasing the jaw aperture width. The purpose of this work was to assess the impact of the variable‐width aperture on delivered dose using measurements and simulations.MethodsLongitudinal beam profile measurements were acquired using an Edge diode with static gantry. Jaw‐offset peak, width, and integral factors were calculated for profiles with the jaws in the extreme positions using both variable‐width (Synchrony) and fixed‐width apertures. Treatment plans with target motion and compensation were compared to planned doses to study the impact of the variable aperture on volumetric dose.ResultsThe jaw offset peak factor (JOPF) for the Synchrony jaw settings were 0.964 and 0.983 for the 1.0‐ and 2.5‐cm jaw settings, respectively. These values decreased to 0.925 and 0.982 for the fixed‐width settings, indicating that the peak value of the profile would decrease by 7.5% compared to centered if the aperture width was held constant. The IMRT dose distributions reveal similar results, where gamma pass rates are above tolerance for the Synchrony jaw settings but fall significantly for the fixed‐width 1‐cm jaws.ConclusionsThe variable‐width behavior of Synchrony jaws provides a larger output correction for the 1‐cm jaw setting. Without the variable‐aperture correction, plans with the 1‐cm jaw setting would underdose the target if the jaws spend a significant amount of time in the extreme positions. This work investigated the change in delivered dose with jaws in the extreme positions, therefore overall changes in dose due to offset jaws are expected to be less for composite treatment deliveries.

A tool for patient-specific prediction of delivery discrepancies in machine parameters using trajectory log files.

Multileaf collimator (MLC) delivery discrepancy between planned and actual (delivered) positions have detrimental effect on the accuracy of dose distributions for both IMRT and VMAT. In this study, we evaluated the consistency of MLC delivery discrepancies over the course of treatment and over time to verify that a predictive machine learning model would be applicable throughout the course of treatment. Next, the MLC and gantry positions recorded in prior trajectory log files were analyzed to build a machine learning algorithm to predict MLC positional discrepancies during delivery for a new treatment plan. An open source tool was developed and released to predict the MLC positional discrepancies at treatment delivery for any given plan. Trajectory log files of 142 IMRT plans and 125 VMAT plans from 9 Varian TrueBeam linear accelerators were collected and analyzed. The consistency of delivery discrepancy over patient-specific quality assurance (QA) and patient treatment deliveries was evaluated. Data were binned by treatment site and machine type to determine their relationship with MLC and gantry angle discrepancies. Motion-related parameters including MLC velocity, MLC acceleration, control point, dose rate, and gravity vector, gantry velocity and gantry acceleration, where applicable, were analyzed to evaluate correlations with MLC and gantry discrepancies. Several regression models, such as simple/multiple linear regression, decision tree, and ensemble method (boosted tree and bagged tree model) were used to develop a machine learning algorithm to predict MLC discrepancy based on MLC motion parameters. MLC discrepancies at patient-specific QA differed from those at patient treatment deliveries by a small (mean=0.0021±0.0036mm, P=0.0089 for IMRT; mean=0.0010±0.0016mm, P=0.0003 for VMAT) but statistically significant amount, likely due to setting the gantry angle to zero for QA in IMRT. MLC motion parameters, MLC velocity and gravity vector, showed significant correlation (P<0.001) with MLC discrepancy, especially MLC velocity, which had an approximately linear relationship (slope = -0.0027, P<0.001, R2 =0.79). Incorporating MLC motion parameters, the final generalized model trained by data from all linear accelerators can predict MLC discrepancy to a high degree of accuracy with high correlation (R2 =0.86) between predicted and actual MLC discrepancies. The same prediction results were found across different treatment sites and linear accelerators. We have developed a machine learning model using trajectory log files to predict the MLC discrepancies during delivery. This model has been a released as a research tool in which a DICOM-RT with predicted MLC positions can be generated using the original DICOM-RT file as input. This tool can be used to simulate radiotherapy treatment delivery and may be useful for studies evaluating plan robustness and dosimetric uncertainties from treatment delivery.

Differences in lung injury after IMRT or proton therapy assessed by 18FDG PET imaging

Background and purposeTo compare lung injury among non-small cell lung cancer (NSCLC) patients treated with IMRT or proton therapy as revealed by 18F-FDG post-treatment uptake and to determine factors predictive for clinically symptomatic radiation pneumonitis. Material and methodsFor 83 patients treated with IMRT or proton therapy, planning CT and follow up 18F-FDG PET-CT were analyzed. Post-treatment PET-CT was aligned with planning CT to establish a voxel-to-voxel correspondence between PET and planning dose images. 18F-FDG uptake as a function of radiation dose to normal lung was obtained for each patient. PET image-derived parameters as well as demographic, clinical, treatment and dosimetric patient characteristics were correlated with clinical symptoms of pneumonitis. ResultsThe dose distributions for the two modalities were significantly different; V5 was higher for IMRT, whereas V60 was higher for protons. The mean lung dose (MLD) was similar for the two modalities. The slope of linear 18F-FDG-uptake – dose response did not differ significantly between the two modalities. The MLD, slope, and 95th percentile of SUV were identified as three major factors associated with radiation pneumonitis. ConclusionsDespite significantly different dose distributions for IMRT and for protons, the slope of the SUV–dose linear regression line previously shown to be associated with RP did not differ between IMRT and protons. Patients who developed radiation pneumonitis had statistically significantly higher MLD and higher slope regardless of treatment modality.

Validation of the performance of 3-D detector arrays for radiotherapy verifications

Introduction Intensity modulated radiotherapy (IMRT/VMAT) is a complex procedure requiring proper dosimetric verification. IMRT dose distributions are characterized by high degree of modulation and by steep dose gradients. The fluence measurements for individual beams or at individual angle are not sufficient for evaluation of total dose distribution and for assuring patient safety. Purpose A number of electronic systems which allow for a pre-treatment plan verification based on reconstruction of the total dose distributions have been developed recently (Delta4, ArcCHECK, Octavius, etc.). A method of validation of such systems tested on the above listed devices is presented here. Materials and methods The method requires anthropomorphic phantoms and dosimetric films. In order to measure dose distributions in various cross-sections of the phantom the film dosimeters were used (Gafchromic EBT-3). The phantoms of the material and shape equivalent to the Delta4, ArcCkeck and Octavious detector arrays were developed. This film dosimetry methodology was used as a benchmark to test and validate the performance of commercially available 3-D verification systems. The gamma formalism was used for evaluation and comparison of the measured and calculated dose distributions. Results The results of comparisons of film measurements performed in test phantoms with dose distribution reconstructed with: Delta4, ArcCHECK and Octavious for a group of patient plans are presented as the percentage of passing points (PPP) for Γ ⩽ 1 criterion. Conclusion The 3-D verification systems and their reconstruction algorithms have to be validated with independent measurements or calculations before use in clinical practice.

SU‐E‐T‐319: Dosimetric Evaluation of IMRT with Mix‐Energy Beam for Deep Seated Targets

Purpose:IMRT is preferred in the range of 6–10MV X‐rays. Partially adding high energy (&gt;10MV) treatment fields, may provide advantage of both higher and lower energies. To study IMRT dose distribution obtained from treatment plans with single (6MV) and mixed‐energy (6MV and 15MV) for deep seated targets (separation more than 30cm).Methods:Five patients of carcinoma of cervix were studied using eclipse planning system. Two different dynamic IMRT plans were generated for Varian CL2300C/D linear accelerator; one is by using 6MV X‐ray with seven equally spaced coplanar beams. In second plan, 2 lateral oblique fields (gantry angle 102°, 255°) beam energy was modified to 15MV by keeping all other parameters and dose volume constraints constant. Dose prescription for the planning target volume (PTV) was (5040cGy/28f). For plan comparison, dose volume histogram (DVH) was used and PTV coverage index (CI=Target volume covered by prescription dose/Target volume), heterogeneity index (D5/D95), mean dose to organ at risk (OAR) and normal tissue integral dose (NTID, liter‐Gray) was also noted. Total monitor unit (MU) required to deliver a plan was also noted.Results:Mixed‐energy plan showed a better conformity and CI values were 0.942±0.032 and 0.960±0.040 for single‐energy and mixed‐energy plan, respectively. In addition, HI value of mixed energy beam is comparable to that of single energy and the values were within 1.084±0.034 and 1.082±0.032 for single energy and mixed‐energy plan, respectively. Variation in mean dose to bladder, rectum and bowel were within 1.05%, 0.87% and 0.90%. NTID was lesser for mixed‐energy beam due to use of two high‐energy fields. NTID were 1573.40±214.60 and 1510.20±249.80 litre‐Gray for single energy and mixed‐energy plan. MU needed to deliver a plan was similar in both plans and MUs were 238±45 and 237±47.Conclusion:Partial use of 15MV treatment fields in IMRT plan for deep seated targets showed dosimetric advantage over 6MV plan.

Development of a dose verification system for Vero4DRT using Monte Carlo method.

Vero4DRT is an innovative image‐guided radiotherapy system employing a C‐band X‐ray head with gimbal mechanics. The purposes of this study were to propose specific MC models of the linac head and multileaf collimator (MLC) for the Vero4DRT and to verify their accuracy. For a 6 MV photon beam delivered by the Vero4DRT, a simulation code was implemented using EGSnrc. The linac head model and the MLC model were simulated based on its specification. Next, the percent depth dose (PDD) and beam profiles at depths of 15, 100, and 200 mm were simulated under source‐to‐surface distance of 900 and 1000 mm. Field size was set to 150×150mm2 at a depth of 100 mm. Each of the simulated dosimetric metrics was then compared with the corresponding measurements by a 0.125 cc ionization chamber. After that, intra‐ and interleaf leakage, tongue‐and‐groove, and rounded‐leaf profiles were simulated for the static MLC model. Meanwhile, film measurements were performed using EDR2 films under similar conditions to simulation. The measurement for the rounded‐leaf profile was performed using the water phantom and the ionization chamber. The leaf physical density and abutting leaf gap were adjusted to obtain good agreement between the simulated intra‐ and interleaf leakage profiles and measurements. For the MLC model in step‐and‐shoot cases, a pyramid and a prostate IMRT field were simulated, while film measurements were performed using EDR2. For the linac head, exclusive of MLC, the difference in PDD was <1.0% after the buildup region. The simulated beam profiles agreed to within 1.3% at each depth. The MLC model has been shown to reproduce dose measurements within 2.5% for static tests. The MLC is made of tungsten alloy with a purity of 95%. The leaf gap of 0.015 cm and the MLC physical density of 18.0g/cm3, which provided the best agreement between the simulated and measured leaf leakage, were assigned to our MC model. As a result, the simulated step‐and‐shoot IMRT dose distributions agreed with the film measurements to within 3.3%, with exception of the penumbra region. We have developed specific MC models of the linac head and the MLC in the Vero4DRT system. The results have demonstrated that our MC models have high accuracy.PACS numbers: 87.55.K‐, 87.56.nk, 87.56.bd

TH‐C‐12A‐03: Development of Expanded Field Irradiation Technique with Gimbaled X‐Ray Head

Purpose:The Vero4DRT has a maximum field size of 150×150 mm2. The purposes of this study were to develop an expanded field irradiation technique using a unique gimbaled x‐ray head of Vero4DRT and to evaluate its dosimetric characteristic.Methods:The expanded field irradiation consisted of four separate fields with 2.39 degree gimbal rotation around orthogonal two axes. The central beam axis for each field shifted 40 mm from the isocenter for longitudinal and lateral directions, and thus, the field size was expanded up to 230×230 mm2. Adjacent region were created at the isocenter (center‐adjacent expanded‐field) and 20 mm from isocenter (offadjacent expanded‐field). To create flat dose distribution in the combined piecewise‐fields, the overlapping and gaps regions on the isocenter plane were adjusted with the gimbal rotating and the MLC. To evaluate dosimetric characteristic of the expanded‐field, films inserted in water‐equivalent phantoms at 50, 100 and 150 mm depth were irradiated and the field size, penumbra, flatness and symmetry were analyzed.In addition, the expandedfield irradiation technique was applied to IMRT. A head and neck IMRT field, which was planned for the conventional linac (Varian Clinac iX), was reproduced with the expanded‐field of the Vero4DRT. The simulated dose distribution for the expanded IMRT field was compared to the measured dose distribution.Results:The field size, penumbra, flatness and symmetry of center‐ and off‐ adjacent expanded‐fields were 230.2–232.1 mm, 7.8–10.7 mm, 2.3–6.5% and –0.5–0.4% at 100 mm depth. The 82.1% area of the expanded IMRT dose distribution was within 5% difference between measurement and simulation, which was analyzed upper 50% dose area, and the 3%/3 mm gamma pass rate was 98.4%.Conclusions:The expandedfield technique was developed using the gimbaled x‐ray head. To extend applied targets, such as whole breast irradiations or head and neck IMRT, the expanded‐field technique would be effective.

Automatic CT simulation optimization for radiation therapy: A general strategy

In radiation therapy, x-ray computed tomography (CT) simulation protocol specifications should be driven by the treatment planning requirements in lieu of duplicating diagnostic CT screening protocols. The purpose of this study was to develop a general strategy that allows for automatically, prospectively, and objectively determining the optimal patient-specific CT simulation protocols based on radiation-therapy goals, namely, maintenance of contouring quality and integrity while minimizing patient CT simulation dose. The authors proposed a general prediction strategy that provides automatic optimal CT simulation protocol selection as a function of patient size and treatment planning task. The optimal protocol is the one that delivers the minimum dose required to provide a CT simulation scan that yields accurate contours. Accurate treatment plans depend on accurate contours in order to conform the dose to actual tumor and normal organ positions. An image quality index, defined to characterize how simulation scan quality affects contour delineation, was developed and used to benchmark the contouring accuracy and treatment plan quality within the predication strategy. A clinical workflow was developed to select the optimal CT simulation protocols incorporating patient size, target delineation, and radiation dose efficiency. An experimental study using an anthropomorphic pelvis phantom with added-bolus layers was used to demonstrate how the proposed prediction strategy could be implemented and how the optimal CT simulation protocols could be selected for prostate cancer patients based on patient size and treatment planning task. Clinical IMRT prostate treatment plans for seven CT scans with varied image quality indices were separately optimized and compared to verify the trace of target and organ dosimetry coverage. Based on the phantom study, the optimal image quality index for accurate manual prostate contouring was 4.4. The optimal tube potentials for patient sizes of 38, 43, 48, 53, and 58 cm were 120, 140, 140, 140, and 140 kVp, respectively, and the corresponding minimum CTDIvol for achieving the optimal image quality index 4.4 were 9.8, 32.2, 100.9, 241.4, and 274.1 mGy, respectively. For patients with lateral sizes of 43-58 cm, 120-kVp scan protocols yielded up to 165% greater radiation dose relative to 140-kVp protocols, and 140-kVp protocols always yielded a greater image quality index compared to the same dose-level 120-kVp protocols. The trace of target and organ dosimetry coverage and the γ passing rates of seven IMRT dose distribution pairs indicated the feasibility of the proposed image quality index for the predication strategy. A general strategy to predict the optimal CT simulation protocols in a flexible and quantitative way was developed that takes into account patient size, treatment planning task, and radiation dose. The experimental study indicated that the optimal CT simulation protocol and the corresponding radiation dose varied significantly for different patient sizes, contouring accuracy, and radiation treatment planning tasks.

Comparison of Volumetric Modulated Arc Therapy (VMAT) and Standard IMRT: GI and GU Toxicity in the Pelvic Nodal Treatment of High-Risk Prostate Cancer

The irradiation of the pelvic nodes in prostate cancer has been discussed controversially for many years. Since the frequency of the gastrointestinal toxicity in patients receiving 4-field conventional or conformal whole pelvis radiation therapy is relevant, the indication for pelvic nodal irradiation is to be considered with caution. The aim of this study was to compare the toxicity in patients with high risk prostate cancer treated with an optimized pelvic Sliding window (SW) to the VMAT technique. Between September 2005 and September 2012, pelvic nodal IMRT to 50.4 Gy had been applied in 229 high-risk prostate cancer patients (pts). In 24% of the pts, an additional boost of 6 to 16 Gy was administered in cases of MRI-staged lymph node metastases. While 123 pts received a 7 or 8-field-IMRT sliding-window technique up to a median total dose to the prostate of 79.2 Gy/1.8 Gy, 108 pts were treated with VMAT technique up to a median total dose to the prostate of 81.0 Gy/1.8 Gy. Acute and late toxicities were prospectively scored by the RTOG/LENT-SOMA morbidity grading scales (until 2009) and a modified CTCAEv3.0 score (since 2009), respectively. The median follow- up time was 48 months (range, 3-87 months). The IMRT dose distribution provided excellent PTV coverage and satisfying protection of all the organs at risk, with less than 2% of all pts experiencing grade (G)3 toxicities but no G4 toxicities were observed. In total 41.8%/4.2%/0.5% of the pts developed acute G1/G2/G3 genitourinary (GU) toxicities. A total of 42.7%/3.7% of pts experienced acute G1/G2 gastrointestinal side effects, no patient developed acute >G2 gastrointestinal symptoms. Maximum late GU toxicities were G1/G2/G3 for 2.4%/0%/0.9% of the pts, respectively. Maximum late gastrointestinal toxicities were G1 for 4.9% of the pts, no patient experienced acute >G1 gastrointestinal symptoms. For acute GU and gastrointestinal toxicity, no significant differences could be seen between pts treated with SW vs VMAT: acute G1/G2/G3 GU toxicities have occurred in 40.4%/5.5%/0% vs 43.4%/2.8%/0.9% of the pts. A total of 44.3%/4.7% of the SW patients developed acute G1/G2 gastrointestinal toxicities compared to 42.4%/4.7% VMAT pts. Maximum late GU toxicities were G1/G2 for 3.2%/0% of SW pts and 4.8%/0% of RA pts, respectively. A total of 1.9% of the SW pts developed late G3 GU toxicities compared to no pts in the VMAT group. Maximum late gastrointestinal toxicities were G1 for 7.6% of SW and VMAT pts, no late >G1 GU symptoms could be observed in either group so far. These data demonstrate that pelvic nodal IMRT is a well-tolerated technique in patients with high-risk prostate cancer. The overall low frequency and severity of side effects do not seem to differ significantly in SW and VMAT technique despite a further dose escalation in the VMAT group by 3.5%.

TH‐F‐105‐01: BEST IN PHYSICS (THERAPY) — The Emergence of Non‐Uniform Spatio‐Temporal Fractionation Schemes Within the Standard BED Model

Purpose: We consider the fractionation problem in intensity‐modulated proton therapy (IMPT). It is known that a low α/β ratio in the normal tissue favors a hyperfractionated regimen. In contrast, a low α/β ratio in the tumor gives rise to hypofractionation. However, in current clinical practice, the patient is treated with the same dose distribution in each fraction. We demonstrate that the standard biologically equivalent dose (BED) model gives rise to non‐uniform fractionation schemes, i.e. for an intermediate range of α/β ratios, it can be optimal to deliver distinct dose distributions in different fractions. Methods: We consider the simultaneous optimization of IMPT dose distributions and the fractionation scheme. This is achieved by simultaneously optimizing multiple (possibly distinct) IMPT plans for different fractions. To that end, we consider an objective function based on the cumulative BED distribution delivered over all fractions. Results: It is shown that the optimal treatment regimen may consist of distinct IMPT plans. For proton therapy this has an intuitive explanation: The dose in the entrance region of the proton beam is mostly independent of the range. Thus, an incident proton beam can primarily irradiate the distal part of the tumor in one fraction, and the proximal part in the subsequent fraction ‐ without significantly changing the dose in the normal tissue. This can be interpreted as the possibility of hypofractionating subregions of the tumor while continuing to hyperfractionate in the normal tissue. Conclusion: To date, the determination of the fractionation scheme and the optimization of the spatial dose distribution in IMRT and IMPT are considered independently. Here, we demonstrate that both problems are interdependent. In particular, the therapeutic ratio can potentially be improved by delivering distinct dose distributions in different fractions.

Effect of metallic expander on dose distributions for breast IMRT

Effect of metallic expander on dose distributions for breast IMRT

SU‐C‐108‐04: To Better Understand IMRT and Planned Dose Distributions: A Measure of Entropy

Purpose: Traditional IMRT QA procedures yield close approximations to the patients planned dose distribution. However, the relationship between QA data and the patients planned distribution is difficult to quantify. This study presents an approach to investigate the relationship between a planar QA distribution and the same doseplane within the patient using complexity analysis. Methods: Shannon entropy was the decided metric to quantify the complexity of three calculated dose distributions: 1) the IMRT QA distribution, 2) the in‐patient dose distributions with heterogeneity correction on, and 3) the in‐patient dose distribution without heterogeneity correction. Varian Eclipse treatment planning system was used to construct the plans and calculate dose. MapCheck 1175 diode array was used for the IMRT QA acquisition. Global entropy was calculated without threshold for each distribution, and local entropies were found for low dose thresholds ranging from 0% to 100% of both prescribed and maximum dose. All calculations were done using MATLAB. Results: The QA data proved to have the highest global entropy of the three dose distributions analyzed. Local entropies, calculated using subsets of all dose points, converged at a low dose threshold of &gt;∼70% of maximum dose. For low dose thresholds &lt;∼70%, the entropies of both patient distributions remained similar, while the entropy for the QA distribution diverged, increasing in value. Conclusion: The greater entropy values for the QA distribution at low dose thresholds &lt;∼70% suggests a more disordered dose distribution than the two planned dose distributions. This is thought to be due to the larger phantom size with respect to the patient, which tends to spread out the dose gradient outside the target. This Result is suggestive that an optimal threshold for meaningful gamma analysis could be &gt;70% of maximum dose, or where the QA and planned dose distributions entropies, or complexities, are accurately matched.

The Influence of Volume Effect in 2D-array Ion Chamber on the Measurement of IMRT Dose Distribution

‡‡ † † , † ‡ We evaluated the influence of volume effect on the measurement of IMRT dose distribution by comparing a 2D-array ion chamber and other dosimeters. Matrix phantom which is a 2D-array ion chamber having volume effect was compared with beam image system and film for the mea surement of dose distribution. Five intensity-modulated radiation therapy plans were created using five fields in thevirtual phantom. The measured dose distribution was compared with the calculated one by radiation treatment planning system and analysis program. We evaluated the conformity of dose distribution by calculating correlation coefficients and gamma values. The highest error rate of 1.3% was associated with matrix phantom in which volume effect in small field sizes was substantial.

Clinical evaluation of a commercial orthopedic metal artifact reduction tool for CT simulations in radiation therapy

Severe artifacts in kilovoltage-CT simulation images caused by large metallic implants can significantly degrade the conspicuity and apparent CT Hounsfield number of targets and anatomic structures, jeopardize the confidence of anatomical segmentation, and introduce inaccuracies into the radiation therapy treatment planning process. This study evaluated the performance of the first commercial orthopedic metal artifact reduction function (O-MAR) for radiation therapy, and investigated its clinical applications in treatment planning. Both phantom and clinical data were used for the evaluation. The CIRS electron density phantom with known physical (and electron) density plugs and removable titanium implants was scanned on a Philips Brilliance Big Bore 16-slice CT simulator. The CT Hounsfield numbers of density plugs on both uncorrected and O-MAR corrected images were compared. Treatment planning accuracy was evaluated by comparing simulated dose distributions computed using the true density images, uncorrected images, and O-MAR corrected images. Ten CT image sets of patients with large hip implants were processed with the O-MAR function and evaluated by two radiation oncologists using a five-point score for overall image quality, anatomical conspicuity, and CT Hounsfield number accuracy. By utilizing the same structure contours delineated from the O-MAR corrected images, clinical IMRT treatment plans for five patients were computed on the uncorrected and O-MAR corrected images, respectively, and compared. Results of the phantom study indicated that CT Hounsfield number accuracy and noise were improved on the O-MAR corrected images, especially for images with bilateral metal implants. The γ pass rates of the simulated dose distributions computed on the uncorrected and O-MAR corrected images referenced to those of the true densities were higher than 99.9% (even when using 1% and 3 mm distance-to-agreement criterion), suggesting that dose distributions were clinically identical. In all patient cases, radiation oncologists rated O-MAR corrected images as higher quality. Formerly obscured critical structures were able to be visualized. The overall image quality and the conspicuity in critical organs were significantly improved compared with the uncorrected images: overall quality score (1.35 vs 3.25, P = 0.0022); bladder (2.15 vs 3.7, P = 0.0023); prostate and seminal vesicles∕vagina (1.3 vs 3.275, P = 0.0020); rectum (2.8 vs 3.9, P = 0.0021). The noise levels of the selected ROIs were reduced from 93.7 to 38.2 HU. On most cases (8∕10), the average CT Hounsfield numbers of the prostate∕vagina on the O-MAR corrected images were closer to the referenced value (41.2 HU, an average measured from patients without metal implants) than those on the uncorrected images. High γ pass rates of the five IMRT dose distribution pairs indicated that the dose distributions were not significantly affected by the CT image improvements. Overall, this study indicated that the O-MAR function can remarkably reduce metal artifacts and improve both CT Hounsfield number accuracy and target and critical structure visualization. Although there was no significant impact of the O-MAR algorithm on the calculated dose distributions, we suggest that O-MAR corrected images are more suitable for the entire treatment planning process by offering better anatomical structure visualization, improving radiation oncologists' confidence in target delineation, and by avoiding subjective density overrides of artifact regions on uncorrected images.

Comparison of dose distribution in IMRT and RapidArc technique in prostate radiotherapy

AimThe aim was to provide a dosimetric comparison between IMRT and RapidArc treatment plans with RPI index with simultaneous comparison of the treatment delivery time. BackgroundIMRT and RapidArc provide highly conformal dose distribution with good sparing of normal tissues. However, a complex spatial dosimetry of IMRT and RapidArc plans hampers the evaluation and comparison between plans calculated for the two modalities. RPI was used in this paper for treatment plan comparisons. The duration of the therapeutic session in RapidArc is reported to be shorter in comparison to therapeutic time of the other dynamic techniques. For this reasons, total treatment delivery time in both techniques was compared and discussed. Materials and methods15 patients with prostate carcinoma were randomly selected for the analysis. Two competitive treatment plans using respectively the IMRT and RapidArc techniques were computed for each patient in Eclipse planning system v. 8.6.15. RPIwin® application was used for RPI calculations for each treatment plan.Additionally, total treatment time was compared between IMRT and RapidArc plans. Total treatment time was a sum of monitor units (MU) for each treated field. ResultsThe mean values of the RPI indices were insignificantly higher for IMRT plans in comparison to rotational therapy. Comparison of the mean numbers of monitor units confirmed that the use of rotational technique instead of conventional static field IMRT can significantly reduce the treatment time. ConclusionAnalysis presented in this paper, demonstrated that RapidArc can compete with the IMRT technique in the field of treatment plan dosimetry reducing the time required for dose delivery.