Terms Of Mu Research Articles

Sub-arc collimator angle optimization based on the conformity index heatmap for VMAT planning of multiple brain metastases SRS treatments

ObjectiveThe aim of this study was to investigate the impact of collimator angle optimization in single-isocenter coplanar volume modulated arc therapy (VMAT) stereotactic radiosurgery (SRS) for multiple metastases with respect to dosimetric quality and treatment delivery efficiency. In particular, this is achieved by a novel algorithm of sub-arc collimator angle optimization (SACAO).MethodsTwenty patients with multiple brain metastases were retrospectively included in this study. A multi-leaf collimator (MLC) conformity index (MCI) that is defined as the ratio of the area of target projection in the beam’s eye view (BEV) to the related area fitted by MLC was applied. Accordingly, for each control point, 180 MCI values were calculated with a collimator angle interval of 1°. A two-dimensional heatmap of MCI as a function of control point and collimator angle for each full arc was generated. The optimal segmentation of sub-arcs was achieved by avoiding the worst MCI at each control point. Then, the optimal collimator angle for each sub-arc would be determined by maximizing the summation of MCI. Each patient was scheduled to undergo single-center coplanar VMAT SRS based on either the novel SACAO algorithm or the conventional VMAT with static collimator angle (ST-VMAT). The dosimetric parameters, field sizes, and the monitoring units (Mus) were evaluated.ResultsThe mean dose-volumetric parameters for the target volume of SACAO were comparable to ST-VMAT, while the conformity index (CI), homogeneity index (HI), and gradient index (GI) were reduced by SACAO. Improved sparing of organs at risk (OARs) was also obtained by SACAO. In particular, the SACAO method significantly (p < 0.01) reduced the field size (76.59 ± 32.55 vs. 131.95 ± 56.71 cm2) and MUs (655.35 ± 71.99 vs. 729.85 ± 73.52) by 41.11%.ConclusionsThe SACAO method could be superior in improving the CI, HI, and GI of the targets as well as normal tissue sparing for multiple brain metastases SRS. In particular, SACAO has the potential of increasing treatment efficiency in terms of field size and MU.

Breast-Conserving Radiotherapy with Simultaneous Integrated Boost—A Dosimetric Comparison of 3DCRT, VMAT and IMRT: Do We Really have a Better Plan?

The availability of more advanced technology like hypo-fractionation has the potential of being the new standard of care in breast cancer. This study evaluates whether 3DCRT with a field-in-field technique (FIF) and a simultaneous integrated boost (SIB) could provide a dosimetrically comparable plan delivered to VMAT or IMRT. 3DCRT-FIF-SIB, VMAT and IMRT-SIB plans were generated for 20 patients. The plans were compared for planning target volume coverage (PTV 95), homogeneity and conformity, dose delivered to lungs, heart and C/L breast. 3DCRT FIF provided better sparing of C/L breast V1 and V5, whole lung V5, p = 0.000. The VMAT plans spared heart V30: (0.1 ± 0.46 vs. 11.5 ± 18.3) p = 0.000 and I/L lung V20: (19.3 ± 5 vs. 32.2 ± 11.1) p = 0.000. It provided a better coverage V95: (97 ± 0.8 vs. 95 ± 2.9) p = 0.002 and sparing of the heart V30: (0.1 ± 0.5 vs. 8.6 ± 11.5) p < 0.002 and lungs I/L V20: (19.3 ± 5.0 vs. 30.7 ± 6.1) p = 0.000. The treatment was faster with less exposure in terms of MU: (529 ± 57.8 vs. 1024 ± 298) p = 0.000. 3DCRT provides a dosimetrically acceptable alternative to more advanced technologies. VMAT and IMRT provide better sparing of heart and lungs. VMAT has a slight benefit of conformity, reduced exposure and shorter treatment time.

Linac-based VMAT radiosurgery for multiple brain lesions: comparison between a conventional multi-isocenter approach and a new dedicated mono-isocenter technique

BackgroundLinac-based stereotactic radiosurgery or fractionated stereotactic radiotherapy (SRS/FSRT) of multiple brain lesions using volumetric modulated arc therapy (VMAT) is typically performed by a multiple-isocenter approach, i.e. one isocenter per lesion, which is time-demanding for the need of independent setup verifications of each isocenter. Here, we present our initial experience with a new dedicated mono-isocenter technique with multiple non-coplanar arcs (HyperArc™, Varian Inc.) in terms of a plan comparison with a multiple-isocenter VMAT approach.MethodsFrom August 2017 to October 2017, 20 patients with multiple brain metastases (mean 5, range 2–10) have been treated by HyperArc in 1–3 fractions. The prescribed doses (Dp) were 18–25 Gy in single-fraction, and 21–27 Gy in three-fractions. Planning Target Volume (PTV), defined by a 2 mm isotropic margin from each lesion, had mean dimension of 9.6 cm3 (range 0.5–27.9 cm3). Mono-isocenter HyperArc VMAT plans (HA) with 5 non-coplanar 180°-arcs (couch at 0°, ±45°, ±90°) were generated and compared to multiple-isocenter VMAT plans (RA) with 2 coplanar 360°-arcs per isocenter. A dose normalization of 100%Dp at 98%PTV was adopted, while D2%(PTV) < 150%Dp was accepted. All plans had to respect the constraints on maximum dose to the brainstem (D0.5cm3 < 18 Gy) as well as to the optical nerves/chiasm, eyes and lenses (D0.5cm3 < 15 Gy). HA and RA plans were compared in terms of dose-volume metrics, by Paddick conformity (CI) and gradient (GI) index and by V12 and mean dose to the brain-minus-PTV, and in terms of MU and overall treatment time (OTT) per fraction. OTT was measured for HA treatments, whereas for RA plans OTT was estimated by assuming 3 min. For initial patient setup plus 5 min. For each CBCT-guided setup correction per isocenter.ResultsSignificant variations in favour of HA plans were computed for both target dose indexes, CI (p < .01) and GI (p < .01). The lower GI in HA plans was the likely cause of the significant reduction in V12 to the brain-minus-PTV (p = .023). Although at low doses, below 2–5 Gy, the sparing of the brain-minus-PTV was in favour of RA plans, no significant difference in terms of mean doses to the brain-minus-PTV was observed between the two groups (p = .31). Finally, both MU (p < .01) and OTT (p < .01) were significantly reduced by HyperArc plans.ConclusionsFor linac-based SRS/FSRT of multiple brain lesions, HyperArc plans assured a higher CI and a lower GI than standard multiple-isocenter VMAT plans. This is consistent with the computed reduction in V12 to the brain-minus-PTV. Finally, HyperArc treatments were completed within a typical 20 min. time slot, with a significant time reduction with respect to the expected duration of multiple-isocenters VMAT.

Does MLC With Finer Leaf Width Improve the Treatment Plan Quality in Stereotactic Body Radiation Therapy of Lung Cancers

To investigate the impact of leaf width on treatment plan quality in stereotactic body radiation therapy (SBRT) of lung cancers. 4DCT scans were acquired for 10 patients. The GTV was delineated on the maximum intensity projection CT, and it was expended by a uniform margin of 5mm to create the PTV. Volumetric Modulated Arc Therapy (VMAT) treatment plans were generated on the average intensity projection CT. The prescription dose was 48 Gy delivered in 4 fractions. For each patient two VMAT plans were created: the first plan (Plan1cm leave) used the 1 cm leaf width MLCi on Elekta Synergy, and the second plan (Plan0.5cm leave) utilized the 0.5 cm leaf width Agility MLC on Elekta Infinity. The output factors, beam energy and profiles of the two treatment machines were matched to within 1%. The two VMAT plans were analyzed and compared using the following dosimetric parameters: For target dose coverage, Quality of Coverage (QI), Homogeneity Index (HI), and Conformity Index (CI) were calculated; For organs at risk, the maximum dose of cord, heart and ribs, and the mean/V5/V10/V20 for the total lung dose were analyzed; The treatment time in terms of MU was also compared. For the dose coverage of GTV, the averaged CI, HI and QI of Plan1cm leave and Plan0.5cm leave are 0.980±0.046 / 0.970±0.043, 1.079±0.053 / 1.084±0.056, and 0.915±0.218 / 0.960±0.092, respectively. The corresponding p-values between the two plans are 0.250, 0.245 and 0.442. For PTV dose coverage, the averaged CI, HI and QI of Plan1cm leave and Plan0.5cm leave are 0.949±0.079 / 0.953±0.076, 1.142±0.083 / 1.142±0.085 and 0.865±0.141 / 0.862±0.154, respectively. The corresponding p-values are 0.465, 0.943 and 0.594 respectively. The mean MU is 2334.3±395.0 for Plan1cm leave and 2442.8±548.5 Plan0.5cm leave, with the p-value of 0.135. For the total lung, the mean lung dose, V5, V10, and V20 of Plan1cm leave are 4.53±1.88 Gy, 24.83±10.70%, 13.97±6.00%, 6.64±3.53% respectively. The corresponding values for Plan0.5cm leave are 4.48±1.79Gy, 25.06±11.53%, 14.27±6.27%, 6.67±3.65% respectively. The p-values for mean lung dose, V5, V10, and V20 are 0.825, 0.751, 0.410, and 0.864 respectively. The averaged maximum dose for cord, heart and ribs are 8.69±4.40, 16.84±13.68, 42.31±4.06 Gy respectively for Plan1cm leave versus 10.29±5.03, 16.46±13.68, 41.67±3.62 for Plan0.5cm leave. The corresponding p-values are 0.034, 0.530 and 0.322 respectively. Treatment plan quality using 0.5 cm leave width MLC is not statically better than the plan with 1 cm leave MLC. This can be explained by the fact that in most SBRT lung cases the tumor has smooth edge and both 1 cm and 0.5 cm leave MLCs are able to conform to the target edges smoothly. However, the maximum cord dose with Agility MLC is statistically lower than that with MLCi because Agility MLC offers lower MLC transmission.

SU‐E‐T‐825: Influence of the Multileaf Collimator Step Size in Step&amp;shoot IMRT for Head&amp;neck Treatments

Purpose: The purpose was to analyze the influence of the step size during head&amp;neck IMRT treatments. Methods: Ten patients with head&amp;neck IMRT treatment plan were selected. Treatment plans were done using nine 6MV photon‐beams on a Primus linac (Siemens) with Optifocus MLC (10mm leaf width) and a dose rate of 200MU/min. The TPS used was Konrad v2.2 (Siemens) with leaves step size of 10, 5 and 3mm. For each step‐size, plans were evaluated in terms of MU, segments number, treatment time, PTVˈs (66Gy and 50Gy) dose uniformity and OAR (parotids and spinal cord) doses. Variations between measured and calculated total plan doses were obtained using solid water phantom and PTW PinPoint ionization chamber. Results: The MU number for 10mm step‐size plans was 624 [495,770] and increased 51% [36%, 87%] for 5mm and 124% [93%, 171%] for 3mm. The segments number for 10mm step‐size plans was 111 [99,129] and increased 41% [27%, 52%] for 5mm and 102% [71%, 125%] for 3mm. The treatment time for 10mm step‐size plans was 11.47min [11.08min, 12.45min] and increased 24% [9%, 33%] for 5mm and 65% [27%, 74%] for 3mm. Reducing the step‐size from 10mm to 5mm leads to better PTV_66Gy dose uniformity within ICRU50 tolerance (no difference observed between 5mm and 3mm). Step‐size diminution from 10mm to 3mm leads to a better coverage of PTV_50Gy especially in patient surface regions. For all the studied plans there are no differences in the parotids mean doses and spinal cord maximum dose. The variation between measured and calculated doses was 1.1% for 10mm step‐size. It increased 1.7% and 2.4% for 5mm and 3mm respectively. Conclusions: Using 10mm step‐size produces plans with adequate OAR doses. PTV Dose uniformity and volume coverage could be improved by 5mm step‐size whereas 3mm step‐size produces larger treatment times without advantages in dose distribution.

SU-E-T-797: Effects of the Number of Intensity Levels in Step&amp;shoot IMRT for Prostate Treatments

Purpose: Step&shoot IMRTTreatment planning requires the conversion of an optimized fluence into deliverable sequences of MLC segments. The objective of this paper is to study the effects of varying the intensity levels (IL's) number in prostate step&shoot IMRT treatments. Methods: Patient data with prostate cancer and IMRT treatment indication was selected. Treatment planning was done using a 6MV 7‐field arrangement (Primus linac, Optifocus MLC, 10mm leaf‐width) and Konrad v2.2 (Siemens) TPS. The treatment plan includes the simultaneous irradiation of the prostate to 82Gy and the seminal vesicles to 64Gy in 41 fractions. Four plans were generated using 5, 7, 10 and 15 IL's and normalized to deliver the same minimum dose to the prostate. Comparisons between plans were done in terms of total segment number, MU, treatment time and DVH for PTVs and OARs (rectum, bladder and femoral heads). Results: The total segment numbers were 44, 60, 84 and 116 from 5 to 15 IL's. The MU's total numbers decreased with the increase of the IL's (527, 522, 518 and 508). Increasing the number of IL's was accompanied by the treatment times raise (11%, 23% and 37% in relation to 5 IL's, 387seg) and resulted in increases of Prostate PTV dose homogeneity and conformity index (CI). The maximum dose (CI) was 87.7Gy (0.957) and 89.7Gy (0.836) for 15 and 5 IL respectively. No difference in minimum dose and CI was found for the seminal vesicle PTV. All plans showed insignificant OAR DVH differences. Conclusions: The increment in the IL's number resulted in an improvement of the Prostate PTV homogeneity and treatment time without any extra sparing to OAR. The benefit of a better uniform dose in the Prostate PTV should be analyzed for each treatment in order to set planning guidelines.

Searching for chameleon-like scalar fields with the ammonia method

(Abridged) In our previous work we found a statistically significant offset Delta V = 27 m/s between the radial velocities of the HC3N(2-1) and NH3(1,1) transitions observed in molecular cores from the Milky Way. This may indicate that the electron-to-proton mass ratio, mu = m_e/m_p, increases by 3x10^{-8} when measured under interstellar conditions with matter densities of more than 10 orders of magnitude lower as compared with laboratory (terrestrial) environments. We now map four molecular cores L1498, L1512, L1517, and L1400K selected from our previous sample in order to estimate systematic effects in Delta V due to possible velocity gradients. We find that in two cores L1498 and L1512 the NH3(1,1) and HC3N(2-1) transitions closely trace the same material and show an offset of Delta V = 26.9 +/- 1.2_stat +/- 3.0_sys m/s throughout the entire clouds. The measured velocity offset, being expressed in terms of Delta mu = (mu_obs - mu_lab)/mu_lab, gives Delta mu = (26 +/- 1_stat +/- 3_sys)x10^{-9}.

SU‐FF‐T‐393: Investigation of Varian's Trilogy LINAC to Deliver Fractional MU

Purpose: To evaluate whether the Trilogy can correctly deliver the fractional MU specified by IMRT plan. Method: We designed a simple dynamic MLC file with 10 step‐and‐shoot IMRT segments to run on a Trilogy to measure dose‐per‐MU for each individual MLC segment. MLC leaves were open during beam‐On segments. After each segment, some of the MLC leaves were programmed to move back‐and‐forth which forced the Trilogy to hold off the beam for about 5 seconds until next beam‐On segment. During this time the dose delivered in the previous segment was recorded using a Farmer chamber and electrometer. 6 MV photon beam with a 400MU/min was used in this experiment. Results: The Trilogy demonstrated excellent dose linearity (r2 = 0.999997), in terms of total dose vs. total MU for each beam, for both static and dynamic MLC beams. Further data analysis on dose per fractional MU in individual segments revealed that the Trilogy tends to over‐shoot in the first segment before stabilizing. To compensate for the overdosing in the first segment, the last segment of a beam is always underdosing because the Trilogy monitor chamber integration circuit stops the beam when it reaches the programmed MU, regardless of the dose delivered in the individual MLC segments. Dose per MU delivered for the segments in the middle of the beam is relatively stable. Conclusions: Although one can not program the Trilogy from the treatment console to run a fractional MU, the Trilogy has the ability to deliver fractional MU during dynamic MLC treatment such as IMRT. It appears that Trilogy has a tendency of overdosing the first segment and underdosing the last segment. Relative error can be 20 to 30% when the segment MU is less than 3.

Construction of a generalized gradient approximation by restoring the density-gradient expansion and enforcing a tight Lieb–Oxford bound

Recently, a generalized gradient approximation (GGA) to the density functional, called PBEsol, was optimized (one parameter) against the jellium-surface exchange-correlation energies, and this, in conjunction with changing another parameter to restore the first-principles gradient expansion for exchange, was sufficient to yield accurate lattice constants of solids. Here, we construct a new GGA that has no empirical parameters, that satisfies one more exact constraint than PBEsol, and that performs 20% better for the lattice constants of 18 previously studied solids, although it does not improve on PBEsol for molecular atomization energies (a property that neither functional was designed for). The new GGA is exact through second order, and it is called the second-order generalized gradient approximation (SOGGA). The SOGGA functional also differs from other GGAs in that it enforces a tighter Lieb-Oxford bound. SOGGA and other functionals are compared to a diverse set of lattice constants, bond distances, and energetic quantities for solids and molecules (this includes the first test of the M06-L meta-GGA for solid-state properties). We find that classifying density functionals in terms of the magnitude mu of the second-order coefficient of the density gradient expansion of the exchange functional not only correlates their behavior for predicting lattice constants of solids versus their behavior for predicting small-molecule atomization energies, as pointed out by Perdew and co-workers [Phys. Rev. Lett. 100, 134606 (2008); Perdew ibid. 80, 891 (1998)], but also correlates their behavior for cohesive energies of solids, reaction barriers heights, and nonhydrogenic bond distances in small molecules.

The minimum effort required to eradicate infections in models with backward bifurcation

We study an epidemiological model which assumes that the susceptibility after a primary infection is r times the susceptibility before a primary infection. For r = 0 (r = 1) this is the SIR (SIS) model. For r > 1 + (mu/alpha) this model shows backward bifurcations, where mu is the death rate and alpha is the recovery rate. We show for the first time that for such models we can give an expression for the minimum effort required to eradicate the infection if we concentrate on control measures affecting the transmission rate constant beta. This eradication effort is explicitly expressed in terms of alpha,r, and mu As in models without backward bifurcation it can be interpreted as a reproduction number, but not necessarily as the basic reproduction number. We define the relevant reproduction numbers for this purpose. The eradication effort can be estimated from the endemic steady state. The classical basic reproduction number R0 is smaller than the eradication effort for r > 1 + (mu/alpha) and equal to the effort for other values of r. The method we present is relevant to the whole class of compartmental models with backward bifurcation.

Revisiting Inhibitory Control Across the Life Span: Insights From the Ex-Gaussian Distribution

Changes in inhibitory control occur across the life span and have been associated with alterations in prefrontal function. In this study, ex-Gaussian analysis was used to reexamine data from an inhibitory control task. Participants (ages 6 to 82 years) composed three groups: children, young adults, and older adults. In fitting the ex-Gaussian distribution to reaction time data, estimates of three parameters were obtained: mu (μ), reflecting average performance; sigma (σ), reflecting variability in performance; and tau (τ), reflecting extremes in performance. Older adults differed from young adults in terms of mu, sigma, and tau. For children, mu and tau values were comparable to those of young adults; sigma, however, was different. Thus, inhibitory changes in older adults were due to slower, more variable, and more extreme responding. Inhibitory changes in children were due only to more variable responding. These findings suggest that different mechanisms underlie age-related changes in inhibitory control during different epochs of the life span. This study demonstrates that the ex-Gaussian approach provides a finer level of analysis than data analytic approaches typically used in neuropsychological research.

Properties of the Preisach model for hysteresis.

As shown by Krasnosel skii, the classical Preisach model allows to construct a hysteresis operator Wbetween spaces of real functions of time. This construction, via the definition of a measure mu in the so-called Preisach plane, is recalled. Characterizations in terms of mu are given for several mapping and continuity properties of W in various function spaces, for the invertibility of W and for the corresponding mapping and continuity properties of the inverse.

Solutions of the Static Theory Integral Equations for Pion-Nucleon Scattering in the One-Meson Approximation

Numerical solutions of the one-meson approximation of the Low equations for elastic pion-nucleon scattering in the fixed-nucleon, extended-source theory are obtained with a Gaussian cutoff function. The validity of the method requires that the scattering amplitudes have no zeros in the complex plane other than at z = plus or minus 1. The functions obtained for the (3,3) and (1,1) states satisfy the Low equations within the accuracy of the method, but the (1,3) and (3,1) states are only approximately given. This difficulty with the (1,3) and (3,1) states is correlated with the development of a zero in the corresponding scattering amplitude well before physically interesting values of the parameters (coupling constant and cutoff) are reached. A best fit to the (3,3)-state data up to 170-Mev pion laboratory energy requires a coupling constant, f/sup 2/, less than 0.08. The solution is consistent with the (3,1)- state data, but gives a (1,1)-state phase shift of larger magnitude than experiment appears to permit. It is found that the cutoff function does not prevent strong interactions at very high energies. Their occurrence appears to be a property of the static model. The contributions of such interactions to various static-theory calculations is briefiy discussed.more » It is shown that in the p-wave part of the relativistic dispersion relations, where there is no cutoff function, if use is made of the low-energy data, then the very high-energy contributions of the static theory are replaced by recoil terms of order mu /M. (auth)« less