Multiple beam two-plasmon decay: linear threshold to nonlinear saturation in three dimensions.

The linear stability of multiple coherent laser beams with respect to two-plasmon-decay instability in an inhomogeneous plasma in three dimensions has been determined. Cooperation between beams leads to absolute instability of long-wavelength decays, while shorter-wavelength shared waves are shown to saturate convectively. The multibeam, in its absolutely unstable form, has the lowest threshold for most cases considered. Nonlinear calculations using a three-dimensional extended Zakharov model show that Langmuir turbulence created by the absolute instability modifies the convective saturation of the shorter-wavelength modes, which are seen to dominate at late times.

Nonlinear development of stimulated Raman scattering from electrostatic modes excited by self-consistent non-Maxwellian velocity distributions.

The parametric coupling involving backward stimulated scattering of a laser and electron beam acoustic modes (BAM) is described as observed in particle-in-cell (PIC) simulations. The BAM modes evolve from Langmuir waves (LW) as the electron velocity distribution is nonlinearly modified to be non-Maxwellian by backward stimulated Raman scattering (BSRS). With a marginal damping rate, BAM can be easily excited and allow an extended chirping in frequency to occur as later SRS pulses encounter modified distributions. Coincident with the emergence of this non-Maxwellian distribution is a rapid increase in BSRS reflectivities with laser intensities. Both the reflectivity scaling with laser intensity and the observed spectral features from PIC simulations are consistent with recent Trident experiments.

Nonlinear spectral signatures and spatiotemporal behavior of stimulated Raman scattering from single laser speckles.

Simulations are reported of the Thomson scatter spectrum of electrostatic waves (ESWs) excited in single laser hot spots by backward stimulated Raman scattering (BSRS). Under conditions similar those in the recent experiments of Kline et al. [Phys. Rev. Lett. 94, 175003 (2005)], a spectral streak, resulting from the trapping-induced frequency shift of the ESW, is found for high wave-number ESWs, similar to the observations. This shift and parametric frequency matching lead to isolated BSRS pulses. Modes with acoustic dispersion, resulting from the trapping-modified electron velocity distribution, can enhance the frequency range of the streak.

Observation of a transition from fluid to kinetic nonlinearities for langmuir waves driven by stimulated Raman backscatter.

Thomson scattering is used to measure Langmuir waves (LW) driven by stimulated Raman scattering (SRS) in a diffraction limited laser focal spot. For SRS at wave numbers klambda(D) less similar 0.29, where k is the LW number and lambda(D) is the Debye length, multiple waves are detected and are attributed to the Langmuir decay instability (LDI) driven by the primary LW. At klambda(D) greater similar 0.29, a single wave, frequency-broadened spectrum is observed. The transition from the fluid to the kinetic regime is qualitatively consistent with particle-in-cell simulations and crossing of the LDI amplitude threshold above that for LW self-focusing.

Transient enhancement and detuning of laser-driven parametric instabilities by particle trapping.

Kinetic simulations of backward stimulated Raman scattering (BSRS), where the Langmuir wave coherence time is greater than the bounce time for trapped electrons, yield transient reflectivity levels far above those predicted by fluidlike models. Electron trapping reduces the Langmuir wave damping and lowers the Langmuir wave frequency, and leads to a secular phase shift between the Langmuir wave and the BSRS beat ponderomotive force. This phase shift detunes and saturates BSRS and a similar effect, due to ion trapping, is the saturation mechanism for backward stimulated Brillouin scattering. Competition with forward SRS is discussed.

3/2omega0 radiation from the laser-driven two-plasmon decay instability in an inhomogeneous plasma.

We present the results of the first reduced model simulations of the nonlinear development of the two-plasmon decay instability in an inhomogeneous plasma, including properties of the 3/2 harmonic emission. A sharp increase in radiation and Langmuir turbulence fluctuation levels occurs above a threshold laser intensity that depends on initial fluctuation levels. We study the competition between the linear propagation of Langmuir waves in the density gradient and the nonlinear saturation due to the Langmuir decay instability. The secondary decay Langmuir waves can provide the dominant source of the radiation and are essential to explain experiments.

CT and MRI derived source localization error in a custom prostate phantom using automated image coregistration.

Dosimetric evaluation of completed brachytherapy implant procedures is crucial in developing proper technique. Additionally, accurate dosimetry may be useful in predicting the success of an implant. Accurate definition of the prostate gland and localization of the implanted radioactive sources are critical to attain meaningful dosimetric data. MRI is recognized as a superior imaging modality in delineating the prostate gland. More importantly, MRI can be used for source localization in postimplant prostates. However, the MRI derived source localization error bears further investigation. We present a useful tool in determining the source localization error as well as permitting the fusion, or coregistration, of selected data from multiple imaging modalities. We constructed a custom prostate phantom of hydrocolloid material precisely implanted with I-125 seeds. We obtained CT, the accepted modality, and MRI scans of the phantom. Subsequently, we developed an automated algorithm that employs a sequential translation of data sets to initially maximize coregistration and minimize error between data sets. This was followed by a noniterative solution for the necessary rotation transformation matrix using the Orthogonal Procrustes Solution. We applied this algorithm to CT and MRI scans of the custom phantom. CT derived source locations had source localization errors of 1.59 mm +/- 0.64. MRI derived source locations produced similar results (1.67 mm +/- 0.76). These errors may be attributed to the image digitization process.

Use of image coregistration in salvage prostate brachytherapy.

PURPOSE: We describe a method of performing salvage prostate brachytherapy on patients whose initial implant was suboptimal. This technique uses an image correlation algorithm only previously used to fuse postimplant magnetic resonance and computed tomographic (CT) images. Here, the initial postimplant CT and the second preimplant volume study are coregistered to plan delivery of the salvage implant. MATERIALS AND METHODS: Two early-stage patients had salvage implants performed with this technique, in which only a limited number of sources were visible on the ultrasound images. The dosimetric results of the first implant were displayed on the preplan generated for the second procedure. The planned total dose then was visualized prior to salvage implant. RESULTS: The implants were performed without complication. Rectum and urethra doses remained acceptable. In each case, the improvement in coverage of the gland was dramatic (V80 coverage improved from 65.2% and 47.3% to 93.1% and 92.2%, respectively), precluding the need for further intervention. CONCLUSIONS: Coregistration of the postimplant CT scan to an ultrasound volume study can be quantifiably and reliably performed. The resulting image set can be used to guide needle placement during a second salvage implant to achieve much improved dosimetric coverage of the gland.

Source localization from axial image sets by iterative relaxation of the nearest neighbor criterion.

The use of axial image sets has become widely used to localize interstitial brachytherapy sources. One application of this method of localization is to perform post-implant dosimetry following transperineal interstitial permanent prostate brachytherapy (TIPPB) where the target structure and the source locations are displayed on the same image. The design of an appropriate scanning sequence often results in abutting slices of an intermediate slice width (3, 4, or 5 mm). Because a single source may be imaged on more than one slice, the resultant scans always show many more source locations than actual sources implanted. The physicist is then faced with the tedious task of determining which sources appear on more than one slice and deciding which source locations to eliminate from the data set. We have developed an algorithm, similar to one employed by Roy et al., which automates this process by relaxing the nearest neighbor criterion until the number of sources is reduced to either the number of sources implanted or the number counted on a projection radiograph. This paper details this algorithm and the results of its application to phantom studies, comparing to known source locations, as well as clinical studies, comparing to orthogonal film source localization, on a series of ten patients. Phantom studies demonstrate the superiority of the algorithm over orthogonal film reconstruction, locating 100% of the sources within 5 mm of the actual location as compared to 66% for the paired radiographs. The clinical study findings are commensurate with these results, with 72% of the sources on average located within 5 mm of the corresponding source in the other data set. The positive correlation of the quality of the orthogonal film reconstruction results with the quality of the coregistration results suggests that differences in registration between the two data sets may be due primarily to the uncertainties in the orthogonal film reconstruction.

Centralized multiinstitutional postimplant analysis for interstitial prostate brachytherapy.

PURPOSE: To investigate the feasibility and utility of performing centralized postimplant analysis for transperineal interstitial permanent prostate brachytherapy (TIPPB) by conducting a pilot study that compares the results obtained from 125I implants conducted at five different institutions. METHODS AND MATERIALS: Dose-volume histogram (DVH) analysis was performed on 10 postimplant CT scans from each of five institutions. This analysis included the total implanted activity of 125I, ultrasound, and CT volumes of the prostate, target-volume ratios, dose homogeneity quantifiers, prostate dose coverage indices, and rectal doses. As a result of the uncertainty associated with the delineation of the prostatic boundaries on a CT scan, the contours were redrawn by a single, study center physician, and a repeat DVH analysis was performed. This provided the basis for comparison between institutions in terms of implant technique and quality. RESULTS: By comparing total activity to preimplant ultrasound volume we clearly demonstrated that differences exist in implant technique among these five institutions. The difficulty associated with determining glandular boundaries on CT scans was apparent, based upon the variability in prostate volumes drawn by the various investigators compared to those drawn by the study center physician. This made no difference, of course, in the TVR or homogeneity quantifiers that are independent of target location. Furthermore, this variability made surprisingly little difference in terms of dose coverage of the prostate gland. Rectal doses varied between institutions according to the various implant techniques. CONCLUSIONS: Centralized, outcome-based evaluation of transperineal interstitial permanent prostate brachytherapy is viable and appropriate. Such an approach could be reasonably used in the conduct of multiinstitutional trials used to study the efficacy of the procedure.

Intraobserver and interobserver variability of MR imaging- and CT-derived prostate volumes after transperineal interstitial permanent prostate brachytherapy.

PURPOSE: To evaluate the relative accuracy and precision of magnetic resonance (MR) imaging and computed tomography (CT) in the assessment of postimplantation prostate volume by determining intraobserver, interobserver, and intermodality variations. MATERIALS AND METHODS: CT and MR images of 41 consecutive patients, after transperineal interstitial permanent prostate brachytherapy, were evaluated by two physicians to determine interobserver and intermodality variability in prostate volume measurements. Repeat evaluation in five randomly selected patients was used to determine intraobserver variability. RESULTS: Observer 1 versus 2 CT-determined mean prostate volume difference was statistically significant (-8.5 cm3 +/- 9.74 [standard deviation], P < .001); observer 1 versus 2 MR-determined mean prostate volume difference was not significant (1.9 cm3 +/- 11.7, P = .492). CT intraobserver range of dimensional errors was 3.5 and 11.4 times that of MR imaging. Observer 1 CT and MR volumes were significantly different (P = .001); observer 2 CT and MR volumes were not significantly different (P = .079). CONCLUSION: With both CT and MR imaging, variation is less when evaluations are conducted by one observer. Variation in one observer may be further reduced by using MR imaging in place of CT.

Comparison of MRI- and CT-based post-implant dosimetric analysis of transperineal interstitial permanent prostate brachytherapy.

The purpose of this work was to investigate how a recently developed MRI-based post-implant dosimetric analysis technique for ultrasound guided transperineal interstitial permanent prostate brachytherapy (TIPPB) compared with the currently accepted CT-based technique. The study was based upon 3-mm MRI and CT scans of 15 patients who had received either 125I or 103Pd implantation. All images were acquired on post-operative day 1 and within 1 hr of each other. Prostate volumes were determined by the same physician. Sources were digitized and calculations performed using an in-house treatment planning system with a nearest neighbor seed sorting routine and AAPM TG43 formalism. Prostate volume, geometric source distribution spread (rcom), dose volume histogram (DVH), and tumor control probability (TCP) calculations were performed from both image sets. Differences in source localization were evaluated by comparing source spread and prescription isodose volumes. Differences in dosimetric analysis were evaluated through prostate-specific DVH and TCP comparisons. Prostate volume as determined from MRI was larger than that of CT by an average of +9.1% (R = 0.70). Calculated rcom was smaller by an average of -0.9 mm (R = 0.81). Isodose volumes at 80, 90, 100, and 150% of the prescription dose differed by an average of +2.5, +2.9, -2.9, and +4.8%, respectively (R = 0.97, 0.98, 0.98, and 0.91). Percentage volume of the prostate encompassed by 80, 100, and 150% of the prescription dose differed by an average of -0.9, -0.9, and -0.1%, respectively (R = 0.34, 0.35, and 0.35). TCP differed by an average of -0.8% (R = 0.37). The results of this study further support our initial findings that MRI may be used to reliably localize the implanted sources for TIPPB. This study also demonstrated that MRI-based post-implant dosimetric analysis is possible. However, it is evident that differences in prostate localization from MRI to CT can result in significantly different assessments of prostate volume coverage. There is clearly a need to further quantify the differences between these two imaging modalities in this application and address whether greater accuracy in describing the dose-volume relationship based on improvements in visualization of the prostate gland from MRI will translate into improved correlation with treatment outcome.

Clinical impact of implementing the recommendations of AAPM Task Group 43 on permanent prostate brachytherapy using 125I. American Association of Physicists in Medicine.

PURPOSE: To determine the clinical impact upon permanent interstitial prostate 125I brachytherapy after conversion to AAPM Task Group 43 (TG 43) guidelines. METHODS: The value of quantities used in the calculation of dose from two institutions, Northwest Tumor Institute (NWTI) and Memorial Sloan-Kettering Cancer Center (MSKCC), which pioneered interstitial techniques for prostate brachytherapy were compared to those recently determined and published by TG 43 of the American Association of Physicists in Medicine (AAPM). Using two different weighting schemes, the change in the commonly prescribed reference dose of 160 Gy was determined and found to be in agreement with that recently suggested. Volumes encompassed by the reference isodose surface were determined from a single source implant and a regularly distributed implant to show the effect of change in reference dose. A comparative analysis on 10 patients was performed to show how this change affected common implant quality descriptors and the effect of changing the calculation formalism without changing the reference dose. RESULTS: Both weighting schemes suggested a change in reference dose from 160 to 144 Gy. Single-source and distributed-source volumetric analysis confirmed this value. The effect on commonly used conformity and uniformity quantifiers for 10 implant patients was tabulated. CONCLUSION: Upon adopting the recommendations suggested by TG 43, institutions that perform permanent 125I prostate implants using calculation methods adapted from the NWTI or MSKCC should revise their treatment prescriptions from 160 to 144 Gy so that the doses delivered to patients remain unaffected. Institutions using other techniques to calculate dose should conduct an analysis similar to the one detailed here.

Source localization following permanent transperineal prostate interstitial brachytherapy using magnetic resonance imaging.

PURPOSE: Dosimetric evaluation of completed brachytherapy implant procedures is crucial in developing proper technique and has prognostic implications. Accurate definition of the prostate gland and localization of the implanted radioactive sources are critical to attain meaningful dosimetric data. Methods using radiographs and CT accurately localize sources, but poorly delineate the prostate gland. MRI has been recognized as a superior imaging modality in delineating the prostate gland, but poor in localizing sources due to lack of source visibility. The purpose of this study was to optimize the visualization of sources using MRI and compare to CT derived source localization. METHODS AND MATERIALS: Multiple MRI scanning techniques were attempted until an acceptable sequence to visualize both the prostate gland and the implanted sources was found. The exams were performed using a pelvic coil only in approximately 15 min. The CT and MRI scans of 20 consecutive patients who had received TRUS-guided permanent transperineal interstitial prostate 125Iodine or 103Palladium brachytherapy were evaluated using an in-house dosimetry system. To eliminate anatomical dependence, the MRI-derived DVHs for the entire calculation volume were then compared to those derived from the CT scans. RESULTS: The differences in isodose volumes, of the calculation volumes, for all implants at all dose levels were not statistically significant at the 95% confidence level. Calculation volume isodose volumes derived from MR images were statistically similar to those derived from CT images at the prescription dose for both 125Iodine (p < 0.01) and 103Palladium (p < 0.026). CONCLUSION: This study presents the first evidence that MRI may be reliably used to identify permanently implanted 125Iodine and 103Palladium sources. Given the advantage of target definition characteristics of MRI, substantially more accurate dosimetric analysis of prostate implants is now possible. The cost of the optimized and abbreviated MR scanning sequence used in this study is comparable to a pelvic CT scan. Postimplant MRI allows more accurate volumetric and anatomically relevant evaluation of permanent prostate implants, which may provide useful clinical correlation.

The coupling of anisotropy and radial dose functions for 103Pd and 125I for use with a commercial treatment planning system.

Many commercial treatment planning systems available today employ traditional dose calculation formulae in their interstitial brachytherapy source calculation algorithms. The 1995 AAPM report on interstitial brachytherapy source dosimetry recommended a new dose calculational formalism and presented a technique for adopting it on systems which embody traditional formalism. In order to comply with these recommendations on our system, the transformations for implementing a one dimensional isotropic point source model were modified by coupling the published anisotropy and radial dose corrections and fitting them to a fifth order polynomial. Using this approach, a more accurate dose calculation is obtained.