Dynamic Intensity Modulation Research Articles

Electrically tunable nonlinear polaritonic metasurface

Nonlinear polaritonic metasurfaces created by the coupling of intersubband nonlinearities in semiconductor heterostructures with optical modes in nanoresonators have recently demonstrated efficient frequency mixings at very low pumping intensities of the order of a few tens of kilowatts per square centimetre. In these subwavelength structures, the efficiency, spectral bandwidth and local nonlinear phase of wave mixing do not depend on phase matching but only on the nonlinear response of the constituent meta-atoms. We exploit this property to demonstrate an electrically tunable nonlinear metasurface that combines a plasmonic nanocavity and a quantum-engineered semiconductor heterostructure, in which the magnitude and phase of the local nonlinear responses are controlled by a bias voltage through the quantum-confined Stark effect. We demonstrate spectral tuning, dynamic intensity modulation and dynamic beam manipulation for second-harmonic generation. Our work suggests a route for electrically reconfigurable flat nonlinear optical elements with versatile functionalities.

Prediction of the individual multileaf collimator positional deviations during dynamic IMRT delivery priori with artificial neural network.

Multileaf collimator (MLC) positional accuracy during dynamic intensity modulation radiotherapy (IMRT) delivery is crucial for safe and accurate patient treatment. The deviations of individual leaf positions from its intended positions can lead to errors in the dose delivered to the patient and hence may adversely affect the treatment outcome. In this study, we propose a state-of-the-art machine learning (ML) method based on an artificial neural network (ANN) for accurately predicting the MLC leaf positional deviations during the dynamic IMRT treatment delivery priori using log file data. Data of ten patients treated with sliding window dynamic IMRT delivery were retrospectively retrieved from a single-institution database. The patients' plans were redelivered with no patient on the couch using a Varian linear accelerator equipped with a Millennium 120 HD MLC system. Then the machine recorded log files data, a total of over 400 files containing 360 800 control points, were collected. A total of 14 parameters were extracted from the planning data in the log files such as leaf planned positions, dose fraction, leaf velocity, leaf moving status, leaf gap, and others. Next, we developed a feed-forward ANN architecture mapping the input parameters with the output to predict the MLC leaf positional deviations during the delivery priori. The proposed model was trained on 70% of the total data using the delivered leaf positional data as a target response. The trained model was then validated and tested on 30% of the available data. The model accuracy was evaluated using the mean squared error (MSE), regression plot, and error histogram. The deviations between the individual MLC planned and delivered positions can reach up to a few millimeters, with a maximum deviation of 1.2mm. The predicted leaf positions at control points closely matched the delivered positions for all MLC leaves during the treatment delivery. The ANN model achieved a maximum MSE of 0.0001mm2 (root MSE of 0.0097mm) in predicting the leaf positions at control points of test data for each leaf. The correlation coefficient, that measures the goodness of fit, was perfect (R=0.999) in all plots indicating an excellent agreement between the predicted and delivered MLC positions for the training, validation, and test data. We successfully demonstrated a proposed ANN-based method capable of accurately predicting the individual MLC leaf positional deviations during the dynamic IMRT delivery priori. Our ML model based on ANN outperformed the reported accuracy in the literature of various ML models. The results of this study could be extended to actual application in the dose calculation/optimization, hence enhancing the gamma passing rate for patient-specific IMRT quality assurance.

Chemoradiotherapy for unresectable locally advanced pancreatic cancer: A pilot study

15630 Background: In pancreatic cancer, surgical treatment at the time of diagnosis is only feasible in 10–20% of patients. We evaluated a chemo-radiotherapy regimen in an attempt to improve resecability. Methods: From August 2004 to November 2007, twenty-seven patients with non resectable, locally advanced pancreatic cancer were prospectively enrolled onto an institutional pilot trial. All patients received chemo-radiotherapy, the former consisting of the GEMOX regimen: Gemcitabine (GEM) 1000 mg/m2 on day 1 and Oxaliplatin (OX) 100 mg/m2 on day 2 every two weeks for 3–4 cycles. Radiotherapy (split-course accelerated hyperfractionation using dynamic intensity modulation and ‘sliding-windows’ dose delivery techniques, 25 Gy in 5 fractions) was administered to all patients 15 days after the 3rd-4th chemotherapy cycle. Patients then received a further 3–4 cycles of GEMOX, underwent restaging and were evaluated for surgery. Potentially resectable patients were submitted to surgery. Unresectable responders received 4 more cycles of GEMOX and further radiotherapy (total of 15 Gy in 3 sessions). Results: Patients received 6–12 cycles of GEMOX. Median duration of treatment was 161 days (range: 76–175). After chemo-radiotherapy, 10/27 (37%) patients obtained a partial tumor response, 10 showed stable disease, 6 progressed, and 1 was not evaluable. Five patients are currently being evaluated for surgery. Another 5 who showed a reduction in vascular infiltration underwent surgical laparotomy: 3 radical pancreatic resection (1 splenopancreatectomy and 2 total pancreatectomy), and 2 explorative laparotomy. R0 margin was achieved in 2 cases submitted to pancreatic resection, while the remaining patient was R1. One patient is still alive without signs of recurrence 2 years after radical pancreatectomy, while the other 2 died from distant metastases 6 and 7 months after the surgical procedure. GEMOX toxicity was similar to that reported in the literature (Louvet et al., J Clin Oncol, 2005). Radiotherapy was well tolerated. Conclusions: Our results show the potential efficacy of the chemo-radiotherapy association. A phase II study is now being planned. No significant financial relationships to disclose.

The leaf sweep algorithm for an immobile and moving target as an optimal control problem in radiotherapy delivery

The variational problem of modelling the optimized dynamic delivery of intensity modulated radiation therapy is investigated. Motions of two jaws are determined that expose each point of the field for a predetermined amount of time and minimize the overall time of the radiation delivery. Variable initial and final conditions for jaw motions are admissible and constraints expressing limitations on speeds of jaw movements are included. Cases of dynamic intensity modulation for immobile as well as moving targets are investigated.

Static and dynamic beam intensity modulation in radiotherapy using a multileaf collimator

Investigations on computer optimization of radiotherapy treatment planning (inverse planning) have demonstrated that dose distributions can often be conformed tightly to a target volume by customizing the beam intensity profiles within the treatment field. The aim of this study was the development and clinical implementation of static and dynamic intensity modulated treatment techniques using a multileaf collimator. Using static beam intensity modulation, a technique for penumbra enhancement at the superior and inferior field edges of axial coplanar treatment plans was developed. Due to penumbra enhancement, the length of all treatment fields could be reduced by typically 1.5 cm, while still achieving an adequate dose in the planning target volume. As a result, the dose delivery to critical structures could often be reduced with respect to our (previous) standard treatment without intensity modulation. A dosimetrical study showed that application of this technique in lung treatments could also compensate for the increased lateral secondary electron transport in lung tissue. For calculation of the required leaf motions to generate optimized intensity modulated beam profiles by means of dynamic multileaf collimation an algorithm was developed that fully avoids tongue-and-groove underdosage effects. Dose measurements showed that, using these leaf motions, the accuracy and stability of intensity modulated profiles was generally within 2%. For individual patients, a fast and accurate method for pretreatment verification of each optimized beam was implemented, based on absolute dose measurements with an electronic portal imaging device. Static and dynamic beam intensity modulation is presently applied routinely in our institution for treatment of head and neck cancer patients and prostate cancer patients.

Matching photon and electron fields with dynamic intensity modulation.

A technique was developed to reduce the size and magnitude of the hot and cold spots in the abutting regions of photon and electron fields. The photon and electron fields were set up such that the photon field extended approximately 2 cm into the electron field in the abutting region. The region of the photon beam that overlapped the electron field was modulated using a multileaf collimator, effectively broadening the photon penumbra to make it complimentary to the electron penumbra. The computer calculations were verified using film measurements for abutting a 6 MV photon beam with a 9 MeV electron beam. A uniform dose was achieved at a prespecified depth of 2 cm, and dose uniformity was improved at the specified depth and beyond compared with unmodulated photon beams. A slight increase in dose inhomogeneity was seen at shallower depths. The overall areas of the hot and cold spots were significantly reduced. The technique also reduced the sensitivity of dose homogeneity to setup errors such that the magnitudes of the hot and cold spots were about half of those produced with unmodulated photon beam when an overlap or gap of 4 mm was introduced. The technique was applied to the treatment of a head and neck cancer and a lymphoma involving the right pleura with markedly reduced dose inhomogeneity in the abutting regions.

A system for three-dimensional dosimetric verification of treatment plans in intensity-modulated radiotherapy with heavy ions.

The introduction of dynamic intensity modulation into radiotherapy using conventional photon beams or scanning particle beams requires additional and efficient methods of dose verification. Dose measurements in dynamically generated dose distributions with a single ionization chamber require a complete application of the treatment field for each single measurement. Therefore measurements are performed by simultaneous use of multiple ionization chambers. The measurement is performed by a computer controlled system and is comprised of the following steps: (a) automated positioning of the ionization chambers, (b) measurement at these points, (c) a comparison with the calculated dose from the treatment planning system, and (d) documentation of the measurement. The ionization chambers are read out by a multichannel electrometer and are densely packed into a mounting of polymethylmetacrylate, which is attached to the arm of a three-dimensional motor-driven water phantom. The measured and planned dose values are displayed numerically as well as graphically. The mean deviation between measured and planned doses as well as their standard deviation are calculated and displayed. Through printouts complete documentation of the measurement is obtained and a quick decision can be made whether the dose distribution is acceptable for the patient. The system is now routinely used for dose verification at the heavy ion therapy project at the Gesellschaft für Schwerionenforschung in Darmstadt. Up to now 242 measurements have been performed for heavy ion treatment of 30 patients. The system allows efficient verification and documentation of carbon ion fields and is in principle also applicable to intensity-modulated photon beams.

Static field intensity modulation to treat a dominant intra-prostatic lesion to 90 Gy compared to seven field 3-dimensional radiotherapy

Purpose/Objective: Recent studies supported by histopathological correlation suggest that the combined use of endorectal magnetic resonance imaging (MRI) and magnetic resonance spectroscopy (MRS) allows differentiation of normal and carcinomatous prostate. The goal of this study was to use static field intensity modulated three-dimensional conformal radiotherapy (SF-IMRT) to treat the entire prostate to a total dose of >70 Gy, while concurrently treating a dominant intraprostatic lesion (DIL) defined by MRI+MRS to 90 Gy while not exceeding normal tissue tolerances. Materials and Methods: For the example chosen, the DIL consisted of a large portion of the peripheral zone of the left lobe of the prostate. University of Michigan (UM-PLAN) three-dimensional treatment planning software was used to design a partially shielded 7 field conformal isodose plan that would treat the entire prostate to >70 Gy at 1.8 Gy per day (80% isodose line), while concurrently treating the DIL to 2.25 Gy per day for a total dose of 90 Gy. Dose volume histograms (DVH) were used to compare the rectal doses to rectum and other adjacent normal tissues using these two techniques. Results: SF-IMRT as described, allowed a total dose of 90 Gy to encompass the DIL, while the rectal dose was slightly lower than that using the standard 7 field technique to the prostate alone. For example, the dose to 30 cm 3 of the rectum was 40 Gy using SF-IMRT and 48 Gy for the standard 7 field technique. Because of differences in the dose per fraction the biologic advantages of the SF-IMRT technique are likely to be even greater. Conclusions: This study demonstrates the feasibility of using SF-IMRT to treat a DIL involving a single lobe of the prostate, as defined by MRI/MRS, to 90 Gy, while simultaneously treating the prostate to >70 Gy without increasing the dose to surrounding normal tissues. A similar approach could be used to treat multifocal disease. This method of treatment is an alternative to dynamic intensity modulation. It is less expensive, and can be adapted to any radiation therapy department without the use of an inverse treatment planning programs.

Design considerations for the sides of multileaf collimator leaves

The use of multileaf collimators (MLCs) in radiation therapy is becoming increasingly widespread as more commercial vendors offer them as options on medical linear accelerators. In most of the commercially available multileaf collimators, the sides of the leaves have a `tongue and groove' (TG) feature, which is necessary for blocking the straight through leakage when the leaves are staggered together. The dosimetric effects of the TG design have not been fully studied for applications in block replacement or in dynamic intensity modulation. This article analytically explains the TG effect by calculating a deficit in photon flux when the side of a leaf is exposed. It is shown that the maximum photon deficit occurs when the leaf thickness is shared equally by the tongue and groove of adjacent leaves. New designs are proposed based on the theoretical analysis and improvements of the new design in leakage and the TG effect are verified with Monte-Carlo simulations. Measurements of the TG effect on an MLC from one manufacturer are also conducted. The results of the measurements confirm the analytical explanations of the TG effect.

The effects of intra-fraction organ motion on the delivery of dynamic intensity modulation

Computer-optimized treatment plans, aimed to enhance tumour control and reduce normal tissue complication, generally require non-uniform beam intensities. One of the techniques for delivering intensity-modulated beams is the use of dynamic multileaf collimation, where the beam aperture moves and the field shape changes during irradiation. When intensity-modulated beams are delivered with dynamic collimation, the problem of intra-fraction organ motion can cause distortions to the desired beam intensities. Unlike static field treatments, where intra-fraction organ motion only affects the boundaries creating broad dose penumbra, the interplay of the movement of the beam aperture and the movement of the patient anatomy can create `hot' and `cold' spots throughout the field. The mechanism for creating these effects is not well understood. This paper provides a simple analytical model which illustrates the fundamental mechanism for creating the dosimetric variations in the target when both the beam aperture and the target move during irradiation. Numerical analysis was carried out which calculates the cumulative primary photon fluence, or beam intensity, received by each point in the target, for a given pattern of motion. The results show that, for clinically realistic parameters, the magnitude of intensity variations in the target can be greater than 100% of the desired beam intensity. The magnitude of the photon intensity variations is strongly dependent on the speed of the beam aperture relative to the speed of the target motion, and the width of the scanning beam relative to the amplitude of target motion. The effects of fractionation as well as methods of minimizing and eliminating the dosimetric effects of intra-fraction organ motion are discussed.

59 Predicting the effects of organ motion on the dose delivered by dynamic intensity modulation

59 Predicting the effects of organ motion on the dose delivered by dynamic intensity modulation

A method for implementing dynamic photon beam intensity modulation using independent jaws and a multileaf collimator

A mathematical model is derived for digitally controlled linear accelerators to deliver a desired photon intensity distribution by combining collimator motion and machine dose rate variations. It shows that, at any instant, the quotient of the machine dose rate and the speed of collimator motion is proportional to the gradient of the desired in-air photon fluence distribution. The model is applicable for both independently controlled collimator jaws and multileaf collimators and can be implemented by controlling different parameters to accommodate linear accelerators from different manufacturers. For independent jaws, each pair of jaws creates photon fluence variations along the direction of the jaw movement. For multileaf collimators, where each leaf is independently controlled, any two-dimensional (2D) photon fluence distribution can be delivered. The model has been implemented for wedged isodose distributions using independent jaws, and 2D intensity modulation using a multileaf collimator. One-dimensional (1D) wedged isodose distributions are created by moving an independent jaw at constant speed while varying machine dose rate. 2D intensity modulation has been implemented using a 'dynamic stepping' scheme, which controls the leaf progression during irradiation at constant machine dose rate. With this automated delivery scheme, the beam delivery time for dynamic intensity modulation, which depends on the complexity of the desired intensity distribution, approaches that of conventional beam modifiers. This paper shows the derivation of the model, its application, and the authors' delivery scheme. Examples of 1D dynamic wedges and 2D intensity modulations will be given to illustrate the versatility of the model, the simplicity of its application, and the efficiency of beam delivery. These features make this approach practical for delivering conformal therapy treatments.