DMLC IMRT Research Articles

TH‐E‐ValA‐07: IMRT Dosimetry with An Active Matrix Flat Panel Dosimeter

Purpose: Dosimetric performance of a fully‐customized Active Matrix Flat Panel Dosimeter (AMFPD) is reported for IMRT measurements in a solidwater phantom. Method and Materials: The AMFPD consists of a‐Si:H photodiodes and thin film transistors deposited on a glass substrate. No scintillator screen or copper plate is present above the photodiodes. The device is operated in a continuous acquisition mode asynchronously with the radiation beam delivery at 0.8 fps. Prior to field delivery, a dark frame was acquired to take into account dark signal contributions to the radiation signal including any lag effects from previous irradiations. Dose was determined by summing the corresponding radiation frames (after subtracting from each frame a dark frame obtained prior to radiation delivery), correcting for defective pixels, applying a pixel‐to‐pixel gain correction, and then applying the measured dose response calibration curve. The response of the AMFPD was evaluated as a function of the applied bias voltage across the photodiodes, as this parameter affects dark signal, lag contributions, and sensitivity. In addition, the AMPFD response was evaluated as a function of dose, dose rate, and energy for static fields at 10 cm depth. For comparison, SMLC and DMLC IMRT fields were measured with the AMFPD and film, using standard methods for reliable film dosimetry. All comparisons were made in absolute dose values of cGy. Results: In continuous acquisition mode, the AMFPD maintained a linear dose response (correlation coefficient r2>0.9999) up to 1040 cGy over the period of study (six months). In order to obtain reliable integrated dose results for IMRT fields, effects of lag on the radiation signal were minimized. Compared to film, the AMFPD results were excellent, generally within 2% /±2 mm. Conclusion: We found that the AMFPD can be used as a dosimeter at multiple depths in phantom for static, SMLC and DMLC IMRT fields.

TH‐E‐ValA‐06: 4D DMLC IMRT Delivery to Targets Moving in Two Dimensions (in BEV)

Purpose: To deliver proper IMRT to moving tumors using DMLC necessitate proper leaf sequencing technique to take care of the dynamic nature of the target. A complete compensation of motion can be achieved only if the compensation is in both the directions (direction along (x) and perpendicular (y) to the leaves motion). A technique to accomplish this is proposed. Method: The motion of the target is divided into two components. Then leaf trajectories to deliver the desired intensity modulated profile is calculated for all leaf pairs (LP) assuming the target is moving in one direction (x direction). Then leaf trajectories of all the leaf pairs are synchronized. Now the motion is compensated in x direction. As the MLC leaf can move only in one direction, the motion compensation in y direction is accomplished by switching the leaf trajectories of each pair appropriately i.e. say if the target is moving upwards in y direction, after a threshold value (value before which motion in y direction is neglected) the leaf trajectory of LPs are switched upwards meaning the leaf trajectory of LP1 is now followed by LP2 and trajectory of LP2 by LP3 and so on. The switching is in the other direction if the target is moving downward. Small dosimetric errors may occur while switching depending upon the time it takes to do the switch and also the threshold value after which switching happens. Results: An example of 4D‐IMRT delivery to an irregular shaped rigid target moving in an elliptical pattern is shown. Other related delivery issues are addressed (dealing with target motion exceeding maximum leaf speed). Conclusions: This method of compensating the two dimensional tumor motion in BEV with one dimensional moving MLC while delivering IMRT meeting all mechanical practical and constraints is possible and promising for 4D‐IGIMRT.

Importing measured field fluences into the treatment planning system to validate a breathing synchronized DMLC–IMRT irradiation technique

Background and purpose Recalculating dose distributions using measured IMRT fluence fields imported into the treatment planning system (TPS) to evaluate the technical feasibility of a prototype developed for breathing synchronized irradiation. Patients and methods DMLC–IMRT fluence patterns acquired on radiographic film, generated by the linac in non-gated and gated mode, have been imported into the TPS. The effect of dose blurring and possible interplay between organ motion and leaf motion, and the efficacy of a breathing synchronized irradiation technique (an adapted version of a commercially available image-guidance system: NOVALIS BODY/ExacTrac4.0, BrainLAB AG) have been evaluated using radiographic film mounted to a simple phantom simulating a breathing pattern of 16 cycles per minute and covering a distance of 4 cm to obtain the resulting fluence maps. Two situations have been investigated to illustrate this principle: (a) a tumor located close to the diaphragm to assess the influence of organ motion on the dose to the target volume as well as to the gastro-intestinal tract that presents a high risk at intersecting with the beam during the breathing cycle. (b) A mediastinal lesion requiring complicated fluence patterns. Results Importing measured fluence maps yielded highly disturbed reconstructed dose distributions in case of the non-gated delivery with the phantom in motion (both orthogonal and parallel to the leaf direction), whereas the measurements from the static (film fixed in space) and the gated delivery showed good agreement with the original theoretical dose distribution. These findings have been confirmed by the dose–volume histograms, corresponding tumor control probabilities, conformity index and dose heterogeneity values. The normal tissue complication probabilities investigated in this study seem to be affected to a lesser degree, which concurs with the observation that the motion effects result in a dose spread in the direction of motion. The applied breathing synchronization technique introduced an increased treatment time with a factor 3–4. Conclusions The use of measured fluence fields, delivered by the linac in non-gated and gated mode, as imported fluence maps for the treatment planning system is an interesting quality assurance tool and revealed the dramatic impact of dose blurring and interplay between DMLC–IMRT dose delivery and organ motion, as well as the potential of breathing synchronization to resolve this issue. The possible advantage of breathing synchronized irradiation is compromised with an increased treatment time.

Dosimetric Errors in Gated DMLC IMRT Delivery to a Moving Target with a 120-Leaf Collimator

Purpose/Objective: The overall impact of intra-fraction, target motion (i.e. respiration) on Gated IMRT dose delivery has not been comprehensively resolved, and recommendations for clinical implementation of gated IMRT are currently inconclusive. A comprehensive experimental strategy is presented in this study to derive all of the clinically relevant operational parameters for Gated IMRT with DMLC, to quantify the expected dose errors in optimal treatment delivery, and identify the expected dose errors for known residual motion.

Real‐time DMLC IMRT delivery for mobile and deforming targets

In numerous cases of radiotherapy delivery to moving targets, simplifying assumptions of identical pattern of motions of tissue for each fraction are not satisfied. Therefore, algorithms capable to respond in real time to motions of target registered at treatment should be developed to improve the precision of radiation intensity delivery. The DMLC delivery of predetermined intensity maps to moving and deforming targets in real time is developed in this paper. Algorithms are constructed so that constraints on maximum admissible speed of leaves are preserved during delivery. A sequence of examples is presented to illustrate behavior of leaf trajectories for representative cases of [dynamic multileaf collimator] (DMLC) [intensity modulated radiation therapy] (IMRT) real-time delivery. The examples presented show real-time deliveries to targets moving as rigid bodies and targets deforming uniformly over their volumes. Examples are admitting random perturbations of predefined target motions that are time dependent only, i.e., target motion perturbations are identical for all target points.

SU-FF-J-58: DMLC Leaf-Pair Optimal Control for Mobile, Deforming Target

Purpose: Tumors in the lung and abdomen tend to move and deform during the course of respiratory cycle. To deliver the intensity modulated therapy, in which intensity map dynamically shifts and deforms in conjunction with the target motion, algorithms have to be developed that control appropriately leaf progression in DMLC IMRT. Description of such algorithms is the purpose of this presentation. Method and Materials: Target motions and deformations are simulated by analytic, periodic functions dependent on position and time. Derived infinitesimal relationships between leaf velocities at all leaf positions assure delivery of the predetermined intensity map. Derived formulas lead to a system of interdependent ordinary, differential equations. When minimization of time of DMLC IMRT delivery is required equations are uniquely defined by scalar fields that are determined by intensity function, target local velocities and target local parameters of deformation, together with constraints on maximum leaf speeds. Integration of differential equations provides trajectories for MLC leaves and thus determines the algorithm for relevant DMLC motion controls. Results: A representative example of DMLC IMRT delivery to deforming target is presented. The example shows the delivery of analytically defined double parabolic intensity function to target undergoing oscillatory type deformation. Target deformation is assumed to be spatially uniform. Deformation of the target, as well as trajectories of leading and following leaves, are calculated and graphically represented for this example. The snapshots of leaf positions at various stages of intensity delivery are also shown. Conclusion: Algorithms are developed for the control of MLC leaf motions that make possible delivery of DMLC IMRT to targets experiencing deforming motions. These realizations of IMRT therapies to deforming targets prevent any discrepancies, rooted in target's dynamic distortions during treatment, between intended and actual intensities delivered to the target.

SU‐DD‐A1‐06: Synchronized Delivery for DMLC IMRT for Stationary and Moving Targets

Purpose: To eliminate tongue and groove (TG) underdosage effects during DMLC IMRT delivery for stationary and moving targets by synchronizing leaf pair trajectories using a non iterative algorithm. Method and Materials: Optimal leaf trajectories to deliver the desired intensities are calculated independently for all leaf pairs. The mid‐time trajectories for all leaf pairs are also determined. From the mid‐time trajectories of all leaf pairs one common synchronized mid‐time (MT) trajectory is determined. Trajectories for all leaf pairs are reconstructed from this synchronized mid‐time trajectory. Properties of the mid‐time trajectory guarantee that no violation of velocity constraints occurs. The same procedure works for both moving and stationary targets. Results: The intensity delivered in the overlapped region without leaf synchronization, and with leaf synchronization, is examined using two examples. One example shows the leaf synchronization of five leaf pairs to deliver clinical intensity for stationary target and other shows ten leaf pair synchronization to deliver clinical intensity for moving target. The no collision property between the leading and following leaves of neighboring leaf pairs for MT‐synchronized trajectories is assured. This property is illustrated in snapshots of the MLC leaves at different times of irradiation for moving targets. Conclusion: The mid time synchronization based solutions remove TG underdosage in DMLC IMRT delivery for stationary and moving targets. The delivery based on these solutions (i) removes TG underdosage in overlapped region in the sense that the intensity in the region is guaranteed to be equal to the lower intensity delivered by any one of two pairs of neighboring leaves (ii) facilitates simultaneous multiple leaf correlation and thus avoids iterative algorithms (iii) minimizes the time of delivery provided all constraints imposed on leaf motions are satisfied (iv) assures that no collisions between leading and following leaves of neighboring leaf pairs occur.

SU‐DD‐A3‐05: Real Time DMLC IMRT Delivery for Mobile and Deforming Target

Purpose: Assume image system in the treatment room provides the information about target motion/deformation during therapy in real time. The ultimate utilization of this information calls for applying it to the intensity modulated therapy so that intensity maps dynamically shift and deform in conjunction with the target motion. This presentation describes algorithms for the control of MLC leaves that deliver real time image guided therapy described above. Method and Materials: Target motions and deformations are modeled from data registered in real time. Real time IMRT aims for (1) delivery of predetermined intensity to target and surrounding tissue, (2) minimized time of delivery and (3) limitation of maximum speed of leaf motions to below the speed of maximum velocity admissible for MLC. As goals (1) and (3) constitute indispensable conditions of IMRT delivery the minimization of the time of delivery (2) has to bee sacrificed. Thus the original set of equations relating leaf positions and velocities to deliver the predetermined intensity is set as the problem with over‐restrictive conditions on leaf speeds. Random perturbations are then imposed over target motions and relationships between leaf velocities are appropriately modified to respond to perturbations. This assures that the slope of local intensity is properly shaped at each point of moving target with unpredictable real time registered motions of the target. Results: A sequence of representative examples delivering real time IMRT to moving/deforming targets is presented. Examples considered are characterized by varied intensities and different pattern of target motions/deformations. Deliveries of planned intensities, with and without corrections for changes in target motions/deformations, are calculated and inter‐compared. Conclusion: MLC leaf motions can be controlled in real time so that image guided DMLC IMRT delivery is feasible provided image system in the treatment room communicates to MLC controller target motion/deformation data during therapy in real time.

Synchronized delivery of DMLC intensity modulated radiation therapy for stationary and moving targets

When delivering intensity modulated treatments the "tongue-and-groove" underdosage effect is a concern that should not be ignored. Algorithms aimed at removing the tongue-and-groove underdosage have been investigated in the past for irradiation of stationary targets. This paper is devoted to algorithms that remove tongue and grove effect for stationary and moving targets. To this end this paper develops original mid-time based algorithms for leaf synchronization. These algorithms exhibit a few additional advantageous properties for DMLC IMRT delivery beyond the removal of tongue-and-grove underdosage. In particular, they safeguard the minimization of time of delivery (for mid-time synchronized algorithms). Moreover, they avoid iterative procedures for synchronization of delivery for multiple pairs of leaves.

TU-C-T-6E-07: Monte Carlo Investigation of Dosimetric Differences Between SMLC and DMLC IMRT Delivery Techniques in Heterogeneous Media

Purpose: To investigate dosimetric differences between SMLC and DMLC IMRT delivery techniques in heterogeneous media using MC. We hypothesize that subtle differences in the energy spectra between SMLC and DMLC, which may not be detected in water, will be accentuated in inhomogeneous, patient-like tissues. With the ability to account for MLC design details and perform accurate transport in heterogeneous media, MC provides a powerful means to evaluate IMRT delivery methods and may be useful as a tool for IMRT QA. Method and Materials: The BEAMnrc code was used to simulate patient independent components of a Varian 21EX linac. The resultant Phase space file was then used as input for the DPM MC code which simulates the jaws, 120-leaf MLC geometry, and the patient or phantom. Transport through the jaws and MLC is accomplished using multiple scatter photon transport. Sampling algorithms have been developed for SMLC and DMLC by randomly sampling leaf segments based on the MU index. Accuracy of the simulations was assessed in a homogeneous media when compared to measurements in a solid water phantom. To assess the impact of MLC design and delivery technique in heterogeneous media, dose was calculated in a phantom consisting of material slabs ranging in density from lung to cortical bone using SMLC and DMLC leaf sequences. Results: Excellent agreement was seen between the Monte Carlo models for SMLC and DMLC sequencing when compared to the respective film measurements in homogenous media. Significant differences, up to 10%, were seen between the delivery techniques in lung equivalent media when calculated in heterogeneous slab geometry. Conclusion: This study provides evidence that heterogeneous tissue may exacerbate energy differences between SMLC and DMLC delivery methods. Such differences may be clinically important considering that they will not be detected during QA which is typically conducted using homogeneous phantoms.

DMLC leaf‐pair optimal control for mobile, deforming target

Existing algorithms of dynamic control of independent pairs of leaves allow optimal DMLC delivery of IMRT to rigid targets translating parallel to leaf trajectories. However, in numerous cases of radiotherapy treatments simplifying assumptions of rigid-like motions of targets and surrounding tissues are clearly not satisfied. Therefore algorithms have to be developed that allow one to control MLC so that predetermined intensities are delivered to various points in targets that experience compression and expansion at the time of irradiation. Moreover, it is desirable for such algorithms to ensure that delivery of modulated intensity map will be done with minimal expense of monitor units. Derivation of the algorithm that optimizes the DMLC IMRT to mobile, deforming target is presented in this paper. [To illustrate the general algorithm two representative examples of DMLC IMRT delivery to deforming targets are presented in full detail.] Finally, similarities and differences between solutions for immobile targets, for moving, rigid targets and for moving, deforming targets are discussed.

23 oral Dosimetric comparisons of static, SMLC, and DMLC IMRT delivery techniques

23 oral Dosimetric comparisons of static, SMLC, and DMLC IMRT delivery techniques