Index of content:
Volume 33, Issue 6, June 2006
- Therapy Scientific Session: Valencia A
- IMRT Delivery
TH‐E‐ValA‐01: On the Dose Delivered to a Moving Target When Employing Different IMRT Delivery Mechanisms33(2006); http://dx.doi.org/10.1118/1.2241970View Description Hide Description
Purpose: To investigate the influence of target motion on dose distributions generated using unmodulated open fields, solid intensity modulator (SIM), Step and Shoot MLC (SMLC) and dynamic MLC (DMLC). Method and Materials: For two lungcancer cases, four treatment plans were generated using Pinnacle3 7.9t consisting of an open field, SIM, SMLC and DMLC delivery on a Varian Clinac 600C/D equipped with a 120 leaf Millennium MLC. The coordinates (x, y, z, t) of the 4D motion trace for each of the tumors were determined using 4D‐CT from which a 4D motion kernel was generated. For each beam used in the experiment, the beams‐eye view tumor motion due to breathing was simulated using a computerized 2D tabletop apparatus. A MAPcheck diode array was incorporated into the apparatus for dose distribution analysis. Each of the four static treatment plans was delivered to the breathing MAPcheck ten times at various points of the breathing cycle. Results: The variation in diode dose readings within the tumor motion envelope was compared for the open field, solid, segmented, and dynamic IMRT deliveries. The open field provided the most uniform dose to the entire set of tumor mimicking diodes followed by SIM, SMLC, and DMLC IMRT, respectively. On an individual diode by diode basis over ten trials, the open field had the smallest average coefficient of variation of 0.122% followed by SIM (0.98%), SMLC (2.22%) and DMLC (3.88%) IMRT delivery, respectively. Conclusion: For the three IMRT delivery methods (SIM, SMLC, and DMLC), SIM consistently provided a more uniform dose to the tumor over many trials. SMLC performed as well as the solid modulators in many cases or was slightly out performed by SIM. DMLC consistently delivered the least uniform dose to the tumor over many trials.
33(2006); http://dx.doi.org/10.1118/1.2241971View Description Hide Description
Purpose: Intensity‐modulated proton therapy (IMPT) will improve the conformality of protonradiotherapy while preserving target homogeneity and low integral dose characteristics. IMPT is currently delivered using a single scanned pencil beam by placing Bragg peak spots at predetermined points in the patient. Very short pulse lengths and low repetition rates will characterize inexpensive compact proton beam systems of the future such as the dielectric wall accelerator. Their pulse structure is not amenable to scanning a single spot. The use of multiple intensity‐modulated pencil beams delivered in a fan beam is a simple approach to IMPT that can be used to upgrade current proton systems or with future accelerators. Methods and Materials: A fan beam is created with a pair of quadrupolemagnets aligned so that the second magnet amplifies beam divergence initiated by the first magnet. A set of multiple range‐shifters simultaneously adjusts penetration of multiple pencil beams. The actuation for the range‐shifters is done out of the plane of the fan beam. The intensity is modulated for any pencil beam by placing a thickness, greater than the range in the protons, in that portion of the fan beam. This achieves binary (on‐off) modulation as is used in x‐ray tomotherapy. Results: Calculations show that a spot delivery rate increase of 8 times can be achieved with this system which would allow larger target volumes to be practically delivered with IMPT or used with pulsed systems with low repetition rates. The system could be used to deliver spot scanning with multiple pencil beams simultaneously or used to deliverproton tomotherapy. Conclusions: An intensity‐modulated proton therapy system, based on multiple pencil beams issuing from a fan beam geometry, would decrease the delivery time for either continuous or low repetition pulse systems.
33(2006); http://dx.doi.org/10.1118/1.2241972View Description Hide Description
Purpose: To develop a leaf‐sequencing algorithm for fixed‐gantry (nonrotational) treatment delivery on a commercial helical tomotherapy system (HI‐ART, TomoTherapy, Inc., Madison, WI). Method and Materials: A genetic algorithm was used to determine the multileaf collimator(MLC) leaf open times for a series of fluence test maps generated from a tomotherapy machine with a fixed gantry angle of 0 degrees (IEC scale). A series of wedge shapes (15, 30, 45, and 60‐degree) were mathematically created to test the algorithm's ability to produce simple modulations, similar to those which would be encountered in breast radiation therapy.Results: In general, the topographic treatment delivery yielded reasonable dose distributions. The agreement for the wedge cases was within ±2%, or 2‐mm distance‐to‐agreement (DTA) in the high dose gradient regions. The central axis measured dose was between 3.6 and 4.2 percent higher than the expected dose for the wedge cases. For double peaks, the agreement was within ±2%, or 2‐mm DTA across the entire measured film. For quadruple peaks, the agreement was within ±2%, or 2‐mm DTA in the high dose gradient regions. At the first peak, calculated and measured agreed to within ±0.5%. The dose gradient between the first peak and the first valley was 5 percent per centimeter. The dose in the first valley agreed to within ±1.6% of the prescribed dose (at the first peak). The maximum error in the quadruple peaks occurred at the second peak, where the measured dose was 3.8% low (relative to the prescribed dose at the first peak). Conclusions: The developed algorithm produced calculated deliverable distributions that agreed well with the artificially constructed distributions. This delivery technique could be used for treatment of a whole intact breast. Additional work is needed to optimize the algorithm to improve agreement between the calculated doses and deliverable dose distributions.
TH‐E‐ValA‐04: IMRT Delivery to a Moving Target by Dynamic MLC Tracking: Delivery for Targets Moving in Two Dimensions in the Beam's‐Eye View33(2006); http://dx.doi.org/10.1118/1.2241973View Description Hide Description
Purpose: To outline a new modification of the dMLC delivery technique enabling the tracking of a target moving through rigid‐body translations in a 2D loop in the beam's eye view and the accuracy of the delivery versus that of deliveries with no tracking and of 1D tracking techniques with patient intensity‐modulated beams (IMB) is quantified.
Method and Materials: Leaf trajectories calculated in the target reference frame were iteratively synchronized assuming regular target motion. This allowed the leaves defined in the lab reference frame to simultaneously follow the target motion and to deliver the required IMB without violation of the leaf maximum‐velocity constraint. The leaves are synchronized until the gradient of the leaf position at every instant is less than a calculated maximum. The delivered fluence in the target reference frame was calculated with a simple primary‐fluence model. The new 2D tracking technique was compared with the delivered fluence produced by no‐tracking deliveries and by 1D tracking deliveries for 33 clinical IMBs. Results: The RMS difference between the desired and the delivered IMB was 15.4±3.3 MU for the case of a no‐tracking delivery; 10.9±2.3 MU for the case where one component of motion was corrected and 6.8±1.6 MU for the 2D tracking delivery. The residual error is due to interpolation and sampling effects. The 2D tracking delivery technique requires an increase in the delivery time evaluated as between 0 and 50% of the unsynchronized delivery time for each beam with a mean increase of 13% for the IMBs tested. Conclusion: The 2D tracking dMLC delivery technique allows optimized IMBs to be delivered to moving targets with increased accuracy and with acceptable increases in delivery time. When combined with realtime knowledge of the target motion at delivery time this technique facilitates improved target conformity relative to no‐tracking deliveries and allows margin reduction.
33(2006); http://dx.doi.org/10.1118/1.2241975View Description Hide Description
Purpose:Photon Sterotactic body radiotherapy has shown a great advantage over conventional treatment for stage I NSCC. Lung SRT has been limited by targets proximally to critical structures. Proton beam therapy can reduce the dose to the healthy tissue. Use of 4DCT introduces a great advantage in delineating the moving target and account for tissue motion in proton beam path. However, design of compensator is a critical factor for dose coverage in 4DCT. Method: We investigated the use of Free Breathing (FB), Maximum Intensity Projection (MIP) and Average CT (ACT) imagedata sets to obtain the best technique for proton beam treatment delivery. The target volume was the union of GTV on all phases contoured on MIP data set. Then MIP‐GTV volume density was set to the average HU of the tumor to account for density variation due to motion on all data sets. The ITV was drawn by expanding 8 mm around the MIPGTV. Apertures were designed by considering only beam penumbra and setup uncertainty. Distal and proximal distances and smearing were added to compensator design as described in reference (1). Compensators were designed on all three data using density corrected MIP‐GTV. Results: The plans were calculated on all data sets. We evaluated compensators that were designed in FB, MIP, and ACT. Dose comparison was done based on the ITV dose coverage. Plans with compensator designed in MIP or ACT produce insufficient dose coverage when calculated in FB. Conclusion: The best coverage is obtained when compensator is designed in the MIP data set and plan is evaluated on ACT data set. The dose coverage was also verified on the inspiration and expiration phases of 4D data set using MIP compensator.
33(2006); http://dx.doi.org/10.1118/1.2241976View Description Hide Description
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 tumormotion in BEV with one dimensional moving MLC while delivering IMRT meeting all mechanical practical and constraints is possible and promising for 4D‐IGIMRT.
33(2006); http://dx.doi.org/10.1118/1.2241977View Description Hide Description
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 transistorsdeposited 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.
- IMRT Optimization and Delivery
33(2006); http://dx.doi.org/10.1118/1.2241568View Description Hide Description
Purpose:IMRT optimization involves several treatment parameters producing a complex, unstable and computationally challenging problem during its search of an optimal plan in a reasonable time. Most parameters have been studied in IMRT optimization except the collimator angle, which is investigated in this study. Method and Materials: Five head‐and‐neck and five prostate cases are selected. The head ‐and‐neck and prostate PTVs range between 79.6–441 cm3 and 86.2–250 cm3 respectively, whereas the OAR volumes vary from 11.0–46.3 cm3 and 41.1–312 cm3 respectively. The patients are treated with five or seven fields equally distributed, 0‐ degree collimator angle, using the 1 cm leaf MLC from a Siemens Primus accelerator based on the plan generated using the Nucletron Oncentra treatment planning system. While dose‐volume constraints are kept the same as in the patient's initial treatment plan, collimator angles are varied systematically (0–90 degrees) and a new treatment plan is optimized for each collimator angle. The number of beamlets, monitor units (MU) and DVHs for each collimator angle are compared. Results: The variation of the total number of beamlets with collimator angle follows the shape of a parabola and peaks at 45° collimator angle for all patients. However, the MUs appear to be relatively independent of the collimator angle. The PTV dose coverage statistics for each patient are relatively independent of the collimator angle. Similar observations are noted for all the OARs, except for the small structures for which differences could be observed in the DVHs between the different collimator angles. Conclusions:Collimator angle does not play a significant role in IMRT optimization, as long as the PTV overage is adequate. This provides an additional freedom to choose from 0–90 degree of the collimator angle for long fields without compromising the coverage with limited MLC range or treatment time.
33(2006); http://dx.doi.org/10.1118/1.2241571View Description Hide Description
Purpose: The essence of inverse planning is how to rank objectively the competitive treatment plans. The purpose of this work is to show the equivalence of recently introduced EUD‐based and the conventional dose‐based objective function and to setup a unified framework in the dose domain for IMRT inverse planning. Methods: The dose‐based objective function such as the quadratic function treats each voxel within a structure equally, whereas the EUD‐based objective function aims to take into account of dose‐volume effect when ranking the candidate plans. For the conventional approach to accomplish what an EUD method does, we partially “break” the implicit constraint that a structure is a mathematically uniform entity with all voxels having the same identity and assign each voxel a voxel‐specific ranking (or penalty scheme). Algorithmically, this is accomplished by assigning a voxel‐specific importance or a voxel specific prescription dose. The seemingly insolvable task of determining a large number of voxel‐specific importance/prescription is dealt by heuristically relating them to the actual local doses. We show that this strategy is a more general ranking scheme, which can not only model the volumetric behavior but also the higher order factors beyond the dose‐volume effect. EUD and other dose‐volume formalisms represent special cases of the general framework. Results: A unified inverse planning framework is established for inverse planning. By assigning higher importance to the voxels within the target volume but close to the margin with a critical structure, while simultaneously assigning higher importance to the voxels within the critical structure close to the target volume, the dose to the target volume is dramatically improved in comparison with the conventional approaches. Conclusions: The EUD‐based approach can be reformulated into the realm of conventional dose‐based formalism without using biological parameters. The formalism allows us to design IMRT plans that would otherwise be unattainable.
33(2006); http://dx.doi.org/10.1118/1.2241572View Description Hide Description
Purpose: In IMRT planning, leaf‐sequencing algorithms are used to translate optimized intensity maps into deliverable aperture shapes. We have developed a novel approach to leaf sequencing called continuous intensity map optimization (CIMO). The CIMO algorithm can significantly reduce the number of segments required for IMRT delivery. A unique feature of the CIMO algorithm is that it operates directly on continuous intensity maps. Consequently, the intensity maps do not need to be divided into discrete levels. Method and Materials: CIMO uses a simulated annealing algorithm to optimize the aperture shapes and weights with a goal of minimizing the discrepancies between the optimized and sequenced fluence maps. We have benchmarked the performance of the CIMO sequencer against the algorithms in the Pinnacle3 and Eclipse treatment planning systems. Results: When the CIMO sequencer was applied to 10 IMRT plans from the Pinnacle3 planning system, the average number of segments was reduced from 133 to 62 (a 54% reduction). The average number of monitor units was reduced from 646 to 555 (a 14% reduction). The CIMO algorithm also provided a 36% reduction in the average root mean square errors between the optimal and sequenced fluence maps. Additionally, the CIMO sequenced plans provided more uniform PTV coverage with comparable sparing of critical structures. The average standard deviation of the PTV dose distribution decreased from 9.6 to 7.4 cGy. When the CIMO algorithm was applied to 5 IMRT patients from the Eclipse planning system, the average number of segments was reduced by 46% and the average number of monitor units was cut by 31%. Equivalent plan quality was observed. Conclusion: As compared with both commercial systems, the CIMO algorithm resulted in equivalent or improved plan quality while providing a significant reduction in the required number of segments and monitor units.
TU‐D‐ValA‐04: Adaptive Diffusion Smoothing: A Novel Method to Control IMRT Field Complexity Based On the Diffusion Equation33(2006); http://dx.doi.org/10.1118/1.2241573View Description Hide Description
Purpose: To introduce and evaluate adaptive diffusion smoothing (ADS), a novel procedure designed to preferentially reduce IMRT beam complexity based on any case related parameter inside the optimization.Method and Materials: The diffusion equation was used to develop a procedure in which IMRT beams are smoothed using coefficients defined for each beamlet. The coefficients can be a function of any parameter and dictate the degree of smoothing allowed for each beamlet. The ADS procedure was incorporated into our optimization system as a weighted objective function penalty and several possible ADS coefficient definitions were investigated. Coefficients were designed to promote 1) uniform smoothing everywhere, 2) smoothing based on beamlet intensities, and 3) smoothing based on beamlet gradients with respect to the plan objectives. The method has been validated on a phantom and studied in clinical sites including prostate. Results: The addition of the ADS penalty in the objective function, for all three coefficient types, produced plans with reduced modulation and minimal dosimetric impact in the phantom. Each ADS coefficient definition had merit, but gradient‐based coefficients showed the most potential for reducing beam modulation without affecting dosimetric plan quality. For example, in a prostate plan, this method reduced MU 40% while preserving full target coverage and increasing mean normal tissue doses by less than 2.2 Gy. Conclusion: A unique method based on the diffusion equation and used within the objective function has been developed to control IMRT beam complexity. This method, called adaptive diffusion smoothing, has been applied to phantom and clinical cases, and was able to reduce modulation significantly while preserving dosimetric plan quality. Adaptive diffusion smoothing is a promising tool for ensuring that only the necessary amount of beam modulation is used, promoting more efficient and accurate IMRT planning, QA, and delivery.
Supported in part by NIH Grant P01‐CA‐59827.
TU‐D‐ValA‐05: Self‐Correction of DMLC Delivery Errors Based On Feedback From On Flight Intensity Calculations33(2006); http://dx.doi.org/10.1118/1.2241574View Description Hide Description
Purpose: To deliver DMLC intensity modulated therapy accurately in the presence of errors related to unaccounted target motion, improperly executed motions of leaves and imprecise control of beam intensity rate during delivery.Method and Materials: The intensity delivered to each target point is calculated on‐the‐fly under the assumption of unidirectional leaf motion in the target frame of reference. The delivered intensity is then compared to the intended intensity. If a discrepancy arises, the leading leaf's trajectory is left unchanged and the following leaf's trajectory is modified to correct the error. Using the derived formulas, the following leaf is sped up or slowed down so that it efficiently corrects the error without unnecessarily compromising the delivered intensity to other points of the target. The formulas only consider the discrepancy between the delivered and intended intensity. They are not directly dependent on the actual and intended leaf positions as a function of time. This means that the leaves will not, in general, return to their originally planned trajectories. After correction is complete however, the correct intensity will be delivered to the rest of the target. Results: Two representative examples of self‐correction are shown. The first uses a static target, and shows how an intensity error due to an unintended leaf velocity can be corrected. The second shows a similar error, but instead uses a rigid moving target. Conclusions: An algorithm is developed to modify the following leaf trajectory when a discrepancy between the delivered and intended intensity arises. The results show that it is feasible to correct some errors in DMLC delivery without interrupting delivery. The tolerance for errors can therefore be set much higher than current control algorithms. This should lead to considerably decreased number of beam interruptions at delivery.
TU‐D‐ValA‐06: Incorporation of Modulation Range Constraints Into a Gradient Search Algorithm for IMRT Optimization33(2006); http://dx.doi.org/10.1118/1.2241575View Description Hide Description
Purpose: Hardware‐sensitive optimization for IMRT results in more conformal dose distributions; however, most planning systems neglect hardware constraints, most likely due to the complexity of incorporating them into gradient search algorithms. An important limitation of delivery systems is the modulation range, which we demonstrate can be incorporated as a hard constraint into gradient search algorithms. Method and Materials: We used a commercial treatment planning system (TPS) for calculation of dose‐deposition coefficients, which were exported to optimization software that we developed. For a beam j comprised of beamlets i, we reformulated the optimization problem such that the fluence of beamlet i, φ i , is the product of the weight of beam j and the transmission of beamlet i, φ i = wjti . A gradient search algorithm determined the optimal values of wj and ti subject to the constraints wj ⩾ 0 and Tminimum ⩽ ti ⩽ 1. The resulting fluence distributions were exported back to the TPS for MLC leaf sequencing and dose calculation. We applied the method to a phantom simulating head and neck re‐irradiation, a situation that requires very low fluence values to protect the spinal cord. Using the same cost function parameters, we varied the minimum transmission constraint. After optimization, leaf sequences for dynamic MLC delivery were determined and the dose calculated. All plans were normalized such that 95% of the target received the prescription dose. Results: Target mean and maximum doses were essentially identical for all transmission constraint values. The maximum spinal cord dose was 33% of the prescription dose for no transmission constraint. The minimum value, 30.8%, occurred for an 8% transmission constraint. Conclusion: Modulation constraint is a good approximation of hardware transmission limits and could be readily incorporated into existing TPSs. Furthermore, initial results suggest that transmission constraint beyond the intrinsic hardware limitation may be beneficial.
TU‐D‐ValA‐07: Prioritized Prescription‐Goal Treatment Planning for IMRT: The Effect of Constraint Moderation33(2006); http://dx.doi.org/10.1118/1.2241576View Description Hide Description
Purpose: Determining the ‘best’ optimization parameters in IMRT planning is typically a time‐consuming trial‐and‐error process with no unambiguous termination point. Recently we and others proposed using a goal‐programming approach which better captures the prioritization of clinical goals without introducing ambiguous user‐input parameters. We consider here the effect of adding ‘slip’ to this method, which allows for slight degradations in metric performance compared to maximum achievable. Method and Materials: In the first phase of the optimization process, only the highest‐order goals are considered (target coverage and dose‐limiting normal structures). In subsequent phases (levels), the achievements of the previous step are turned into hard constraints and lower‐order goals are optimized subject to these constraints. Slip factors were introduced to allow for slight violations of the constraints. Linear as well as quadratic goal terms were evaluated for performance as well as dosimetric ‘steerability.’ The resulting constraints can also be expressed as linear or quadratic equations.Results: Focusing on head and neck cases, we present several examples of treatment plans using prioritized optimization. These are compared to conventional IMRT plans in terms of dosimetric properties and optimization speed. The main advantages of the new optimization method are (1) its ability to generate plans that meet the clinical goals/prescriptions without tuning any weighting factors or dose‐volume constraints, and (2) the ability to conveniently include more terms which represent elements such as beam weight smoothness. Lower level goals can be optimized to the achievable limit without compromising higher order goals. Modest slip factors improved overall performance. Conclusion: The prioritized prescription‐goal planning method including slip factors allows for a more intuitive and human‐time‐efficient way of dealing with conflicting goals compared to the conventional trial‐and‐error method of varying weighting factors.
This research was supported by a NIH grant R01 CA90445 and a grant from TomoTherapy, Inc.
TU‐D‐ValA‐08: Geometric Considerations for Optimizing Beam Directions for IMAT Treatment of Lung Cancer with Mediastinum Nodal Irradiation33(2006); http://dx.doi.org/10.1118/1.2241577View Description Hide Description
Purpose: Based on geometric considerations, we optimized beam directions and arc ranges for intensity‐modulated arc therapy (IMAT) to maximize the sparing of lung and spinal cord while treating the mediastinum nodes and gross disease. Methods and Materials: A phantom with multiple planning target volumes (PTV) and lung with varying geometric shapes was created on Pinnacle 7.6c (Philips Medical Systems). A spinal cord was placed posteriorly. Mean lung and maximum cord dose were obtained for the various geometries for AP/PA and IMAT fields that were designed to spare the cord. We combined AP/PA and IMAT fields in order to achieve an optimal plan for each geometry, and seek correlations between patient geometry and their optimal relative weights. This combination of AP/PA and the IMAT fields were used to treat a patient in 2004 diagnosed with non‐small cell lungcancer with concurrent chemotherapy.Results: For a circular PTV in the phantom, the mean lungdose varied for AP/PA between 50% and 29% of the mean PTV dose, depending on the lung geometry, and for IMAT, between 50% and 46%. The maximum cord dose was 107% for AP/PA versus 33% for IMAT. Optimized composite plan of AP/PA and IMAT were created for each phantom geometry, balancing PTV dose,lung and cord tolerance. For the patient plan, the prescription dose was 60Gy, and the composite AP/PA‐IMAT plan gave a maximum cord dose of 45Gy with mean lungdose =16.1Gy and V20Gy =23%. Patient completed treatment and had no acute or late lung or spinal cord toxicities. Conclusions: IMAT was found to be useful in sparing the cord but not the lung, while AP/PA maximizes lung sparing but not the cord. A combination of AP/PA and IMAT provides an optimal class solution for treatment of lungcancer where mediastinum nodal irradiation is indicated.
TU‐D‐ValA‐09: Optimized Removal of the Tongue‐And‐Groove Underdose Via Constrained Partial Synchronization and Variable Depth Recursion33(2006); http://dx.doi.org/10.1118/1.2241578View Description Hide Description
Purpose:IMRT leaf sequencers that remove tongue‐and‐groove underdosages (TGU) significantly increase the number of segments by using full synchronization (FS). This work aims to minimize the increase in the number of segments by using Constrained Partial Synchronization (CPS) and Variable Depth Recursion (VDR). Method and Materials: Adjacent leaves simultaneously expose cells in an intensity map to deliver Vs, determined by cell values V0 and V1 (V0 <= V1). The tongue‐and‐groove ratio, TGR, is (V1‐Vs)/(V0‐Vs). In FS, TGR = infinity, while for CPS it is constrained to be greater than the minimum required to remove TGUs. The TGU for various TGRs were measured with film to determine this minimum TGR. The extraction and sweep leaf sequencing processes used in VDR optimization were modified to use CPS. The algorithm was tested on 1400 random maps (3 to 16 levels) and 42 clinical maps. Results: For a Siemens MLC, the TGUs are 18%, 4%, and 0, for TGRs of 1, 1.5, and 2, respectively. For the clinical cases, the average increase in the number of segments relative to VDR with no synchronization was 7% for TGR=1.5, and 32% for VDR with FS. A fully synchronized sweeping window produces 170% more segments. For the random maps, an unsynchronized sweeping window produced 1.5 to 2.5 times as many segments as VDR with CPS. Similar results can be obtained for interdigitating MLCs. This has implications for direct aperture optimization (DAO) algorithms that use the sweeping window as a starting point (Pinnacle), for which TGU has been observed. The concept of CPS can be applied to DAO algorithms, by choosing appropriate levels for each segment. Conclusion: CPS combined with VDR removes the tongue‐and‐groove underdose while minimizing the number of segments.
- Teletherapy Delivery III and Dose Calculation
TH‐D‐ValA‐01: Preliminary Investigation of Multi‐Pass Respiratory Gated Helical Tomotherapy (MRG‐HT)33(2006); http://dx.doi.org/10.1118/1.2241923View Description Hide Description
Purpose: To investigate the feasibility of multi‐pass respiratory gated helical tomotherapy (MRG‐HT) technique by measuring the dosimetric degradation caused by target motion, and the resulting dosimetric improvements made by the MRG‐HT technique Method and Materials: MRG‐HT was simulated on the HT unit (Tomotherapy Inc.) by delivering a sequence of HT plans modified by blocking beam projections remaining outside the gating window (50 % duty cycle, end‐expiration gating). After each plan delivery, the couch was reset and the succeeding plan was commenced at a different respiratory phase to allow the previously “blocked” beam projections to fall within the gating window. EDR2 film was placed inside a moving phantom (Modus Medical Devices Inc.), which was driven sinusoidally in the longitudinal direction. For each of the two scenarios with different experimental parameters, the resulting dose profiles along the axis of gantry rotation were obtained for a stationary target treated with the original plan, a moving target treated with the original plan, and a moving target treated with the MRG‐HT technique. Results: In the first scenario, target motion led to underdosing 7.5 +/− 0.5 mm of the target and overdosing additional 2.0 +/− 0.5 mm of normal tissues. With the gating technique, these values were reduced to 4.5 +/− 0.5 mm and 1.0 +/− 0.5 mm, respectively. Further improvement could be achieved using a smaller gating window. In the second scenario, selected parameters caused significant dose modulations of 8.5 +/− 1.0 % inside the target, which was effectively eliminated by the MRG‐HT technique. Conclusion: MRG‐HT seems to be a very promising technique for gated HT. Its dosimetric improvement was demonstrated in the form of reducing target underdosing and dose modulations. In future, more sophisticated experiments will be made to simulate lungcancer treatment, including the effect of non‐sinusoidal motion and lung inhomogeneity.
33(2006); http://dx.doi.org/10.1118/1.2241924View Description Hide Description
Purpose: In recent years there has been an explosion of research work concerning the topic of charged ion acceleration using high‐power lasers. The maximum particle energy and the shape of the distribution function are the two main parameters influencing the potential utilization of the new technology in radiation therapy. The energy spectrum depends on the quality of the laser beam as well as the geometric shape of the interaction target. Recent experiments have shown that it is possible to generate quasi‐monoenergetic protons using a double target system. However, influence of the laser prepulse has not yet been fully quantified. The purpose of this study is to find how the laser prepulse changes the property of the target and under which conditions can one expect the highest proton energy. Method and Materials: A radiation‐magnetodynamics code with inline atomic kinetic modeling was used to simulate interaction of the laser prepulse with initially cold double‐layer target. The simulated target system consists of an aluminum substrate on which a thin layer of hydrogen is located. Different initial substrate thicknesses and laser contrast ratios were simulated to find an optimal interaction conditions. Results of hydrodynamic simulations were used as initial conditions for the simulation of the main pulse interaction with the altered target. Results: It is shown that interaction of the prepulse with the target leads to its partial ionization with subsequent formation of a shock wave. Propagating shock wave destroys initial configuration of matter in a substrate, greatly expanding it in laser's propagation direction. Results of simulations suggest that maximal proton energy is achieved using lasers with smallest contrasts, incident on targets of thicknesses 1–2 μm. Conclusions: Presence of laser prepulse impedes acceleration efficiency of protons. Minimizing the contrast ratio or shortening the duration of the prepulse should result in higher maximal proton energies.
TH‐D‐ValA‐03: An Improved Lung Model, Incorporating Realistic Random Anatomical Features, for Monte Carlo‐Based Dosimetry33(2006); http://dx.doi.org/10.1118/1.2241925View Description Hide Description
Purpose: To investigate the effects of incorporating a more physically‐realistic lung model, preserving random anatomical features of the lung, on MC‐based dose distributions. Methods: A random lung model was built based on morphological data. The model homogenizes the lung parenchyma with structures of “chunk” sizes less than 0.05 cm, and models all larger chunks (branches of the bronchial and vessel trees, up to ∼1.5 cm) as randomly‐positioned 2‐D cylinders. The MC code PENELOPE was employed to calculate dose distributions in a water phantom containing a lung region, modeled by either a homogenized lung (as used in conventional planning) or the random lung model. Dose calculations used 6 and 18 MV photon beams with four different field sizes.Results: Depth dose curves in the random lung model illustrate significant perturbations when the structure size is comparable to the field size. For the 1×1 cm field size, large differences (up to 34% of Dmax) exist in the largest structures due to the loss of CPE with small field size. For large field sizes (10×10 cm or higher), little difference is observed between the random and the homogeneous models. The additional attenuation of the large structures also results in a region of dose reduction behind the lung.Conclusion: A new random lung model reveals significant dose perturbations from the homogeneous model, and shows that the homogeneous model breaks down when the field size is comparable to the structure size. This work is of importance in IMRT planning, where beamlets are used, or in the treatment of small tumors, where small field sizes are used in the planning. This work suggests that in such cases, a more precise description of the lung geometry, e.g. a high resolution CT‐based pixel‐by‐pixel density map, may be necessary for accurate dosimetry.
33(2006); http://dx.doi.org/10.1118/1.2241926View Description Hide Description
Purpose: To quantitatively evaluate the consistency of cone beam CT(CBCT) data and the deviation from helical CT if used for calculating dose to heterogeneous material. Methods and Materials: A Gammex RMI 467 tissue characterization phantom was used to generate CT numbers for both the Elekta XVI CBCT and the Picker 5000CT. For both devices, the RMI phantom was positioned using the manufacturer's recommendations. Hounsfield numbers (HU) were obtained using a default window and level and a ROI of 1cm diameter over the center of the rod (insert). The numbers were recorded and a HU vs. electron density correction curve was generated. For the cone beam portion, we used the Elekta XVI CBCT with M10 collimator and no filter. CBCT data, conventional CT data and correction curves were transferred to our CMS XIO treatment planning system. To verify the consistency of CBCT numbers, we added a 1cm of bolus to the phantom and repeated the procedure. A single field treatment plan was generated with heterogeneity and non heterogeneity corrections for CBCT and CT datasets, and for nonbolused and bolused geometries. The Gammex phantom tissue equivalent rods were modified to accept TLD for measurement verification. Results: Calculated doses for treatment plans generated without heterogeneity correction for data sets from CT and CBCT agreed within ∼1%. CBCT numbers for the same insert differed with a small change in setup (ie. adding 1cm bolus). When heterogeneity correction was turned on, a difference of 14% was found between CT and CBCT nonbolused plans. TLD measured doses agreed with those from the calculated CT set. Conclusion: Without heterogeneity correction, the dose calculation with CT and CBCT has excellent agreement suggesting a consistency in contour acquisition. One has to be extremely cautious in using CBCT data for heterogeneity corrections since significant error can occur.