Inverse Planning Software Research Articles

SU‐FF‐T‐416: The Use of ModuMLC® in Combination with IG‐IMRT for Treating a Very Small Eye Tumor

Introduction: IMRT employing a conventional MLC (1 cm leaf width at isocenter) for small eye tumors may be suboptimal because of its relatively large leaf width compared to the tumor size. The combination of moduleaf (MMLC, 0.25 cm leaf width at isocenter), head immobilization and IG‐IMRT may offer a better alternative. In this study, two IMRT plans are generated for the treatment of a small eye tumor using a conventional MLC (MLC82, 1 cm leaf width), and MMLC with IGRT. Method and materials: A young patient with choroidal hemangioma was treated with a Primatom‐based IG‐IMRT technique. The size of the tumor was delineated to be 4 cm3. A total of 12 Gy in 8 fractions was prescribed to the CTV, and the dose allowed to the right lens (0.12 cm3) was constraint to ⩽ 5 Gy. Treatment planning was carried out on the KonRad inverse planning software. Two plans were generated for a five‐field setup. One was optimized for dose delivery with the MLC82, while the other is optimized for the MMLC. The patient was setup in the treatment couch immobilized with the customized mask and a bite block. Image guidance was performed with a CT‐on‐rail system. Dosimetric measurements were carried out to verify the accuracy of dose delivery for the MMLC plan. Results: The plan for the MLC82 achieves a minimum dose of 7.53 Gy to the PTV and the maximum dose to the right lens is 8.2 Gy. With MMLC, the minimum dose is 12.3 Gy to the PTV while the maximum dose to the right lens is 5.15 Gy. Dosimetric measurement for the MMLC plan yielded good agreement with calculation. Conclusion: The use of IGRT, MMLC and customized mask improves the overall dose delivery accuracy and appears to be a superior choice for treatment of small tumors.

Equivalent uniform dose concept evaluated by theoretical dose volume histograms for thoracic irradiation

Background and Purpose The goal of our study was to quantify the limits of the EUD models for use in score functions in inverse planning software, and for clinical application. Materials and Methods We focused on oesophagus cancer irradiation. Our evaluation was based on theoretical dose volume histograms (DVH), and we analyzed them using volumetric and linear quadratic EUD models, average and maximum dose concepts, the linear quadratic model and the differential area between each DVH. Results We evaluated our models using theoretical and more complex DVHs for the above regions of interest. We studied three types of DVH for the target volume: the first followed the ICRU dose homogeneity recommendations; the second was built out of the first requirements and the same average dose was built in for all cases; the third was truncated by a small dose hole. We also built theoretical DVHs for the organs at risk, in order to evaluate the limits of, and the ways to use both EUD(1) and EUD/LQ models, comparing them to the traditional ways of scoring a treatment plan. For each volume of interest we built theoretical treatment plans with differences in the fractionation. Conclusion We concluded that both volumetric and linear quadratic EUDs should be used. Volumetric EUD(1) takes into account neither hot–cold spot compensation nor the differences in fractionation, but it is more sensitive to the increase of the irradiated volume. With linear quadratic EUD/LQ, a volumetric analysis of fractionation variation effort can be performed.

Multiobjective optimization with a modified simulated annealing algorithm for external beam radiotherapy treatment planning

Inverse planning in external beam radiotherapy often requires a scalar objective function that incorporates importance factors to mimic the planner's preferences between conflicting objectives. Defining those importance factors is not straightforward, and frequently leads to an iterative process in which the importance factors become variables of the optimization problem. In order to avoid this drawback of inverse planning, optimization using algorithms more suited to multiobjective optimization, such as evolutionary algorithms, has been suggested. However, much inverse planning software, including one based on simulated annealing developed at our institution, does not include multiobjective-oriented algorithms. This work investigates the performance of a modified simulated annealing algorithm used to drive aperture-based intensity-modulated radiotherapy inverse planning software in a multiobjective optimization framework. For a few test cases involving gastric cancer patients, the use of this new algorithm leads to an increase in optimization speed of a little more than a factor of 2 over a conventional simulated annealing algorithm, while giving a close approximation of the solutions produced by a standard simulated annealing. A simple graphical user interface designed to facilitate the decision-making process that follows an optimization is also presented.

TU-D-224C-02: Monte Carlo Direct Aperture Optimization (MC-DAO) for IMRT

Purpose: To improve the accuracy/efficiency of IMRT planning by combining Monte Carlo (MC) dose calculation with direct aperture optimization (DAO). Method and Materials: A 6 MV beam arrangement is applied to an IMRT phantom and patient examples. A phase space is calculated below the secondary jaws of a virtual Varian 21EX linac by MC simulation (BEAMnrc code (NRC, Canada)). The phase space is subdivided into 2.5×5.0 mm2 beamlets and the dose distribution from each beamlet is calculated to organs‐of‐interest within the patient/phantom using DOSXYZnrc. This information is input into DAO inverse planning software. The DAO includes multileaf collimator transmission and leaf motion limitations as it modifies the shape/weight of the treatment apertures. The optimized leaf sequence requires no additional leaf motion calculation step. A final forward MC dose calculation is performed. The MC doses are verified with ion chamber and film measurement. MC‐DAO is applied to a difficult phantom geometry, namely a c‐shaped target with embedded organ‐at‐risk located directly adjacent to a 5.0cm‐thick air slab. Clinical sites include nasopharynx and lung.Results: The MC optimization allows for accurate modeling of the electronic disequilibrium introduced by the air cavities. For the phantom example, MC reveals that the plan optimized with a pencil beam (PB) algorithm fails to provide adequate coverage to the PTV close to the air cavity, whereas the MC‐DAO plan demonstrates adequate coverage. For the nasopharynx, the PB plan showed errors during ion chamber/film verification, probably due to the small (∼5×4 cm2) fields whereas the MC‐DAO plan showed good agreement. The reduction in monitor units for MC‐DAO plans is 20 – 40% compared to a commercial fluence‐based (PB) treatment planning system. Conclusion:MC simulation generates accurate input data for IMRT inverse treatment planning in difficult‐to‐calculate regions. The addition of DAO results in a more efficient treatment plan delivery.

TU‐E‐ValB‐07: A Segmentation and Leaf Sequencing Algorithm for IMAT

Purpose: To develop an intensity segmentation and leaf sequencing algorithm specifically for intensity‐modulated arc therapy (IMAT), which can be applied to optimized intensity patterns derived from existing commercial IMRT inverse planning software. Methods and Materials: Three phantom cases, as well as a clinical case were planned using a Hi‐Art II (Tomotherapy Inc, WI.) planning station. The end of planning sinograms were then extracted and inputted into our IMAT conversion algorithm. The number of required arcs, deliverable MLC segments for each arc and the relative intensity weighting of each arc were outputted. The number of arcs (modulation) could be controlled by a user parameter, α. The resulting MLC segments were then fed into a fast monte‐carlo dose calculation algorithm, NXEGS (NumeriX, LLC) to obtain 3D dose distributions. Dose statistics (max, min, mean) and dose volume histograms of relevant structures were calculated and compared against the results generated by the Hi‐Art II system. Results: Each plan was converted in under three minutes on a typical desktop PC, with the arc numbers varying between 4 and 15 360° arcs. Qualitatively, the dose distributions obtained from the IMAT plans were similar to the tomotherapy results, as well as planned doses. Quantitatively, the IMAT plans were slightly degraded, with the average dose to normal structures being 7.5% higher for IMAT vs. tomotherapy. However, the IMAT plans generally met planned values, being 9.1% below for maximum doses to normal structures. The number of arcs and therefore the resulting dose distribution could be varied according to α. Conclusions: IMAT segmentation and leaf sequencing produced deliverable IMAT MLC segments and relative arc weights directly from Hi‐Art II optimized plans. The algorithm was computationally efficient, and produced similar dose distributions. Additional optimization could improve resulting dose distributions further. IMAT back‐up for tomotherapy is another potential application.

The incorporation of SPECT functional lung imaging into inverse radiotherapy planning for non-small cell lung cancer

Background and purpose Patients with non-small cell lung cancer (NSCLC) often have inhomogeneous lung perfusion. Radiotherapy planning computed tomography (CT) scans have been accurately co-registered with lung perfusion single photon emission computed tomography (SPECT) scans to design radiotherapy treatments which limit dose to healthy ‘perfused’ lung. Patients and methods Patients with localised NSCLC had CT and SPECT scans accurately co-registered in the planning system. The SPECT images were used to define a volume of perfused ‘functioning’ lung (FL). Inverse planning software was used to create 3D-conformal plans, the planning objective being either to minimise the dose to whole lungs (WL) or to minimise the dose to FL. Results Four plans were created for each of six patients. The mean difference in volume between WL and FL was 1011.7 cm 3 (range 596.2–1581.1 cm 3). One patient with bilateral upper lobe perfusion deficits had a 16% reduction in FLV 20 (the percentage volume of functioning lung receiving ≥20 Gy). The remaining patients had inhomogeneous perfusion deficits such that inverse planning was not able to sufficiently optimise beam angles to avoid functioning lung. Conclusion SPECT perfusion images can be accurately co-registered with radiotherapy planning CT scans and may be helpful in creating treatment plans for patients with large perfusion deficits.

386 The comparison of dose distribution calculated by inverse planning software of Cadplan® and Eclipse® in the treatment of anal carcinoma

386 The comparison of dose distribution calculated by inverse planning software of Cadplan® and Eclipse® in the treatment of anal carcinoma

Optimization of radiobiological effects in intensity modulated proton therapy

Today, inverse treatment planning for intensity modulated proton therapy (IMPT) usually employs a constant relative biological effectiveness (RBE). In this paper, the potential clinical relevance of RBE variations for scanning techniques in IMPT is investigated, and a new strategy to include the RBE into the inverse planning process is presented. Three-dimensional RBE distributions are calculated based on a phenomenological model that describes the RBE as a function of dose, linear energy transfer (LET) and tissue type in the framework of the linear-quadratic model. This RBE model is integrated into the optimization loop of inverse planning by using a modified version of the standard quadratic objective function, where the physical dose is replaced by the biological effect. This system for "biological optimization" was implemented into a research version of the inverse planning software KonRad and allows the direct optimization of the product of RBE and physical dose. Several treatment plans for a prostate case are presented, which compare the biological with the conventional physical dose optimization for IMPT scanning techniques, in particular distal edge tracking (DET) and the full three-dimensional (3D) modulation of beam spots. Mainly due to their different LET distributions, the RBE effects for these two techniques are quite different: while the RBE distribution was more or less homogeneous in the planning target volume (PTV) for 3D modulation, considerable RBE variations within the PTV were observed for DET. These unfavorable effects could be compensated for by employing the new biological objective function, which led to a more homogeneous distribution of the product of RBE and physical dose in the PTV. The computation time increased by a factor of 2 compared to the optimization of the physical dose. In conclusion, the proposed method allows the simultaneous multifield optimization of the biological effect in a reasonable time, and is therefore well suited for studying the influence of a variable RBE in IMPT as well as for minimizing potentially adverse effects.

Bouquet brachytherapy: Feasibility and optimization of conically spaced implants

Purpose To examine the dosimetric feasibility of a conical implantation approach to robotic-assisted prostate brachytherapy. Methods and materials An in-house inverse planning software based on the genetic algorithm (GA) was used to optimize the needle angulations and the seed positions along needles that form one or two bouquets. Volume data from 20 prostate seed implant patients (six 125I and 14 103Pd) previously treated using the conventional rectilinear template approach were used. The dosimetry outcomes of the optimized treatment plan in the conical approach were compared with those from the original treatment plans based on the conventional rectilinear template approach. Results When seed spacing is restricted to nominal 1 cm center-to-center spacing, dosimetry results in the conical approach suffer from a higher urethra dose and higher dose heterogeneity compared with the original rectilinear template plans. When the seed loading patterns are optimized as part of inverse planning, the resulting dosimetry plans exhibit adequate dose coverage and uniformity through the target volume, as well as satisfactory sparing of the urethra and rectum. Conclusions Conically spaced implantation for prostate brachytherapy with 125I and 103Pd seeds is feasible in terms of dosimetry outcomes. Techniques for optimized inverse planning for this approach have been developed.

Evaluation of radiobiological effects in intensity modulated proton therapy: New strategies for inverse treatment planning

Currently, treatment planning for intensity modulated proton therapy (IMPT) usually disregards variations of the relative biological effectiveness (RBE). To investigate the potential clinical relevance of a variable RBE for beam scanning techniques, new strategies for the evaluation of radiobiological effects and for the incorporation of the RBE into the inverse planning process are presented. These strategies are based on a fast algorithm for three‐dimensional calculations of the dose averaged linear energy transfer (LET) as a measure of the local radiation quality, and on a simple phenomenological approach for the RBE as a function of dose, LET and tissue type. It was found that the biological effect depended strongly on the type of scanning technique used, mainly due to differences in the LET distributions. New objective functions that account for LET and RBE were integrated into an inverse planning software, which now allows simultaneous multifield optimization of the biological effect in a reasonable time. With these methods, unfavorable RBE effects can be identified and compensated for by direct optimization of the product of RBE and dose, which is demonstrated for several clinical examples. The proposed strategies are therefore valuable tools to evaluate and improve the quality of treatment plans in IMPT.

Comparison of dynamic and step-and-shoot intensity-modulated radiation therapy planning and delivery

Intensity-modulated radiation therapy (IMRT) is commonly delivered using the dynamic or segmental mode of multileaf collimators (DMLC or SMLC). Both methods are designed to deliver intensity-modulated beams as determined by inverse planning software. In this study, we have used the Helios IMRT planning system to generate ideal treatment plans for 10 cases of 2 common treatment sites (prostate and head and neck) and have investigated the actual treatment fluence distributions generated for each of the MLC leaf motion choices. The 2 dose delivery techniques were dosimetrically compared to each other and to the treatment plans. For each technique, point doses were measured in a water phantom using ionization chambers. Also for each technique, 2-dimensional dose distributions at a selected depth in a plastic phantom were obtained, using extended range film. The total delivery time and the number of monitor units (MU) delivered by each method were also compared. Our results indicate that the 2 delivery methods produce comparable results dosimetrically. For the cases reviewed, the delivery time was an average of 15% longer for SMLC deliveries, while the number of MUs (beam-on time) required by SMLC was an average of 15% fewer, than that for the DMLC. In the interest of simplicity, lower beam-on time, and potentially fewer mechanically-related problems, we think that the SMLC delivery technique may be the better choice when Helios is used for planning and Varian linear accelerators are used for delivery.

Is there a preferred strength for regularly spaced 125I seeds in inverse-planned prostate implants?

Purpose: To determine whether a preferred seed strength exists for 125I prostate implants preplanned using a fixed intraneedle seed spacing of 1 cm and an objective needle placement strategy within the planning target volume (PTV), and incorporating explicit dose-volume constraints for the PTV and tissues at risk. Methods and Materials: Prostate, urethra, and rectum contours for 10 patients were obtained from transrectal ultrasound studies. The PTV was defined in accordance with Radiation Therapy Oncology Group (RTOG) 0019 protocol. Inverse planning software was used to optimally arrange seeds of strength 0.3–0.8 U to cover the PTV to D Rx = 145 Gy, and limit urethra and rectum doses to 150% and 100% of D Rx, respectively. Isodose distributions and dosimetric indices were calculated: V 200, V 150, V 100, V 90, D 100, D 90 for PTV; V 150 for urethra; and V 100 for rectum. For seeds of strength 0.414 and 0.6 U and three prostate sizes, the sensitivity of V 90 and D 90 to elementary perturbations of the optimal seed arrangement were examined. Results: For our planning scenario, 125I seeds of strength 0.5–0.6 U provided the best possible PTV coverage while maintaining V 200 at ∼25%. The source arrangement for 0.6-U seeds was only modestly more sensitive to perturbations than that for 0.414-U seeds. These findings may not be applicable to implants planned manually or that involve needle placement outside the PTV. Conclusion: Given a particular source arrangement, inverse planning aimed at maximizing dosimetric coverage of the prostate while limiting doses to the urethra and rectum can be used to search for a preferred seed strength. For regularly spaced sources within the PTV, higher strength seeds can provide better dose coverage and better urethral protection than lower strength seeds.

Inverse and forward optimization of one- and two-dimensional intensity-modulated radiation therapy-based treatment of concave-shaped planning target volumes: the case of prostate cancer.

Intensity-modulated radiation therapy (IMRT) was suggested as a suitable technique to protect the rectal wall, while maintaining a satisfactory planning target volume (PTV) irradiation in the case of high-dose radiotherapy of prostate cancer. However, up to now, few investigations tried to estimate the expected benefit with respect to conventional three-dimensional (3D) conformal radiotherapy (CRT). Estimating the expected clinical gain coming from both 1D and 2D IMRT against 3DCRT, in the case of prostate cancer by mean of radiobiological models. In order to enhance the impact of IMRT, the case of concave-shaped PTV including prostate and seminal vesicles (P+SV) was considered. Five patients with concave-shaped PTV including P+SV were selected. Two different sets of constraints were applied during planning: in the first one a quite large inhomogeneity of the dose distribution within the PTV was accepted (set (a)); in the other set (set (b)) a greater homogeneity was required. Tumor control probability (TCP) and normal tissue control probability (NTCP) indices were calculated through the Webb-Nahum and the Lyman-Kutcher models, respectively. Considering a dose interval from 64.8 to 100.8 Gy, the value giving a 5% NTCP for the rectum was found (D(NTCP(rectum)=5%)) using two different methods, and the corresponding TCP(NTCP(rectum)=5%) and NTCP(NTCP(rectum)=5%) for the other critical structures were derived. With the first method, the inverse optimization of the plans was performed just at a fixed 75.6 Gy ICRU dose; with the second method (applied to 2/5 patients) inverse treatment plannings were re-optimized at many dose levels (from 64.8 to 108 Gy with 3.6 Gy intervals). In this case, three different values of alpha/beta (10, 3, 1.5)were used for TCP calculation. The 3DCRT plan consisted of a 3-fields technique; in the IMRT plans, five equi-spaced beams were applied. The Helios Inverse Planning software from Varian was used for both the 2D IMRT and the 1D IMRT inverse optimization, the last one being performed fixing only one available pair of leaves for modulation. A previously proposed forward 1D IMRT 'class solution' technique was also considered, keeping the same irradiation geometry of the inversely optimized IMRT techniques. With the first method, the average gains in TCP(NTCP(rectum)=5%) of the 2D IMRT technique, with respect 3DCRT, were 10.3 and 7.8%, depending on the choice of the DVHs constraints during the inverse optimization procedure (set (a) and set (b), respectively). The average gain (DeltaTCP(NTCP(rectum)=5%)) coming from the inverse 1D IMRT optimization was 5.0%, when fixing the set (b) DVHs constraints. Concerning the forward 1D IMRT optimization, the average gain in TCP(NTCP(rectum)=5%) was 4.5%. The gain was found to be correlated with the degree of overlapping between rectum and PTV. When comparing 2D IMRT and 1D IMRT, in the case of the more realistic set (b) constraints, DeltaTCP(NTCP(rectum)=5%) was always less than 3%, excepting one patient with a very large overlap region. Basing our choice on this result, the second method was applied to this patient and one of the remaining. Through the inverse re-optimization of the treatment plans at each dose level, the gain in TCP(NTCP(rectum)=5%) of the inverse 2D technique was significantly higher than the ones obtained by applying the first method (concerning the two patients: +6.1% and +2.4%), while no significant benefit was found for inverse 1D. The impact of changing the alpha/beta ratio was less evident in the patient with the lower gain in TCP(NTCP(rectum)=5%). The expected benefit due to IMRT with respect to 3DCRT seems to be relevant when the overlap between PTV and rectum is high. Moreover, the difference between the inverse 2D and the simpler inverse or forward 1D IMRT techniques resulted in being relatively modest, with the exception of one patient, having a very large overlap between rectum and PTV. Optimizing the inverse planning at each dose level to find TCP(NTCP(rectum)=5%)e level to find TCP(NTCP(rectum)=5%) can improve the performances of inverse 2D IMRT, against a significant increase of the time for planning. These results suggest the importance of selecting the patients that could have significant benefit from the application of IMRT.

344 poster Dosimetric accuracy of Nucletron's inverse treatment planning (ITP) software

344 poster Dosimetric accuracy of Nucletron's inverse treatment planning (ITP) software

The use of a semi-customized phantom for verification of conformal plans

We have developed a technique for inverse treatment planning of prostate therapy designed to improve the degree of conformation between the dose distribution and the target volume. We compared the inverse plan with a “standard” four-field box technique as well as a four-field technique using oblique fields (“cross technique”). We validated the dosimetry of the inverse plan using Fricke gel solution in phantom specifically designed for this purpose. The phantom is a Plexiglas tank with a cross section, which approximates the dimensions of the pelvis. Anatomical data from computed tomography (CT) images of a patient were used to simulate organs in our phantom. This allows us to calculate dose distributions with the external geometry of the phantom and internal anatomy of the patient. Dose–volume histograms (DVHs) for the three different plans were calculated. The phantom containing the Fricke gel was irradiated according to the inverse plan. Magnetic resonance (MR) images was used to determine the dose distribution delivered to the phantom. We observe, on DVHs, that the inverse plan significantly reduces the dose to the rectum and the bladder but slightly increases the inhomogeneity inside the target volume. Correlation is good between isodoses on MR images and calculated isodoses. We conclude that inverse planning software can greatly improve the conformal degree of treatment to the prostate. This technique could be applied to other complex anatomic sites at which dose to organs at risk is a limiting factor and increased dose to the target volume is indicated. Our phantom and the Fricke gel solution are convenient to carry out validation of conformal treatments.