Direct Machine Parameter Optimization Plans Research Articles

187. Evaluation of intrarater and interrater VMAT plans obtained with MCO algorithm and comparison with DMPO plans

Purposeto evaluate Multicriteria Optimization Algorithm (MCO) and its reliability in providing VMAT treatment plans through the technique of Pareto surfaces navigation. Methodsten patients who were previously treated with VMAT for prostate cancer were selected and replanned five times each by three different operators (total 50 plans) in RayStation treatment planning system using MCO [1] that provides a real-time assessment of the tradeoff between different clinical goals. For the optimization of each plan we started from the same template derived according the target coverage and the spare of healthy tissue used in our clinical protocols. Furthermore we calculate also a manual plan for each patient using the algorithm based on Direct Machine Parameter Optimization (DMPO); the quality of the plans was evaluated through D2%, D98%, Conformity Index (CI = [volume that receives 95% of dose prescription/target volume]) and Homogeneity index (HI = [dose that covers 5% of the target/dose that covers 95% of the target]). We evaluated intrarater and interrater reliability of the MCO plans through Intraclass Correlation Coefficient (ICC). Finally we compared the best MCO plan (assigning a score to each of the four indexes) with the one obtained with DMPO optimization estimating Pearson and Linn’s Correlation Coefficients (PCC and LCC). ResultsThe average intrarater ICC over the three operators for CI index was “fair” (0,50). Interrater ICC showed “poor” similarity between operators for CI (0,35). Both PCC and LCC between DMPO plan and MCO best plan were relevant (⩾0,7) for CI for op3. ConclusionsDespite some “poor” statistical similarity found in the analysis, the MCO optimization has the potential to provide high-quality plans comparable to the DMPO algorithm in terms of PTV coverage especially in isodoses conformity achievement.

Physician Driven Multicriteria Optimization of Advanced Lung Cancer Patients VMAT Plans

Multi-Criteria Optimization (MCO), a novel commercially available optimization method for intensity modulated and volumetric arc therapies (IMRT and VMAT) has the potential to improve treatment planning techniques and workflows. By navigating between Pareto solutions, MCO allows planners and physicians to assess in real time the impact and tradeoffs between all clinical goals and organ constraints. The purpose of our work is to investigate a universal set of objectives and constraints for lung cancer MCO VMAT plans and the impact of involving the Physician in the navigation of the generated Pareto plans. The central hypothesis is that physician-driven MCO VMAT plans will allow choice of optimal dosimetric trade-offs for a given individual patient. We randomly selected ten lung cancer patients (Stage II-IV) planned with a two arcs VMAT technique. Prescription doses ranged from 50.4Gy to 66Gy in 1.8 or 2Gy per fraction, with one case of dose painting. Target volumes were generated per clinical standard of care. The ITVs ranged from 18cc-387cc. Using the clinically delivered isocenter and beam set-up, we retrospectively generated MCO plans with a universal set of constraints and objectives that was developed by trying different combinations in a few patients. Two versions of the MCO plan scores (one navigated by the dosimetrist [MCOd] and one by the physician [MCOp]) were compared with clinical plans generated with Direct Machine Parameter Optimization (DMPO). Dosimetric and biological parameters evaluated were PTV V95%, Conformity Index (CI), PTV Dmax and the un-complicated probability (UP) representing the absence of lung and esophagus complications. The cord dose was not evaluated because almost all plans had Dmax doses of less than 45Gy, with negligible probability of complications. MCOd vs clinical comparison: the values for PTV V95 were MCOd: 99%(95-100) vs clinical plans: 98% (94-100),p-value= 0.2, the CI were similar in both cases (p-value=0.5) but the Dmax was higher for the MCO plans (average 109% vs. 107%, p =0.004), the lung-esophagus UP was similar between MCOd (Mean: 74%, range: 55-87%) and the clinical pla(Mean:75%, range: 60-89%), p= 0.1. Corresponding MCOd versus MCOp comparisons were similar for the parameters tested. However, using real time assessment of dosimetric trade-offs, in each individual plan the physician was able to actively choose a preferred combination of dose to target vs organs. When evaluating individual patients, there were five patients for which MCO plans had a clear advantage in reducing dose to lung and/or esophagus while improving/maintaining target coverage, four patients with comparable plans and one patient where MCO was worse. MCO VMAT lung plans obtained with standardized objectives and constraints generated comparable or better plans than the clinical DMPO plans while allowing physicians to make real time comparisons of dosimetric trade-offs and accept a plan optimal for an individual patient.

SU-E-T-719: Dosimetric Differences of Post HDR Prostate IMRT Plans Generated From RayStation Multi-Criteria Optimization (MCO) and Pinnacle Direct Machine Parameter Optimization (DMPO)

Purpose: To evaluate dosimetric differences of post HDR prostate IMRT plans generated from RayStation Multi‐Criteria Optimization (MCO) and Pinnacle3 Direct Machine Parameter Optimization (DMPO) Methods: Ten patients previously treated with DMPO plans on Pinnacle3 v9.0 were selected. CT images with same RT structures from Pinnacle were sent to RayStation v2.4.8.180 for re‐planning using MCO Pareto surface plans with the same prescription and beam angles. The prescription was 50.4 Gy to 95% of PTV for all cases. Planning outcomes such as D95 (95% of volume receiving the prescribed dose), D 5, D2, D1 for PTV, D 53 for bladder, D53 , D30, and D1cc for rectum, D1cc for small and large bowel, conformity index (CI = PIV / PTV), and homogeneity index (HI = D5 / D95) were evaluated according to the clinical protocol. Results: The mean volume of PTV was 222.6 cm3 (range from 178.5 to 276.3 cm3). The D95 of PTV coverage of RayStation vs. Pinnacle was improved and more uniform with 49.8 Gy vs. 49.7 Gy, respectively. D5, D2, and D1 of PTV were lower with RayStation. So were D53 for bladder and rectum, D30 for rectum (p = 0.017), D1cc for rectum and D1cc for small bowel, D1cc for large bowel except one case on RayStation. CI was better with RayStation compared to Pinnacle (1.01 vs. 1.07). HI was better with RayStation compared to Pinnacle (1.06 vs. 1.07). Conclusion: MCO planning automatically generates a set of Pareto optimized solutions for given objectives to allow tradeoffs between targets and critical organs. RayStation can achieve better uniform tumor coverage with fewer hot spots while sparing more critical structures in post HDR prostate plans. MCO Pareto based plan is helpful in determining the best optimized dosimetry solutions.

Relationship of segment area and monitor unit efficiency in aperture‐based IMRT optimization

In step‐and‐shoot IMRT plans, aperture‐based optimization (or one‐step optimization) has been considered as a means of improving monitor unit (MU) efficiency compared to fluence‐based optimization (or two‐step optimization). However, the extent of improvement on MU efficiency varies, depending on the implementation and design of one‐step optimization. In this paper, we attempted to investigate MU efficiency issue in two methods of one‐step optimization implemented in two commercial treatment planning systems (TPSs). Five patients with nasopharyngeal cancer and five patients with advanced prostate cancer were selected for this study. For these patients, clinically used IMRT plans were generated using the Direct Machine Parameter Optimization (DMPO) in the Pinnacle TPS. New IMRT plans were created using the Direct Aperture Optimization (DAO) method in the Panther TPS. For the purpose of this study, we used the similar planning dose objectives and beam configurations with a similar total number of segments in each pair of DMPO and DAO plans. With similar plan quality, DMPO plans required more MUs than DAO plans. The average number of MUs (expressed in mean ±1 SD) for the DMPO and DAO plans was 1,169±186 and 671±135 for the nasopharynx cases, and 711±48 and 400±65 for the prostate cases, respectively. The average segment areas (expressed in mean ±1 SD) for the DMPO plans were smaller than those for the DAO plans: 46.0±7.6 cm2 vs. 100.9±32.3 cm2 for the nasopharynx cases, and 58.3±17.2 cm2 vs. 97.4±35.0 cm2 for the prostate cases, respectively. In conclusion, two one‐step optimization algorithms, DMPO and DAO, resulted in much different MU efficiency with the similar number of segments and optimization parameters. This MU difference is largely attributed to the fact that large area segments are used more often in DAO plans than in DMPO plans.PACS number: 87.55.de

Considerations on IMRT for quasi-isotropic non-coplanar irradiation

The purpose of this study was the mathematical analysis of IMRT with many non-coplanar fields for planning target volumes (PTV) surrounding nearly spherical organs at risk (OAR). Our approach is partially analogous to the well known inverse planning for a cylindrically symmetric (CS) case (Brahme et al 1982 Phys. Med. Biol. 27 1221–9) and leads to a spherically symmetric (SS) solution. For the planning study we approximated isotropic 4 Pi irradiation by a quasi-isotropic non-coplanar IMRT technique with 16 fields which we compared to a coplanar IMRT technique with 15 equidistant fields. A virtual spherical phantom contained a spherical central organ at risk which was surrounded by a PTV shaped like a spherical shell with a gap towards the spherical OAR. We compared three types of plans: (1) non-segmented inversely planned fluence distributions prior to sequencing, (2) plans obtained by direct machine parameter optimization (DMPO) with up to 120 segments (good approximation of non-segmented fluence) and (3) more practical DMPO plans with up to 64 segments. In this study we sought an analytical SS solution for the non-segmented fluence distribution in 4 Pi-geometry. For the CS case Brahme et al found that a special narrow fluence peak (‘Brahme peak’) has to be applied to improve dose uniformity in PTV areas adjacent to the OAR. We showed that in the SS case the peak was steeper but the area under the peak was smaller. The relevance of the peak decreased for increasing gap between the OAR and the PTV. The plan quality of the non-segmented SS plans was higher albeit the fluence distributions were less uniform. The plan quality of the segmented plans degraded if the allowed number of segments was reduced; the degradation was quicker for the SS beam arrangement than for the CS beam arrangement. For 64 segments, the SS plans delivered less uniform and more conformal dose distributions than the CS plans, ensuring better sparing of the healthy tissue. Also, the SS plans always needed less monitor units than the CS plans. In conclusion, due to substructures or steeper fluence gradients, the improved potential of quasi-isotropic SS-plan quality can only be exploited, if many segments are allowed. SS plans seem to spare normal tissue better. Further analysis of non-coplanar beam arrangements with less degree of symmetry is planned, followed by a study on non-coplanar intensity modulated arc techniques.

Fast IMRT by increasing the beam number and reducing the number of segments

PurposeThe purpose of this work is to develop fast deliverable step and shoot IMRT technique. A reduction in the number of segments should theoretically be possible, whilst simultaneously maintaining plan quality, provided that the reduction is accompanied by an increased number of gantry angles. A benefit of this method is that the segment shaping could be performed during gantry motion, thereby reducing the delivery time. The aim was to find classes of such solutions whose plan quality can compete with conventional IMRT.Materials/MethodsA planning study was performed. Step and shoot IMRT plans were created using direct machine parameter optimization (DMPO) as a reference. DMPO plans were compared to an IMRT variant having only one segment per angle ("2-Step Fast"). 2-Step Fast is based on a geometrical analysis of the topology of the planning target volume (PTV) and the organs at risk (OAR). A prostate/rectum case, spine metastasis/spinal cord, breast/lung and an artificial PTV/OAR combination of the ESTRO-Quasimodo phantom were used for the study. The composite objective value (COV), a quality score, and plan delivery time were compared. The delivery time for the DMPO reference plan and the 2-Step Fast IMRT technique was measured and calculated for two different linacs, a twelve year old Siemens Primus™ ("old" linac) and two Elekta Synergy™ "S" linacs ("new" linacs).Results2-Step Fast had comparable or better quality than the reference DMPO plan. The number of segments was smaller than for the reference plan, the number of gantry angles was between 23 and 34. For the modern linac the delivery time was always smaller than that for the reference plan. The calculated (measured) values showed a mean delivery time reduction of 21% (21%) for the new linac, and of 7% (3%) for the old linac compared to the respective DMPO reference plans. For the old linac, the data handling time per beam was the limiting factor for the treatment time reduction.Conclusions2-Step Fast plans are suited to reduce the delivery time, especially if the data handling time per beam is short. The plan quality can be retained or even increased for fewer segments provided more gantry angles are used.

SU‐FF‐T‐106: Does Intrafraction Motion Increase Dose to the Inframammary Fold Areas in Whole Breast Treatment Using Direct Machine Parameter Optimization IMRT?

Purpose: Dosimetric uncertainties in the inframmamary fold due to intrafraction breast motion are major clinical concerns for treating large or irregularly shaped breast patients as skin break‐downs commonly occurred near the regions during the course of the treatment. Direct Machine Parameter Optimization (DMPO) IMRT has been well reported for whole breast treatments. The purpose of this study is to investigate whether intrafraction motion affects skin sparing for patients treated with this technique. Method and Materials: A pair of open tangential beams (6 MV) combined with a maximum of 20 DMPO segments of mixed photon energies (6 or 18 MV) was used to create DMPO IMRT plan for a large shaped breast case which was then benchmarked against a standard wedge‐pair plan. In order to constrain the dose to the inframamary fold, a region‐of‐interest starting near the inferior border and extending 3–4 cm superiorly was contoured. Using recorded intrafraction breast movements obtained from an image‐tracked CyberKnife system, transformed treatment plans were created while scaling the average time patterns from the CyberKnife treatment to simulate DMPO IMRT delivery. Results: Transformed DMPO plans received lower dose (90.6±3.1)% to the inframammary region compared to the original DMPO plan (96.0%). V100% for the PTV was slightly reduced from the original 97.5% to 95.0±1.5%. The treatment planning effort and delivery efficiency of DMPO IMRT were found to be similar to the wedge‐pair technique. However, the DMPO method significantly reduced the maximum dose and improved the overall dose uniformity to the inframmamary region. Conclusions: DMPO is an effective and efficient planning technique in skin sparing particularly in the inframammary fold areas for the whole breast IMRT treatments. Our study found no negative impact on such advantage due to intrafraction patient's breathing motion. In addition, the PTV dose coverage was also found to be insensitive to such motion.

SU‐FF‐T‐129: Comparison of IMRT Planning with Two‐Step and One‐Step Optimization: Strategy for Improving Therapeutic Gain Reducing Integral Dose

Purpose: Intensity‐modulated radiotherapy (IMRT) plans are often complex and time consuming both in the optimization and delivery phase, the latter increasing the potential for dosimetric errors. The purpose of this study is to evaluate the effectiveness and efficiency of one‐step optimization with step‐and‐shoot technique as compared to conventional two with sliding windows technique optimization in inverse IMRT planning. Material and methods: The approach generally employed to generate the photon fluence maps is a two‐step process: first the optimization problem is solved generating an ideal matrix of photon fluence, then the fluence profiles are converted into a deliverable photon fluences through a sequencer software using step‐and shoot or sliding window modality of multi‐leaf collimators. Machine parameters can be considered directly in direct machine parameter optimization (DMPO) or in the conversion phase in the two step approach. The Pinnacle IMRT TPS allows both approach. One‐step plans had the maximum number of segments set at 50, 70, 90. Two‐step plans were produced using ideal fluence, with or without smooth filter, converted into a sliding windows sequence. Moreover, the plans were generated allowing or not the split of the beams. Results: All plans showed similar target coverage. Compared to traditional two‐step optimization and dynamic modality, DMPO plans resulted in lower MUs, and lower doses to sensitive structures as well as Normal Tissue Complication Probability. The total MUs and the integral dose were lower for plans with the splitted than the no splitted beams. Conclusion: One‐step optimization is an effective and efficient method for simplifying IMRT plans.

Parameter optimization in HN‐IMRT for Elekta linacs

Planning and delivery in HN‐IMRT has been challenging for the Elekta linac because of numerous machine limitations. Direct aperture optimization (DAO) algorithms have had success in simplifying the planning process and improving plan quality. Commercial adaptations of DAO allow for widespread use in many clinics; however clinical validation of these methods is still needed. In this work we evaluated Pinnacle3 commercial software for HN‐IMRT on the Elekta linac. The purpose was to find a set of planning parameters that are applicable to most patients and optimal in terms of plan quality, delivery efficiency, and dosimetric accuracy. Four types of plans were created for each of 12 patients: ideal fluence optimization (FO), conventional two‐step optimization (TS), segment weight optimization (SW), and direct machine parameter optimization (DMPO). Maximum number of segments (NS) and minimum segment area (MSA) were varied in DMPO. Results showed DMPO plans have the best optimization scores and dosimetric indices, and the most consistent IMRT output among patients. At larger NS (≥80), plan quality decreases with increasing MSA as expected, except for MSA<8 cm2, suggesting presence of local minima in DMPO. Segment area and MUs can vary significantly between optimization methods and parameter settings; however, the quantity ‘integral MU’ remains constant. Irradiation time is linearly proportional to total plan segments, weakly dependent on MUs and independent of MSA. Dosimetric accuracy is independent of DMPO parameters. The superior quality of DMPO makes it the choice for HN‐IMRT on Elekta linacs and its consistency allows development of ‘class solutions’. However, planners should be aware of the local minima issue when pushing parameters to the limit such as NS<80 and MSA<8 cm2. The optimal set of parameters should be chosen to balance plan quality and delivery efficiency based on a systematic evaluation of the planning technique and system constraints.PACS number: PACS: 87.55.D, 87.55.de

SU‐GG‐J‐140: Online Re‐Planning Using Direct Machine Parameter Optimization: A Non‐Human Primate Lung Irradiation Study

Purpose: Establish the feasibility of CBCT‐based, online re‐planning using direct machine parameter optimization (DMPO) to irradiate the total lung parenchyma in non‐human primates (NHPs). A study of acute‐radiation‐exposure‐ induced pneumonitis in NHPs involves total lung irradiation (TLI) to 12.5 Gy in a single session, where acute radiation exposure may also lead to the lethal hematopoeitic and gastrointestinal subsyndromes. The latter syndromes occur at 7–8 Gy and at 12–14 Gy, respectively, 1 and 2 weeks after irradiation. Hence, high dose conformity is essential to spare bone marrow, cord and bowel. Method and Materials: An offline DMPO plan is created on a CBCT scan of the NHP several days prior to the TLI session; using seven, 6MV photon beams. TLI targets a PTV from a 1 cm expansion of lung contours, sparing tissues outside the PTV. On the day of the irradiation a new CBCT is acquired, loaded into the planning system, and contours and DMPO dose objectives are propagated from the offline plan. DMPO is executed online using the original off‐line plan as starting points and the resultant on‐line plan is compared to the offline plan, on a Philips Pinnacle 8.1v workstation. Results: Online‐ and offline‐planned DMPO plans achieve V95 &gt; 95% coverage of the PTV, and excellent bowel sparing. However, offline‐planned DMPO delivers &gt; 7 Gy to more than 50% of the cord volume, while online‐planned DMPO delivers &lt; 6 Gy to more than 50 % of the cord. Online DMPO re‐optimization from scanning to the start of irradiation takes less than 45 minutes. Conclusion: Online DMPO‐based re‐planning of NHP total lung irradiations is established in support of a study of radiation‐induced pneumonitis in non‐human primates. Online‐planning on NHPs is valuable experience prior to clinical implementation in human patients.

SU‐DD‐A1‐02: Parameter Optimization in Head and Neck IMRT for Elekta Linacs

Purpose: Planning and delivery in HN‐IMRT is challenging for Elekta linacs because of numerous constraints on beam delivery systems. The purpose of this study is to find a set of planning parameters that are applicable to most patients and optimal in terms of plan quality, delivery efficiency and dosimetric accuracy. Method and Materials: Four types of plans were created for each of 12 patients: ideal fluence optimization (FO), conventional two‐step optimization (TS) consisting of FO followed by MLC conversion, segment weight optimization (SW) and direct machine parameter optimization (DMPO). Maximum number of segments (NS) and minimum segment area (MSA) were varied in DMPO. Plan quality was evaluated based on score, dose distributions and dosimetric indices. Delivery efficiency was evaluated by irradiation time, and dosimetric accuracy by Mapcheck. Results: Plan quality deviates most from ideal FO for TS, with slight improvement for SW. DMPO is the closest to FO with the least variation among patients. NS of 80–160 in DMPO yield optimal plans. At larger NS (⩾80), plan quality decreases with MSA as expected, except for MSA &lt;8cm2, which suggests presence of local minima in the DMPO algorithm. The irradiation time is strongly dependent on the plan segments (NSactual), weakly dependent on MUs, and independent of MSA. Typical plans with 79 segments and 8cm2 MSA have ∼747 MU and take ∼ 8 minutes to deliver; this increases to ∼13 minutes for 158 segments. Dosimetric accuracy is independent of DMPO parameters. Conclusion: The superior quality of DMPO plans makes it ideal for planning HN‐IMRT on Elekta linacs and its consistency allows development of class solutions. However, the vulnerability of local minima warrants such a study to systematically evaluate the effect of parameters in new planning techniques. The optimal set of parameters should be chosen to balance plan quality and delivery efficiency.

WE-E-AUD A-03: Evaluation of Dosimetric Margins in Prostate IMRT Treatment Plans Generated with Pinnacle DMPO

Purpose: To quantify ‘dosimetric margins’ existing in prostate IMRT plans generated with Pinnacle Direct Machine Parameter Optimization (DMPO), due to imperfect conformance of the planned dose distribution to delineated targets. To demonstrate that when these margins are accounted for, the van Herk margin framework provides accurate target coverage estimates. Method and Materials: DMPO was performed on 27 prostate plans with 5mm PTV margins. Setup errors were simulated via fluence convolution and sampling from a systematic error distribution, and resulting dose volume histograms (DVHs) were used to generate van Herk style Dose Population Histograms (DPHs). The dosimetric margin distribution (DMD) is the 3D margin distribution between the clinical target volume (CTV) and the treated volume (TV). Consistent with ICRU guidelines, the TV was the volume enclosed by the planning target volume (PTV) minimum dose isodose surface. Due to imperfect conformance of the planned dose distribution, the DMD can extend beyond the PTV. This creates an additional ‘buffer zone’ around targets, which can absorb setup and other geometric errors. The DMD was measured by exporting CTV and TVs from Pinnacle as meshes, and measuring collision distances. Results: DPH analysis showed the DMPO plans could tolerate random plus systematic setup errors having standard deviations (SDs) up to 3mm. Naive application of the van Herk margin formula suggests that 5mm PTV margins should absorb errors with SDs up to only 1.6mm. Coverage calculations based on measured DMDs were in agreement with the DPH analysis: the presence of dosimetric margins allows the prostate plans to absorb larger errors than one would naively expect. Conclusion: These DMPO results agree with those previously obtained using an in‐house optimizer. Both optimizers create similarly large dosimetric margins. Accounting for these margins enables accurate coverage estimates, without the need for simulations. (Supported by NIH R01CA98524 and NIH P01CA116602).

Clinical Evaluation of Direct Aperture Optimization When Applied to Head-And-Neck IMRT

Direct Machine Parameter Optimization (DMPO) is a leaf segmentation program released as an optional item of the Pinnacle planning system (Philips Radiation Oncology Systems, Milpitas, CA); it is based on the principles of direct aperture optimization where the size, shape, and weight of individual segments are optimized to produce an intensity modulated radiation treatment (IMRT) plan. In this study, we compare DMPO to the traditional method of IMRT planning, in which intensity maps are optimized prior to conversion into deliverable multileaf collimator (MLC) apertures, and we determine if there was any dosimetric improvement, treatment efficiency gain, or planning advantage provided by the use of DMPO. Eleven head-and-neck patients treated with IMRT had treatment plans generated using each optimization method. For each patient, the same planning parameters were used for each optimization method. All calculations were performed using Pinnacle version 7.6c software and treatments were delivered using a step-and-shoot IMRT method on a Varian 2100EX linear accelerator equipped with a 120-leaf Millennium MLC (Varian Medical Systems, Palo Alto, CA). Each plan was assessed based on the calculation time, a conformity index, the composite objective value used in the optimization, the number of segments, monitor units (MUs), and treatment time. The results showed DMPO to be superior to the traditional optimization method in all areas. Considerable advantages were observed in the dosimetric quality of DMPO plans, which also required 32% less time to calculate, 42% fewer MUs, and 35% fewer segments than the conventional optimization method. These reductions translated directly into a 29% decrease in treatment times. While considerable gains were observed in planning and treatment efficiency, they were specific to our institution, and the impact of direct aperture optimization on plan quality and workflow will be dependent on the planning parameters, planning system, and linear accelerators used by a particular institution.

SU-EE-A1-01: Dosimetric Comparisons of DMPO and Two-Step Approach Step-And-Shoot IMRT Plans

Purpose: To systematically evaluate step-and-shoot Intensity-Modulated-Radiation-Therapy (IMRT) plans generated by Direct-Machine-Parameter-Optimization (DMPO) and by Two-Step-Approach (TSA) using identical optimization parameters in Pinnacle 3 treatment planning system. Method and Materials: Using Pinnacle 3 version7.6c, TSA plans of total eight patients with Head-and-Neck, Prostate and Lung cancers were generated using identical optimization parameters from clinical plans used DMPO. The dose of planned-target-volume (PTV) in TSA plan was scaled to closely match at prescribed dose volume in the DMPO plan. Three PTV dosimetric indices: dose-coverage, dose-conformity and dose-inhomogeneity, were generated for each plan. Dosimetric comparisons were performed for organ-at-risk (OAR) with both “maximum-dose-objectives” and “dose-volume-based-objectives”. Final dose recalculation using EGS4-based in-house Monte-Carlo program for each plan was performed and corresponding dosimetric data were obtained. Film-based IMRT QA was performed for three patients. Results: On average, total monitor-units (MUs) are about 25% higher of TSA than DMPO. The averaged segment-numbers and PTV dosimetric indices are almost identical between plans from DMPO and TSA. The maximum-dose (defined at 0.1cc) of Head-and-Neck and Lung OARs with “maximum-dose-objectives” of TSA are, on average, ∼2.5Gy and ∼0.9Gy lower than those of DMPO, respectively. The averaged dose difference in prostate OARs with “maximum-dose-objectives” is small. For OARs with “dose-volume-based-objectives”, there is little difference between TSA and DMPO for all sites. The Monte-Carlo dose recalculations showed similar trends. The agreement between Pinnacle3 calculations and film measurements is 99% for all fields using 3%–3mm criteria. Conclusion: Dosimetric comparisons between DMPO and TSA IMRT plans demonstrated that using identical optimization parameters, DMPO plans have less total MUs and similar averaged segment-number as well as almost identical PTV dosimetric index values as TSA plans. For Head-and-Neck and lung plans, TSA has noticeable better sparing of OARs with “maximum-dose-based-objectives”, which is confirmed by Monte-Carlo recalculations. Film QA demonstrated both TSA and DMPO plans are very accurate.

Intensity-modulated radiotherapy of breast cancer using direct aperture optimization

Background and purpose To design a clinically reliable and efficient step-and-shoot IMRT delivery technique for the treatment of breast cancer using direct aperture optimization (DAO). Using DAO, segments are created and optimized within the same optimization process. Patients and methods The DAO technique implemented in the Pinnacle treatment planning system, which is called direct machine parameter optimization (DMPO), was used to generate IMRT plans for twelve breast cancer patients. The prescribed dose was 50 Gy. Two DMPO plans were generated. The first approach uses DMPO only; the second technique combines DMPO with two predefined segments (DMPO segm), having shapes identical to the conventional tangential fields. The weight of these predefined segments is optimized simultaneously with DMPO. The DMPO plans were compared with normal two-step (TS) IMRT, creating segments after optimizing the intensity. Results Dose homogeneity within the target volume was 4.8±0.6, 4.3±0.5 and 3.8±0.5 Gy for the TS, DMPO and DMPO segm plans, respectively. Comparing the IMRT plans with an idealized dose distribution obtained using only beamlet optimization, the degradation of the dose distribution was less for the DMPO plans compared with the two-step IMRT approach. Furthermore, this degradation was similar for all patients, while for the two-step IMRT approach it was patient specific. Conclusions An efficient step-and-shoot IMRT solution was developed for the treatment of breast cancer using DMPO combined with two predefined segments.