Progressive Resolution Optimizer Research Articles

Evaluation of improvements in plan quality with Photon Optimizer v16.1 for single brain lesion SRS treatment.

The purpose of this study is to compare the performance of the Photon Optimizer (PO) version 16.1 algorithm with its earlier version PO v13.6 and with Progressive Resolution Optimizer (PRO) version 13.6 algorithms. 20 patients with single brain lesions treated with the stereotactic radiosurgery (SRS) technique were retrospectively selected for this study. Initially, for all patients volumetric modulated arc therapy (VMAT) SRS plans were generated with the PRO v 13.6 algorithm. Then, all the plans were re-generated with two versions 13.6 and 16.1 of PO algorithm using the same setup and dose-volume optimization objectives as that of PRO with a similar planning approach. The quality of the generated plans was analysed using ICRU 91 plan evaluation parameters and also using dice similarity co-efficient (DSC), centre of mass distance (CMD) between target and prescription isodose line, Monitor units (MU) and brain-gross tumor volume (GTV) 12 Gy volume. Paired Student t-test was used for statistical analysis with 0.05 as a significant value. PO v16.1 improved all the dosimetric parameters studied compared to PO 13.6, the difference is statistically significant for all the parameters (p < 0.05), except for median dose and brain-GTV 12 Gy volume. PO v16.1 also showed statistically significant improvement for all the dosimetric parameters evaluated, except DSC and conformity index (CI), compared to PRO v13.6. The PO v16.1 generated plans are dosimetrically superior to PO v13.6 and PRO v13.6 in terms of target dose coverage and dose gradient with lesser beam modulation and plan complexity for single brain lesion SRS.

Comparison of Progressive Resolution Optimizer and Photon Optimizer algorithms in RapidArc delivery for head and neck SIB treatments.

The aim of this study is to analyze and verify characterization of two different algorithms using simultaneous integrated boost (SIB) in head and neck (H&N) plans. In our study 15 patients were selected, who received radiation therapy by using Eclipse volumetric modulated arc therapy (VMAT) Progressive Resolution Optimizer (PRO) algorithm 15.1. The same cases were re-optimized using a Photon Optimizer (PO) algorithm 15.6.A total of 30 treatment plans (15 PRO-VMAT plans and 15 PO-VMAT plans) were produced in the present study. All plans were created using double full arcs, keeping the identical constraints, cost functions and optimization time. Plan evaluation was done using planning target volume (PTV) parameters (D98%, D95%, D50%, D2% mean dose and V105%), homogeneity index (HI), conformity index (CI), Monitor unit (MU) per degree with control points (CP), organ at risk (OAR) doses and gamma verification (Portal dosimetry and ArcCHECK) values were evaluated. Treatment was delivered in Varian Truebeam 2.5, energy 6 MV with Millennium 120 multileaf collimator (MLC). The PTV coverage (D95%) for PRO and PO were 98.7 ± 0.8 Gy, 98.8 ± 0.9 Gy, HI were 0.09 ± 0.02 and 0.09 ± 0.02, CI were 0.98 ± 0.01 and 0.99 ± 0.01. Monitor units (MU) for PRO and PO were 647.5 ± 137.9, 655.2 ± 138.4. The Portal dose results were [3%, 3mm (%) & 1 %, 1 mm (%)] for PO and PRO 100 ± 0.1, 95.1 ± 1.4 and 100 ± 0.1, 95.2 ± 1.3. For ArcCHECK were 99.9 ± 0.1, 94.7 ± 3.0 and 99.9 ± 0.1, 93.5 ± 3.9, respectively. Results showed that PTV coverage and OAR doses were comparable. For individual patients CI and HI of PO showed slightly higher values than PRO. MUs for PO were slightly increased as compared to PRO. MU per degree with each individual control points generated by PO showed a high degree of modulation compared to PRO. Hence, new PO optimizer can produce a comparable degree of plan while using the same PRO objectives.

Progressive resolution optimizer (PRO) predominates over photon optimizer (PO) in sparing of spinal cord for spine SABR VMAT plans

Backgroundwe assessed the performance of the optimization algorithms by comparing volumetric modulated arc therapy generated by a progressive resolution optimized (VMATPRO) and photon optimizer (VMATPO) in terms of plan quality, MU reduction, sparing of the spinal cord (or cauda equina), and plan complexity.MethodsFifty-seven patients who received spine stereotactic ablative radiotherapy (SABR) with tumors located in the cervical, thoracic, and lumbar spine were retrospectively selected. For each patient, VMATPRO and VMATPO with two full arcs were generated with using the PRO and PO algorithms. For dosimetric evaluation, the dose-volumetric (DV) parameters of the planning target volume (PTV), organs at risk (OARs), the corresponding planning organs at risk (PRV), and 1.5-cm ring structure surrounding the PTV (Ring1.5 cm) were calculated for all VMAT plans. The total number of monitor units (MUs) and the modulation complexity score for the VMAT (MCSv) were compared. To investigate the correlations of OAR sparing to plan complexity, Pearson’s and Spearman’s correlation tests were conducted between the two algorithms (PO – PRO, denoted as Δ) in the DV parameters for normal tissues, total MUs, and MCSv.ResultsFor the PTVs, Target conformity and dose homogeneity in the PTVs of VMATPRO were better than those of VMATPO with statistical significance. For the spinal cords (or cauda equine) and the corresponding PRVs, all of the DV parameters for VMATPRO were markedly lower than those for VMATPO, with statistical significance (all p < 0.0001). Among them, the difference in the maximum dose to the spinal cord between VMATPRO and VMATPO was remarkable (9.04 Gy vs. 11.08 Gy with p < 0.0001). For Ring1.5 cm, no significant difference in V115% for VMATPRO and VMATPO was observed.ConclusionsThe use of VMATPRO resulted in improved coverage and uniformity of dose to the PTV, as well as OARs sparing, compared with that of VMATPO for cervical, thoracic, and lumbar spine SABR. Better dosimetric plan quality generated by the PRO algorithm was observed to result in higher total MUs and plan complexity. Therefore, careful evaluation of its deliverability should be performed with caution during the routine use of the PRO algorithm.

Clinical evaluation of photon optimizer (PO) MLC algorithm for stereotactic, single-dose of VMAT lung SBRT

Recently implemented photon optimizer (PO) MLC optimization algorithm is mandatory for RapidPlan modeling in Eclipse. This report quantifies and compares the dosimetry and treatment delivery parameters of PO vs its predecessor progressive resolution optimizer (PRO) algorithm for a single-dose of volumetric modulated arc therapy (VMAT) lung stereotactic body radiation therapy (SBRT). Clinical SBRT treatment plans for 12 early-stage non–small-cell lung cancer patients receiving 30 Gy in 1 fraction using PRO-VMAT were re-optimized using the PO-VMAT MLC algorithm with identical planning parameters and objectives. Average planning target volume derived from the 4D CT scans was 13.6 ± 12.0 cc (range: 4.3 to 41.1 cc) Patients were treated with 6 MV flattening filter free beam using Acuros-based calculations and 2.5 mm calculation grid-size (CGS). Both treatment plans were normalized to receive same target coverage and identical CGS to isolate effects of MLC positioning optimizers. Original PRO and re-optimized PO plans were compared via RTOG–0915 protocol compliance criteria for target conformity, gradient indices, dose to organs at risks and delivery efficiency. Additionally, PO-VMAT plans with a 1.25 mm CGS were evaluated. Both plans met RTOG protocol requirements. Conformity indices showed no statistical difference between PO 2.5 mm CGS and PRO 2.5 mm CGS plans. Gradient index (p = 0.03), maximum dose to 2 cm away from planning target volume in any direction (D2cm) (p < 0.05), and gradient distance (p < 0.05) presented statistically significant differences for both plans with 2.5 mm CGS. Some organs at risks showed statistically significant differences for both plans calculated with 2.5 mm CGS; however, no clinically significant dose differences were observed between the plans. Beam modulation factor was statistically significant for both PO 1.25 mm CGS (p = 0.001) and PO 2.5 mm CGS (p < 0.001) compared to clinical PRO 2.5 mm CGS plans. PO-VMAT plans provided decreased beam-on time by an average of 0.2 ± 0.1 minutes (up to 1.0 minutes) with PO 2.5 mm and 1.2 ± 0.39 minutes (maximum up to 3.22 minutes) with PO 1.25 mm plans compared to PRO 2.5 mm plans. PO-VMAT single-dose of VMAT lung SBRT plans showed slightly increased intermediate-dose spillage but boasted overall similar plan quality with less beam modulation and hence shorter beam-on time. However, PO 1.25 mm CGS had less intermediate-dose spillage and analogous plan quality compared to clinical PRO-VMAT plans with no additional cost of plan optimization. Further investigation into peripheral targets with PO-MLC algorithm is warranted. This study indicates that PO 1.25 mm CGS plans can be used for RapidPlan modeling for a single dose of lung SBRT patients. PO-MLC 1.25 mm algorithm is recommended for future clinical single-dose lung SBRT plan optimization.

Improving treatment efficiency via photon optimizer (PO) MLC algorithm for synchronous single-isocenter/multiple-lesions VMAT lung SBRT.

PurposeElderly patients with multiple primary or oligometastases (<5 lesions) lesions with associated co‐morbidities may not retain their treatment position for the traditional long SBRT treatment time with individual isocenters for each lesion. Treating multiple lesions synchronously using a single‐isocenter volumetric arc therapy (VMAT) plan would be more efficient with the use of the most recently adopted photon optimizer (PO) MLC algorithm and improve the patient comfort. Herein, we quantified the clinical performance of PO versus its predecessor progressive resolution optimizer (PRO) algorithm for single‐isocenter/multiple‐lesions VMAT lung SBRT.Materials and methodsFourteen patients with metastatic non‐small‐cell lung cancer lesions (two to five, both uni‐ and bilateral lungs) received a highly conformal single‐isocenter co/non‐coplanar VMAT (2–6 arcs) SBRT treatment plan. Patients were treated with a 6X‐FFF beam and Acuros algorithm with a single‐isocenter placed between/among the lesions, using PO for MLC optimization. Average isocenter to tumor distance was 5.5 ± 1.9 cm. Mean combined PTV derived from 4D‐CT scans was 38.7 ± 22.7 cc. Doses were 54 Gy/50 Gy in 3/5 fractions prescribed to 70%–80% isodose line so that at least 95% of the PTV receives 100% of prescribed dose. Plans were re‐optimized using PRO algorithm. Plans were compared via ROTG‐0915 protocol criteria for target conformity, heterogeneity and gradient indices, and dose to organs‐at‐risk (OAR). Additionally, total number of monitor units (MU), modulation factor (MF) and beam‐on time were compared.ResultsAll plans met SBRT protocol requirements for target coverage and OAR doses. Comparison of target coverage and dose to the OAR showed no statistical significance between the two plans. PO had 1042 ± 753 (P < 0.001) less MU than PRO resulting in a beam‐on time of about 0.75 ± 0.5 min (P < 0.001) less, on average. For similar dose distribution, a significant reduction of beam delivery complexity was observed with PO (average MF = 3.7 ± 0.7) vs PRO MLC algorithm (average MF = 4.4 ± 1.3) (P < 0.001).ConclusionsPO MLC algorithm improved treatment efficiency without compromising plan quality when compared to PRO algorithm for single‐isocenter/multi‐lesions VMAT lung SBRT. Shorter beam‐on time can potentially reduce intrafraction motion errors and improve patient compliance. PO MLC algorithm is recommended for future clinical lung SBRT plan optimization.

EP-1809 Comparison of Photon Optimizer (PO) and Progressive Resolution Optimizer (PRO) for SRS VMAT Plans

EP-1809 Comparison of Photon Optimizer (PO) and Progressive Resolution Optimizer (PRO) for SRS VMAT Plans

21 How can we improve treatment plan quality by statistical reporting of patient data?

Introduction A statistical reporting is intended to guarantee the results of treatment plans and to help detect the change of practice. The aim of this study is to obtain an indicator in order to know if the optimization was successful and to maintain a good clinical practice. Methods In our department, the medical prescription for patients treated for prostate cancer is a 75 Gy dose (2.5 Gy per fraction) on the prostate and a 46 Gy dose (2 Gy per fraction) on the iliac lymph nodes. Plans with VMAT (arcs of 6 MV, dose rate: 600 UM/min) are calculated in the Eclipse Treatment Planning System (AAA algorithm, Varian) version 13.6. The coverage of the 95% of PTV with 97% of the prescribed dose is always respected. With the analysis of 32 treatment plans, DVH values for organs at risk (OAR) are collected and are treated in the ascending order. -Bladder: V57Gy, V70Gy, V75Gy (%) -Bladder wall: V67Gy (cc) -Rectum: V46Gy, V60Gy, V70Gy, V75Gy (%) -Rectum wall: V67Gy (cc) These values are divided into four quartiles: Q1, Q2, Q3, and Q4. For each plan, a homemade dosimetric score is computed. If dosimetric indice is smaller than Qn, the score is n. If dosimetric indice is between n and (n + 1), the score is (n + 1). An average score on each OAR is calculated: 1 is the best and 4 the worst achievable score. These scores have been determined for 45 treatment plans calculated with Progressive Resolution Optimizer (PRO13) algorithm and 45 treatment plans calculated with Photon Optimizer (PO13) algorithm. A paired Student’s t-test ( α = 0.05) was used to cross-check all plans. Finally, the possibility of improving a plan was studied for a plan calculated with PO13 and a score higher than 4. Results Results reported in Table 1 show our values of Q1, Q2, Q3, and Q4. With the new algorithm PO13, the average score is reduced from 3.8 to 3.2, with a more important benefit on the rectum because the score is decreased in average from 4 to 3.2. And, the differences were significant (p Conclusions Combined with the dosimetric indices, these scores are useful for improving our clinical practice. Our service has set new standards: now, values Q2 are our optimization constraints and we continue to optimize if the score is higher than 4. The values of quartiles will be reviewed if you change your practice, with an improvement. The second aim of this study is to configure a knowledge-based planning (Rapidplan) with a robust and representative sample of plans. Download : Download high-res image (194KB) Download : Download full-size image

A quantitative assessment of the consequences of allowing dose heterogeneity in prostate radiation therapy planning.

Target dose uniformity has been historically an aim of volumetric modulated arc therapy (VMAT) planning. However, for some sites, this may not be strictly necessary and removing this constraint could theoretically improve organ‐at‐risk (OAR) sparing and tumor control probability (TCP). This study systematically investigates the consequences of PTV dose uniformity that results from the application or removal of an upper dose constraint (UDC) in the inverse planning process for prostate VMAT treatments. OAR sparing, target coverage, hotspots, and plan complexity were compared between prostate VMAT plans with and without the PTV UDC optimized using the progressive resolution optimizer (PRO, Varian Medical Systems, Palo Alto, CA). Removing the PTV UDC, the median D1cc reached 144.6% for the CTV and the PTV, and an average increase of 3.2% TCP was demonstrated, while CTV and PTV coverage evaluated by D99% was decreased by less than 0.6% with statistical significance. Moreover, systematic improvement in the rectum dose volume histograms was shown (a 5–10% decrease in the volume receiving 50% to 75% prescribed dose), resulting in an average decrease of 1.3% (P < 0.01) in the rectum normal tissue complication probability. Additional consequences included potentially increased dose to the urethra as evaluated by PTV D0.035cc (median: 153.4%), delivering 283 extra monitor units (MUs), and slightly higher degrees of modulation. In general, the results were consistent when a different optimizer (Photon Optimizer, Varian Medical Systems) was used. In conclusion, removing the PTV UDC is acceptable for localized prostate cases given the systematic improvement of rectal dose and TCP. It can be particularly useful for cases that do not meet the rectum dose constraints with the PTV UDC on. This comes with the foreseeable consequences of increased dose heterogeneity in the PTV and an increase in MUs and plan complexity. It also has a higher requirement for reproducing the position and size of the target and OARs during treatment. Finally, with the PTV UDC completely removed, in some cases the maximum doses within the PTV did approach levels that may be of concern for urethral toxicity and therefore in clinical implementation it may still be necessary to include a PTV UDC, but one based on limiting toxicity rather than enforcing dose homogeneity.

P184] Proposed solution to improve the protection of rectum in VMAT prostate treatments

Purpose Volumetric modulated arc therapy (VMAT) is a technique widely used in prostate cancer nowadays, which decreases the delivery time and the dose in the organs at risk. The goal of this work is to achieve a higher dose gradient between the PTV and rectum, meanwhile, decrease the optimization time in our treatment planning system (TPS). Methods Our TPS is Eclipse (Varian Medical System, Palo Alto, California). The VMAT optimizer is Progressive Resolution Optimizer (PRO v13.06.26) and the calculation algorithm is Anisotropic Analytical Algorithm (AAA v13.06.26). Instead the initial state of the multi-leaf collimator (MLC) given by the optimizer, we try to find one more adapted to the organs delimited by the physician. To obtain this initial state, an arc therapy treatment was generated fitting the MLC to the PTV in the beam’s eye view (BEV) with a 5 mm of margin and protecting the rectum without margin, as well. We propose to use this arc therapy plan as an initial plan when optimization starts and only perform the last stages of VMAT optimization, assigning priority only to the objectives of the PTV and the bladder. Results Our group has used this method for more than 20 concomitant and single volume treatments. We have compared the results of our method with those obtained by the usual method of optimization. In both cases, we delivered the dose with a single arch. The coverage of the PTV is similar in both solutions, but the protection of the rectum in low and medium doses is notably better with our technique. The MLC transitions between adjacent Gantry angles become smoother using our solution. The differences between the dose to the bladder and to the femoral heads in both methods are not remarkable. The time used to perform an optimization with the new technique is significantly lower. Conclusions The proposed method increases the protection of the rectum in prostate treatments, decreases the optimization time and simplifies the table of restrictions in the optimizer.

Dosimetric and Radiobiological Evaluation of Dose Volume Optimizer (DVO) and Progressive Resolution Optimizer (PRO) Algorithm against Photon Optimizer on IMRT and VMAT Plan for Prostate Cancer

Dosimetric and Radiobiological Evaluation of Dose Volume Optimizer (DVO) and Progressive Resolution Optimizer (PRO) Algorithm against Photon Optimizer on IMRT and VMAT Plan for Prostate Cancer

26. Implementation of a new optimization algorithm in VMAT for the treatment of prostate cancer

Introduction The aim of the current study is to evaluate a new optimization algorithm available since the Eclipse’s version 13.6 (Varian): Photon Optimizer version 13 (PO13). The purpose is to obtain a treatment planning protocol for prostate cancer treatments, reproducible which can be delivered by the treatment machine (patient’s quality assurance QA) and evaluate the dosimetric gain for patients (better organ at risk protection). Methods In our department, the medical prescription for the patients treated for a prostate cancer is a 75 Gy dose (2.5 Gy per fraction) on the prostate (PTVPRO) and a 46 Gy dose (2 Gy per fraction) on the right and left iliac lymph nodes. Treatments planning are made of 2 treatment plans: a first treatment plan in integrated boost, delivering a dose of 57.5 Gy on the PTVPRO and of 46 Gy on the PTVGGs in 23 fractions, and the 2nd treatment plan with a dose of 17.5 Gy on the PTVPRO in 7 fractions. Plans are optimized with PO13 and are calculated in the Eclipse’s Treatment Planning System (AAA algorithm, Varian) version 13.5 in order to be delivered on a Truebeam (Varian) version 2.5 MR2 in VMAT (arcs of 6 MV, dose rate: 600 UM/min). Several adjustable parameters with PO13 are tested on one patient: Normal Tissue Optimization NTO’s influence, calculation resolution during the optimization (1.25 mm, 2.5 mm and 5 mm), one or more arcs, optimization volumes, PTV’s priority compared to OAR priority, PTV’s priority compared to NTO priority, use of the intermediate dose calculation. The methodology used is to obtain the best PTV’s cover, changing a single parameter at each optimization without optimising relatively to the OAR at first. Parameters chosen and fixed, the constraints on the OAR are introduced. These parameters will be tested with several patients (reproducibility). The plan’s dosimetric evaluation is realised with the following Dose Volume Histogram (DVH): - For PTV: D95%, D98%, D2%, Dmin and Confomation Number (CN)1. - For OAR: bladder and rectum (Dmax, V35Gy, V50Gy), femoral heads (Dmax and Dmean). The monitor units number (MU) per arc is also noted. The dosimetric benefit for the patient will be evaluated in comparison with the real treatment plan calculated with the PRO13 algorithm (Progressive Resolution Optimizer algorithm version 13). The validation of the patient QA of these plans will be realised with the portal imager (Gamma index, PDIP Varian). Results The protocol will be based on the plan presenting a sufficient coverage for the PTV with the best CN taking into account the DVH values on the OAR. The 57.5 Gy plan chosen is Plan_1 (Table 1) with 2 arcs, a priority of 200 on the PTV for a priority of 500 on the NTO (automatic), a calculation resolution of 2.5 mm, an intermediate dose calculation. Plan_2 with a single arc has D98% and D95% reduced by 1 Gy compared to the other plans and Plan_4 has a CN too weak on the two PTV. Plan_3 with a calculation resolution of 1.25 mm seems the same as Plan_1 but the optimization time is multiplied by 2, a resolution of 2.5 mm is therefore preferred. The first results including the OAR’s constraints (priority of 100) shows that there is a significant reduction of DVH values V35Gy and V50Gy between the PRO13 and the new protocol in PO13 (Table 1). Download : Download high-res image (261KB) Download : Download full-size image Conclusions This study allowed us to simplify our practices with the use of the NTO as a replacement for optimization volumes (rings). It appears that the treatment plans obtained with PO13 allow a better saving of OAR with an equivalent coverage of PTV.

Photon optimizer (PO) vs progressive resolution optimizer (PRO): a conformality- and complexity-based comparison for intensity-modulated arc therapy plans

This study aimed to provide guidance on the advantages and limitations of a new optimizer, “photon optimizer” (PO), when compared with its predecessor, “progressive resolution optimizer” (PRO), for intensity-modulated arc therapy (IMAT) plans.Eleven study plans that included a cohort of prostate, head and neck, and brain treatment sites were optimized using both PRO and PO algorithms. A plan template using the same objectives for the same number of iterations was used for each optimized plan to obtain hypothetical treatment plans that would be comparable with a clinical plan. Analysis was performed using plan conformity-based parameters such as target volume coverage factor, conformation number and homogeneity indices, and plan complexity assessment parameters such as small aperture score, modulation indices, and monitor unit variation with arc angle for prostate, brain and head, and neck IMAT treatment plans.Plan conformality analysis demonstrated that conformation numbers, target volume coverage factors, and homogeneity indices produced by the 2 optimizers were comparable for most anatomic sites. IMAT treatment plans produced using the PRO optimizer were found to be less complex than plans produced using the PO optimizer, in terms of multileaf collimator (MLC) leaf position variability and modulation complexity scores. Similarly, the PRO optimizer was shown to produce treatment plans that used fewer monitor units (and generally fewer monitor unit per degree of arc rotation) than PO optimizer.This study demonstrated that the PO optimizer can produce IMAT treatment plans with a similar degree of dose conformity to the target volume and generally improved organ at risk sparing, compared with the PRO optimizer. Better coverage to organs at risk produced by plans optimized using PO was observed to have higher MLC variability and monitor units. Therefore, careful evaluation of treatment plan conformity and complexity before assessing its deliverability is recommended when implementing the routine use of PO optimizer.

Trajectory optimization in radiotherapy using sectioning (TORUS).

A challenging problem in trajectory optimization for radiotherapy is properly handling the synchronization of the medical accelerators dynamic delivery. The initial coarse sampling of control points implemented in a Progressive Resolution Optimization type approach (VMAT) routinely results in MLC aperture forming contention issues as the sampling resolution increases. This work presents an approach to optimize continuous, beam-on radiation trajectories through exploration of the anatomical topology present in the patient and formation of a novel dual-metric graph optimization problem. This work presents a new perspective on trajectory optimization in radiotherapy using the concept of sectioning (TORUS). TORUS avoids degradation of 3D dose optimization quality by mapping the connectedness of target regions from the BEV perspective throughout the space of deliverable coordinates. This connectedness information is then incorporated into a graph optimization problem to define ideal trajectories. The unique usage of two distance functions in this graph optimization permits the TORUS algorithm to generate efficient dynamic trajectories for delivery while maximizing the angular flux through all PTV voxels. 3D dose optimization is performed for trajectories using a commercial TPS progressive resolution optimizer. The TORUS algorithm is applied to three example treatments: chest wall, scalp, and the TG-119 C-shape phantom. When static collimator coplanar trajectories are generated for the chest wall and scalp cases, the TORUS trajectories are found to outperform both 7 field IMRT and 2 arc VMAT plans in delivery time, organ at risk sparing, conformality, and homogeneity. For the TG-119 phantom, when static couch and collimator non-coplanar trajectories are optimized, TORUS trajectories have superior sparing of the central core avoidance with shorter delivery times, with similar dose conformality and homogeneity. The TORUS algorithm is able to automatically generate trajectories having improved plan quality and delivery time over standard IMRT and VMAT treatments. TORUS offers an exciting and promising avenue forward toward increasing dynamic capabilities in radiation delivery.

Assessment of monitor unit limiting strategy using volumetric modulated arc therapy for cancer of hypopharynx

PurposeTo quantify relative merit of MU deprived plans against freely optimized plans in terms of plan quality and report changes induced by progressive resolution optimizer algorithm (PRO3) to the dynamic parameters of RapidArc. Materials and methodsTen cases of carcinoma hypopharynx were retrospectively planned in three phases without using MU tool. Replicas of these baseline plans were reoptimized using “Intermediate dose” feature and “MU tool” to reduce MUs by 20%, 35%, and 50%. Overall quality indices for target and OAR, integral dose, dose-volume spread were assessed. All plans were appraised for changes induced in RapidArc dynamic parameters and pre-treatment quality assurance (QA). ResultsWith increasing MU reduction strength (MURS), MU/Gy values reduced, for all phases with an overall range of 8.6–34.7%; mean dose rate decreased among plans of each phase, phase3 plans recorded greater reductions. MURS20% showed good trade-off between MUs and plan quality. Dose-volume spread below 5Gy was higher for baseline plans while lower between 20 and 35Gy. Integral dose was lower for MURS0%, not exceeding 1.0%, compared against restrained plans. Mean leaf aperture and control point areas increased systematically, correlated negatively with increasing MURS. Absolute delta dose rate variations were least for MURS0%. MU deprived plans exhibited GAI (>93%), better than MURS0% plans. ConclusionBaseline plans are superior to MU restrained plans. However, MURS20% offers equivalent and acceptable plan quality with mileage of MUs, improved GAI for complex cases. MU tool may be adopted to tailor treatment plans using PRO3.

Photon Optimizer (PO) prevails over Progressive Resolution Optimizer (PRO) for VMAT planning with or without knowledge‐based solution

The enhanced dosimetric performance of knowledge‐based volumetric modulated arc therapy (VMAT) planning might be jointly contributed by the patient‐specific optimization objectives, as estimated by the RapidPlan model, and by the potentially improved Photon Optimizer (PO) algorithm than the previous Progressive Resolution Optimizer (PRO) engine. As PO is mandatory for RapidPlan estimation but optional for conventional manual planning, appreciating the two optimizers may provide practical guidelines for the algorithm selection because knowledge‐based planning may not replace the current method completely in a short run. Using a previously validated dose–volume histogram (DVH) estimation model which can produce clinically acceptable plans automatically for rectal cancer patients without interactive manual adjustment, this study reoptimized 30 historically approved plans (referred as clinical plans that were created manually with PRO) with RapidPlan solution (PO plans). Then the PRO algorithm was utilized to optimize the plans again using the same dose–volume constraints as PO plans, where the line objectives were converted as a series of point objectives automatically (PRO plans). On the basis of comparable target dose coverage, the combined applications of new objectives and PO algorithm have significantly reduced the organs‐at‐risk (OAR) exposure by 23.49–32.72% than the clinical plans. These discrepancies have been largely preserved after substituting PRO for PO, indicating the dosimetric improvements were mostly attributable to the refined objectives. Therefore, Eclipse users of earlier versions may instantly benefit from adopting the model‐generated objectives from other RapidPlan‐equipped centers, even with PRO algorithm. However, the additional contribution made by the PO relative to PRO accounted for 1.54–3.74%, suggesting PO should be selected with priority whenever available, with or without RapidPlan solution as a purchasable package. Significantly increased monitor units were associated with the model‐generated objectives but independent from the optimizers, indicating higher modulation in these plans. As a summary, PO prevails over PRO algorithm for VMAT planning with or without knowledge‐based technique.

32. Comparison between multi-criteria optimization (MCO) (Raystation®) and Progressive Resolution Optimizer (PRO) (Eclipse®) for the dosimetry of breast cancer with prophylactic nodal irradiation treated with volumetric modulated arc therapy (VMAT)

Introduction Since 2012 in our centers, VMAT with ECLIPSE® Treatment Planning System (TPS) v10 (Varian®) is routinely used to treat patients with breast cancer requiring prophylactic nodal irradiation. In 2015, we acquired the RayStation® TPS v4.7.2 (RAYSEARCH®), with the new dosimetric approach based on MCO concept. The aim of the study is to demonstrate that the use of the MCO reduces learning time by obtaining faster treatment plans respecting the clinical dosimetric constraints commonly published in the literature. Methods This retrospective study included 10 patients, randomly Selected, treated for breast cancer (5 left breasts and 5 right breasts) with nodal irradiation (internal mammary chain, supraclavicular area and axillar third area of Berg when clinically relevant) using VMAT. For each patient, the initial dosimetry was performed using the Eclipse TPS with PRO optimization algorithm V3 and the calculation of the dose with the algorithm “Analytic Anisotropic Algorithm” V10 and 2 mm grid. The prescribed doses were 50 Gy on the mammary gland and from 44 to 50 Gy to the nodes, within 25 fractions. For each patient, a new plan was performed using the RayStation® TPS, maintaining ballistic parameters and the computing grid (2 mm), but with the MCO optimization and the algorithm for dose calculation “Collapsed Cone Convolution”. PTV coverage was evaluated in terms of the homogeneity index (HI), conformity index (CI), DMEAN (Gy), V95% and doses to organs at risks were compared to each other. Results The Homogeneity index was significantly better in the MCO algorithm arm with V95% = 95.39 ± 1.96% versus 85.15 ± 8.48% (p = 0.005) in the PRO arm. Even finding for the HI index = 0.15 ± 0.03 vs 0.21 ± 0.03 (p = 0.007)) and HI index = 0.18 ± 0.06 vs 0.24 ± 0.05, respectively for the PTVBreast and PTVNodes. The analysis of doses to organs at risk for the 10 patients shows that there was no significant difference between the two algorithms (p > 0.05). Conclusions: The MCO algorithm can be used to generate efficiently treatment plans for complex breast cancer. The TPS RayStation with the MCO optimization can quickly and easily obtain comparable or even higher treatment plans than the PRO algorithm. This new TPS was easily and quickly adopted by our team for this location.

Assessment and performance evaluation of photon optimizer (PO) vs. dose volume optimizer (DVO) for IMRT and progressive resolution optimizer (PRO) for RapidArc planning using a virtual phantom

Purpose: The purpose of the study was to present the quantitative and qualitative evaluation of newly incorporated photon optimizer (PO) versus previously was used independent dose volume optimizer (DVO) for intensity modulated radiation therapy (IMRT) and progressive resolution optimizer (PRO) for Rapid-arc/ volumetric modulated arc therapy (VMAT) in version 13.5 of Eclipse treatment planning system (ETPS). Methods: We accomplished this study with the help of cylindrical virtual phantom created in the ETPS. Six individual phantoms study sets (PSS) were generated and different material density value was assigned in order to evaluate the behavior optimizers in the presence of tissue heterogeneity. Several independent plans were generated for IMRT and Rapid-arc by changing optimizer module PO, DVO, and PRO for 6 MV, 15 MV flattened beam and 6 MV-flattening filter free (FFF) beam. Results: The self-governing evaluations of PO versus DVO for IMRT plan and PO versus PRO for Rapid-arc/VMAT plans were performed. We estimated and compared various distinct parameters such as maximum dose, minimum dose, mean dose, conformity index (CI), quality index (QI), homogeneity index (HI), integral plan monitor unit (MU) and dose volume histogram (DVH). The percentages of the average variation over all PSS and beam energy between PO versus DVO optimized plan quality parameters such as planning target volume (PTV) maximum, minimum, mean doses, CI, QI and HI were 0.23%, 1.67%, 0.09%, 20.4%, 0.77% and 0.52% , respectively, whereas for PO versus PRO were 1.18%, 3.38%, 0.19%, 8.11%, 2.78%, and 1.28%, respectively. Conclusion: The results presented in this study showed that PO generates plans with better quality in shorter time compared to DVO and PRO for both IMRT and Rapid-arc/VMAT, respectively.

SU‐F‐T‐412: Spatiotemporal Distribution of Dose Rates in VMAT Plans

Purpose:To assess dose rates in volumetric modulated arc therapy (VMAT) plans.Methods:For dose rate analysis, using Monte Carlo methods, dose distributions are calculated in sectors of consecutive DICOM control points and each multiplied with the MU‐rate obtaining dose rate distributions for every voxel at every time‐point during a treatment fraction. Dose rate distributions were calculated for a clinical head and neck, cranial (2 Gy fraction dose) and a SBRT liver treatment case (5 Gy per fraction). To assess optimizer influence on dose rate distributions, five conventionally fractionated head and neck cases were optimized using the progressive resolution optimizer (PRO) versions 2 and 3 (Varian Medical Systems, Palo Alto, CA).Results:Firstly, histograms of dose rates over the course of treatment delivery in volumes of interest were generated for the PTV and spine or brainstem of the clinical treatment plans. Secondly, dose rates were assessed as color wash overlay on top of CT slices, allowing the user to explore the dose rate variation over time. For the cases considered, dose rates were spread out over a continuous spectrum with a mean value in the lower half. PRO3 generated plans showed higher maximum dose rates compared to PRO2 (699.80 cGy/min vs. 543.46 cGy/min) and modulation of MU rates in blocks as opposed to gradual modulation.Conclusion:Calculation and visualization of dose rate distributions in VMAT have been performed. Dose rates in clinical relevant VMAT cases are spread out over a continuous spectrum with a mean in the lower part for both conventional and SBRT test cases. Different optimization algorithms may lead to substantially different dose rate distributions in the same cases. The development of the Swiss Monte Carlo Plan framework has been partially supported by Varian Medical Systems

SU‐F‐T‐417: Dosimetric Evaluation of Dose‐To‐Medium Vs. Dose‐To‐Water Using Acuros XB for Lung, Spine and Head‐And‐Neck Plans

Purpose:This study evaluates the dosimetric differences between dose‐to‐medium (Dm) and conventional dose‐to‐water (Dw) reporting using Varian's Acuros XB (AXB) dose calculation algorithm with actual patient treatment plans.Methods:Twelve patients were selected: four head‐and‐neck (HN) (2 Gy × 33–35 fractions), four lung SBRT (10–12 Gy × 5 fractions) and four spine SBRT (6–8 Gy × 5 fractions) cases. All treatment plans were optimized using the Varian Eclipse Progressive Resolution Optimizer (version 13.6) to meet objectives of institutional guidelines. After the optimization process, each plan was calculated using both Dw and Dm options within AXB. The calculated treatment plans were normalized such that the prescribed dose covered at least 95% of the PTV. The dose‐volume histograms were evaluated for various dosimetric parameters. The percent difference between the two options is calculated as (AXB_Dw − AXB_Dm)/AXB_Dm *100.Results:The largest difference between AXB_Dw and AXB_Dm was seen in the mandible structure for HN cases, where AXB_Dw was up to 10% higher than AXB_Dm results (5.3%–10.2%). AXB_Dw was also higher for PTV maximum dose in these cases, ranging from 2.5%–5.7%. Among all other dosimetric parameters in HN, Spine and lung SBRT plans; the differences between AXB_Dw and AXB_Dm were within 5%. We also noted that AXB_Dw was consistently higher (1.8–3.8%) for cord in lung SBRT, and was consistently lower for esophagus/bowel in spine SBRT (−5% − −2.9%).Conclusion:The preliminary dosimetric results from our clinical study show that the selection of either Dm or Dw in AXB is not likely to produce significant dosimetric differences in most clinical cases. However, Dw can be different by up to 10% in bony anatomy compared to Dm. Therefore, Dm is the preferred option to achieve the greatest accuracy in both target and critical structure, providing a smooth transition from conventional to state‐of‐the‐art treatment planning.

Improved VMAT planning for head and neck tumors with an advanced optimization algorithm

ObjectiveIn this study, the “Progressive Resolution Optimizer PRO3“ (Varian Medical Systems) is compared to the previous version „PRO2“ with respect to its potential to improve dose sparing to the organs at risk (OAR) and dose coverage of the PTV for head and neck cancer patients. Materials and MethodsFor eight head and neck cancer patients, volumetric modulated arc therapy (VMAT) treatment plans were generated in this study. All cases have 2-3 phases and the total prescribed dose (PD) was 60-72Gy in the PTV. The study is mainly focused on the phase 1 plans, which all have an identical PD of 54Gy, and complex PTV structures with an overlap to the parotids. Optimization was performed based on planning objectives for the PTV according to ICRU83, and with minimal dose to spinal cord, and parotids outside PTV. In order to assess the quality of the optimization algorithms, an identical set of constraints was used for both, PRO2 and PRO3. The resulting treatment plans were investigated with respect to dose distribution based on the analysis of the dose volume histograms. ResultsFor the phase 1 plans (PD=54Gy) the near maximum dose D2% of the spinal cord, could be minimized to 22±5 Gy with PRO3, as compared to 32±12Gy with PRO2, averaged for all patients. The mean dose to the parotids was also lower in PRO3 plans compared to PRO2, but the differences were less pronounced. A PTV coverage of V95%=97±1% could be reached with PRO3, as compared to 86±5% with PRO2. In clinical routine, these PRO2 plans would require modifications to obtain better PTV coverage at the cost of higher OAR doses. ConclusionA comparison between PRO3 and PRO2 optimization algorithms was performed for eight head and neck cancer patients. In general, the quality of VMAT plans for head and neck patients are improved with PRO3 as compared to PRO2. The dose to OARs can be reduced significantly, especially for the spinal cord. These reductions are achieved with better PTV coverage as compared to PRO2. The improved spinal cord sparing offers new opportunities for all types of paraspinal tumors and for re-irradiation of recurrent tumors or second malignancies.