Standard Volumetric Modulated Arc Therapy Research Articles

Comparative Dosimetry for Hippocampal-Avoidant Whole Brain Radiotherapy with Helical Tomotherapy and VMAT Planning Techniques

The advent of hippocampal-avoidant whole brain radiotherapy (HA-WBRT) has allowed for substantial improvements in cognition without compromising the efficacy of palliative whole brain radiotherapy. Volumetric modulated arc therapy (VMAT), a form of rotational intensity-modulated radiation therapy (IMRT), allows for rapid treatment delivery and minimizes fractional treatment time. However, given the intrinsic complexity of HA-WBRT planning, standard VMAT techniques require significant departmental resources and often fail to achieve planning objectives. We hypothesize that treatment planning and delivery on a helical tomotherapy (HT) unit improves OAR dosimetry compared to standard VMAT techniques. Patients with multiple brain metastases who received WBRT with palliative intent were included in this single institution dosimetric study. Treatment objectives and dose constraints for HA-WBRT from NRG CC001 were utilized. Three separate plans were generated for each patient including Dual-Arc Conventional VMAT (DAC), Split-Arc Partial-Field VMAT (SAPF), and HT for dosimetric comparison. VMAT plans were generated using 6-MV photon beams with a maximum dose rate of 600 MU/min with a 120-leaf MLC. DAC plans utilized 2 coplanar arcs each with jaw tracking. SAPF plans used four partial arcs, and the field size of each beam was reduced to allow the MLC to block the centrally located hippocampus without sacrificing the whole brain PTV coverage. HT plans with a dose rate of 1000MU/min and Helical Delivery mode used a 2.5cm dynamic Jaw setting. Mean differences in target volume coverage and OAR dosimetry between planning approaches were calculated. Two-tailed, paired Student's t-tests were employed to determine statistically significant differences between DAC, SAPF, and HT plans. A total of 15 treatment plans were generated for five patients (5 DAC, 5 SAPF, and 5 HT plans). HT was seen to significantly reduce hippocampal D100% compared to both DAC (∆-114.16 cGy, p = 0.001) and SAPF (∆-125.76 cGy, p < 0.001). Moreover, HT hippocampal D0.03cc was significantly lower than DAC (∆-80.26 cGy p = 0.003) and SAPF (∆-174.40 cGy, p < 0.001). At the same time, PTV coverage as determined by D98% was significantly higher compared to both DAC (∆ +372.66 cGy, p = <0.001) and SAPF (∆ +304 cGy, p = <0.001). Compared to both DAC and SAPF, HT planning for HA-WBRT provides significant improvements in target coverage and OAR sparing. Additional research is warranted to determine whether further reductions in dose to the hippocampi provide additional clinical benefit.

Dosimetric evaluation of accelerated partial breast irradiation (APBI) using volumetric modulated arc therapy (VMAT) technique

Purpose: Five-fraction radiation therapy treatment regimens for accelerated partial breast irradiation (APBI) using intensity modulated radiotherapy (IMRT) have become increasingly used after several phase three trials demonstrated similar or reduced toxicity compared with whole breast irradiation. Improving the treatment efficiency of this technique could significantly improve delivery accuracy and tolerability, especially for treatment plans utilizing deep inspiration breath hold (DIBH) technique. The aim of this study was to determine the optimal technique for APBI, with a focus on volumetric modulated arc therapy technique (VMAT) and flattening-filter free (FFF) delivery. Methods: Ten APBI cases, five cases each left and right breast, were randomly selected. Each case was contoured following guidelines included in the APBI-IMRT-Florence Trial and planned to use both non-coplanar static-field IMRT with standard flattened-filter (FF) beam and VMAT techniques with both FF and FFF beam. For each VMAT plan, two partial arcs were used with patient-specific start and stop angles. Arc extents emulated opposed tangential fields and were chosen based on target and organ at risk (OAR) locations. Evaluated OARs included the heart, ipsilateral and contralateral lung, and normal breast. All plans were generated using the Eclipse treatment planning system (Version 15.6). A prescription dose of 30Gy delivered in 5 fractions was used for all cases. Plans were compared and evaluated using several dose metrics as well as treatment time. Results: For targets, VMAT exhibited similar coverage (V95%) but higher Dmax compared to IMRT (105% (IMRT) v.s .111% (VMAT), p=0.002. VMAT provided similar OAR avoidance compared to IMRT for the heart (Dmean, V3Gy, V0.5Gy), ipsilateral lung (V10Gy), contralateral lung (V5Gy) and contralateral breast Dmax within 10cGy/2% (p~0.005). VMAT contributed slightly lower dose outside the target in the ipsilateral breast, with an average difference in V15Gy of approximately 8% (37% (IMRT) v.s. 29% (VMAT)). Finally, because IMRT delivery is non-coplanar, and typically requires at least 5 treatment fields, co-planar VMAT reduced the total treatment significantly. VMAT with FF beam energies also used fewer monitor unites compared to IMRT with FF beam energies. The utilization of FFF beam energies for VMAT further improved delivery time compared to IMRT. Overall, VMAT with FFF beam energies reduced treatment time by approximately 20% compared to IMRT with FF beam. Conclusions: Co-planar partial-arc FFF VMAT technique produced equivalent target coverage, improved efficiency, better normal tissue sparing, and shorter delivery time compared to non-coplanar IMRT technique. Shorter delivery time also assists in the reduction of patient motion associated with breath hold. VMAT technique with FFF delivery is a suitable replacement for IMRT in APBI.

Complexity analysis of VMAT prostate plans: insights from dimensionality reduction and information theory techniques

Abstract Introduction: Volumetric Modulated Arc Therapy (VMAT) is a state-of-the-art prostate cancer treatment, defined by high dose gradients around targets. Its unique dose shaping incurs hidden complexity, impacting treatment deliverability, carcinogenesis, and machine strain. This study compares various aperture-based VMAT complexity indices in prostate cases using principal component and mutual information analyses. It suggests essential properties for an ideal complexity index from an information-theoretic viewpoint. Material and methods: The following ten complexity indices were calculated in 217 VMAT prostate plans: circumference over area (CoA), edge metric (EM), equivalent square field (ESF), leaf travel (LT), leaf travel modulation complexity score for VMAT (LTMCSV), mean-field area (MFA), modulation complexity score (standard MCS and VMAT variant MCSV), plan irregularity (PI), and small aperture score (SAS5mm). Principal component analysis (PCA) was applied to explore the correlations between the metrics. The differential entropy of all metrics was also calculated, along with the mutual information for all 45 metric pairs. Results: Whole-pelvis plans had greater complexity across all indices. The first three principal components explained 96.2% of the total variance. The complexity metrics formed three groups with similar conceptual characteristics, particularly ESF, LT, MFA, PI, and EM, SAS5mm. The differential entropy varied across the complexity metrics (PI having the smallest vs. EM the largest). Mutual information analysis (MIA) confirmed some metrics’ interdependence, although other pairs, such as LTMCSV/SAS5mm, LT/MCSV, and EM/SAS5mm, were found to share minimal MI. Conclusions: There are many complexity indices for VMAT described in the literature. PCA and MIA analyses can uncover significant overlap among them. However, this is not entirely reducible through dimensionality reduction techniques, suggesting that there also exists some reciprocity. When designing predictive models of quality assurance metrics, PCA and MIA may prove useful for feature engineering.

Dosimetric Comparision of Coplanar versus Noncoplanar Volumetric Modulated Arc Therapy for Treatment of Bilateral Breast Cancers.

The purpose of this study was to compare the dosimetric parameters of volumetric modulated arc therapy (VMAT) treatment plans using coplanar and noncoplanar beams in patients with bilateral breast cancer/s (BBCs) in terms of organ at risk sparing and target volume coverage. The hypothesis was to test whether VMAT with noncoplanar beams can result in lesser dose delivery to critical organs such as heart and lung, which will result in lesser overall toxicity. Data of nine BBC cases treated at our hospital were retrieved. Computed tomography simulation data of these cases was used to generate noncoplanar VMAT plans and the parameters were compared with standard VMAT coplanar plans. Contouring was done using radiation therapy oncology group guidelines. Forty-five Gray in 25 fractions was planned followed by 10 Gy in five fractions boost in breast conservation cases. No significant difference in planning target volume (PTV) coverage was found for the right breast/chestwall (P = 0.940), left breast/chestwall (P = 0.872), and in the total PTV (P = 0.929). Noncoplanar beams resulted in better cardiac sparing in terms of Dmean heart. The difference in mean dose was >1 Gy (8.80 ± 0.28 - 7.28 ± 0.33, P < 0.001). The Dmean, V20 and V30 values for total lung slightly favor noncoplanar beams, although there was no statistically significant difference. The average monitor units (MUs) were similar for coplanar plans (1515 MU) and noncoplanar plans (1455 MU), but the overall treatment time was higher in noncoplanar plans due to more complex setup and beam arrangement. For noncoplanar VMAT plans, the mean conformity index was slightly better although the homogeneity indices were similar. VMAT plans with noncoplanar beam arrangements had significant dosimetric advantages in terms of sparing of critical organs, that is Dmean of heart doses with almost equivalent lung doses and equally good target coverage. Larger studies with clinical implications need to be considered to validate this data.

Normal Tissue Integral Dose as a Result of Prostate Radiation Therapy: A Quantitative Comparison Between High-Dose-Rate Brachytherapy and Modern External Beam Radiation Therapy Techniques

Quantification of integral radiation dose delivered during treatment for prostate cancer is lacking. We performed a comparative quantification of dose to nontarget body tissues delivered via 4 common radiation techniques: conventional volumetric modulated arc therapy, stereotactic body radiation therapy, pencil-beam scanning proton therapy, and high-dose-rate brachytherapy. Plans for each radiation technique were generated for 10 patients with typical anatomy. For brachytherapy plans, virtual needles were placed to achieve standard dosimetry. Standard planning target volume margins or robustness margins were applied as appropriate. A "normal tissue" structure (entire computed tomography simulation volume minus planning target volume) was generated for integral dose computation. Dose-volume histogram parameters for targets and normal structures were tabulated. Normal tissue integral dose was calculated by multiplying normal tissue volume by mean dose. Normal tissue integral dose was lowest for brachytherapy. Pencil-beam scanning protons, stereotactic body radiation therapy, and brachytherapy resulted in 17%, 57%, and 91% absolute reductions compared with standard volumetric modulated arc therapy, respectively. Mean nontarget tissues receiving 25%, 50%, and 75% of the prescription dose were reduced by 85%, 76%, and 83% for brachytherapy relative to volumetric modulated arc therapy, by 79%, 64%, and 74% relative to stereotactic body radiation therapy, and 73%, 60%, and 81% relative to proton therapy. All reductions observed using brachytherapy were statistically significant. High-dose-rate brachytherapy is an effective technique for reducing dose to nontarget body tissues relative to volumetric modulated arc therapy, stereotactic body radiation therapy, and pencil-beam scanning proton therapy.

A Dosimetric Analysis of Modified Volumetric Modulated Arc Therapy for Bone Marrow Sparing Radiotherapy in Cervical Cancer-An alternative Approach to Conventional VMAT

External beam radiotherapy remains the primary treatment modality in cervical cancer. Nowadays Intensity Modulated Radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) are increasingly being used to reduce normal tissue toxicity. The drawback of conventional VMAT is that a considerable volume of pelvic bone marrow receives a low dose. We analyzed whether there was a way to reduce the volume of the low dose regions of bone marrow, and assessed the potential benefit of conventional-4Arc (C-4Arc VMAT), and Modified-4Arc (M-4Arc VMAT) over the conventional 2 ARC VMAT. Twelve clinically proven locally advanced cervical cancer patients treated with concurrent chemo-radiotherapy by Conventional VMAT (RapidArc) in dual rotation mode (C-2Arc VMAT) were selected for this study.C-4Arc VMAT and M-4Arc VMAT dose plans were generated for these twelve patients and these three different types of plans were evaluated for the quality and compared dosimetrically. M-4Arc VMAT designs exhibited a greater bone marrow sparing when compared with conventional VMATs with respect to volume receiving 5Gy to 35Gy without compromising PTV dose coverage. M-4Arc VMAT plans, the bone marrow volume receiving 30 Gy (V30Gy),40Gy (V40Gy), and mean doses were lower than the C- 4 Arc plan and a similar result was observed for V50(Gy) also when comparing with the standard 2 Arc plan. In modified VMAT plans, the rectum and bladder dose volumes were lower than standard VMAT. Similarly, the bowel bag V35(Gy), V40(Gy), V50(Gy), mean doses. The right and left femoral head doses were reduced significantly when compared to conventional VMAT plans. The M-4Arc VMAT plans are better than the C-2Arc and C-4Arc VMAT plans for reducing the dose to bone marrow by limiting the MLC field width travel.

Planning and dosimetric evaluation of three total body irradiation techniques: Standard SSD VMAT, Extended SSD VMAT and Extended SSD Field-in-Field.

The aim of this study was to dosimetrically compare three total body irradiation (TBI) techniques which can be delivered by a standard linear accelerator, and to deduce which one is preferable. Specifically, Extended Source to Surface Distance (SSD) Field-in-Field (FiF), Extended SSD Volumetric Modulated Arc Therapy (VMAT), and Standard SSD VMAT TBI techniques were dosimetrically evaluated. Percent depth dose and dose profile measurements were made under treatment conditions for each specified technique. After having generated treatment plans with a treatment planning system (TPS), dose homogeneity and critical organ doses were investigated on a Rando phantom using radiochromic films and optically stimulated luminescence dosimeters (OSLDs). TBI dose of 12Gy in six fractions was prescribed for each technique. The gamma index (5%/5mm) was used for the analysis of radiochromic films. Passing rates for Extended SSD FiF, Extended SSD VMAT and Standard SSD VMAT techniques were found to be 90%, 87% and 94%, respectively. OSLD measurements were within ± 5% agreement with TPS calculations for the first two techniques whereas the agreement was found to be within ± 3% for the Standard SSD VMAT technique. TPS calculations demonstrated that mean lung doses in the first two techniques were around 8.5Gy while it was kept around 7Gy in Standard SSD VMAT. It is concluded that Standard SSD VMAT is superior in sparing the lung tissue while all three TBI techniques are feasible in clinical practice with acceptable dose homogeneity. In the absence of VMAT-based treatment planning, Extended SSD FiF would be a reasonable choice compared to other conventional techniques.

Comparison of anatomically informed class solution template trajectories with patient-specific trajectories for stereotactic radiosurgery and radiotherapy.

Class solution template trajectories are used clinically for efficiency, safety, and reproducibility. The aim was to develop class solutions for single cranial metastases radiotherapy/radiosurgery based on intracranial target positioning and compare to patient-specific trajectories in the context of 4π optimization. Template trajectories were constructed based on the open-source Montreal Neurological Institute (MNI) average brain. The MNI brain was populated with evenly spaced spherical target volumes (2 cm diameter, N = 243) and organs-at-risk (OARs) were identified. Template trajectories were generated for six anatomical regions (frontal, medial, and posterior, each with laterality dependence) based on previously published 4π optimization methods. Volumetric modulated arc therapy (VMAT) treatment plans generated using anatomically informed template 4π trajectories and patientspecific 4π trajectories were compared against VMAT plans from a standard four-arc template. Four-arc optimization techniques were compared to the standard VMAT template by placing three spherical targets in each of six anatomical regions of a test patient. This yielded 54 plans to compare various plan quality metrics. Increasing plan technique complexity, the total number of OAR maximum dose reductions compared to the standard arc template for the 6 anatomical classes was 4+/-2 (OFIXEDc) and 7+/-2 (OFIXEDi). In 65.6% of all cases, optimized fixed-couch positions outperformed the standard-arc template. Of the three comparisons, the most complex (OFIXEDi) showed the greatest statistical significance compared to the least complex (VMATi) across 12 plan quality metrics of maximum dose to each OAR, V12Gy, total plan Monitor Units, conformity index, and gradient index (p < 0.00417). In approximately 70% of all cases, 4π optimization methods outperformed the standard-arc template in terms of maximum dose reduction to OAR, by exclusively changing the arc geometry. We conclude that a tradeoff exists between complexity of a class solution methodology compared to patient-specific methods for arc selection, in the context of plan quality improvement.

Novel VMAT Planning Technique Improves Dosimetry for Head and Neck Cancer Patients Undergoing Definitive Chemoradiotherapy

<h3>Purpose/Objective(s)</h3> Intensity modulated radiotherapy (IMRT) allows for substantial sparing of organs-at-risk (OARs) without compromising delivery of curative-intent doses in patients with head and neck cancers. Volumetric modulated arc therapy (VMAT) allows for rapid treatment delivery and minimizes fractional treatment time. Given the intrinsic complexity of VMAT planning, standard two-arc techniques often fail to achieve planning objectives. We hypothesize that a simple, institutional approach improves OAR dosimetry compared to a standard two-arc technique. <h3>Materials/Methods</h3> Head and neck cancer patients who received definitive chemoradiotherapy were included in this single institution study. VMAT plans were generated per institutional protocol using 6 MV photon beams. Briefly, three single arcs were employed with assigned collimator angles of (C) 0°, (C) 90°, and (C) 90°. The maximum jaw extension of the (C) 0° field encompassed the entirety of the treatment volume. The inferior jaw on the first collimated arc was set with a maximum extension of 1 cm beyond the isocenter, while the superior jaw of the second collimated arc was set with a maximum extension of 1 cm beyond the isocenter. Comparison plans were generated for each patient using the manufacturer's recommended beam arrangement, comprising two collimated arcs - (C) 30° and a reciprocal (C) 330° - with jaw tracking enabled. Mean differences in target volume and OAR dosimetry between planning approaches were calculated. A two-tailed, paired Student's t-test was employed to determine statistically significant differences in dosimetry between standard and institutional plans. <h3>Results</h3> Ten patients were planned using institutional and standard two-arc protocols. Statistically significant mean improvements were observed in ipsilateral parotid mean dose (∆314.7 cGy, p = 0.007) and V30Gy (∆6.62%, p = 0.004), contralateral parotid mean dose (∆292.6 cGy, p = 0.006) and V30Gy (∆5.36%, p = 0.004), parotid volume receiving less than 20 Gy (∆7.36 cc, p = 0.010), pharyngeal constrictor mean dose (∆269.3 cGy, p = 0.003), ipsilateral brachial plexus maximum dose (∆96.0 cGy, p = 0.036), and global maximum dose (∆1.73%, p = 0.047) with the three-arc technique. No significant differences in PTV, spinal cord, mandible, cricopharyngeus, cochlear, or contralateral brachial plexus dosimetry were identified. Subjectively, dosimetrists reported shorter planning time and easier achievement of objectives with the three-arc approach. <h3>Conclusion</h3> Our institutional approach to head and neck VMAT conveys significant dosimetric improvements compared to a standard VMAT beam arrangement. This approach represents a simple and reproducible solution to planning for head and neck cancer patients which may reduce planning time. Additional research is warranted to further optimize this technique and quantify reductions in planning time.

A dosimetric comparison of 3D DCAT vs VMAT for palliative and early-stage liver lesions using eclipse TPS.

Volumetric modulated arc therapy (VMAT) and 3D dynamic conformal arc therapy (DCAT) are 2 methods proven useful for the clinical implementation of stereotactic body radiation therapy (SBRT) for lung lesions however, similar comparisons of SBRT liver lesions are lacking. The purpose of this study was to determine if the conformity of dose, irradiated volume, and dose to organs at risk (OAR) are equivalent or improved with the use of DCAT as an alternative treatment method when compared to standard VMAT for SBRT delivery of palliative and early-stage liver lesions. Twenty patients with liver lesions sized 2.0 to 5.0 cm were selected for this study. Plans were created with both DCAT and VMAT techniques for each patient. Metrics evaluated included the mean heart, kidney, large bowel, small bowel, esophagus, and stomach doses, the lung volume receiving 20 Gy (V20), the volume of the normal liver receiving 15 Gy (V15), conformity index (CI), heterogeneity index (HI), and the irradiated volume or volume receiving 25 Gy (V25). The p-values for the mean dose to kidneys, small bowel, esophagus, and the lung V20 were greater than 0.05, and no statistical difference could be determined between DCAT and VMAT. The p-values for the mean heart, large bowel, stomach, and liver V15 were less than 0.05, indicating statistical significance and superiority of VMAT for minimizing dose to these organs, especially V15 of the liver. The DCAT technique produced CI greater than 1.0 for all patients proving superior coverage, while standard VMAT produced significantly improved V25 with p-values less than 0.0001, and consequently higher HI.

Flattening Filter-Free Volumetric-Modulated Arc Radiotherapy for Left-Sided Whole-Breast, Partial-Breast, and Postmastectomy Irradiations.

Purpose:Unflattened photon beams exhibit many benefits over traditional flattened beams for radiotherapy (RT), but comprehensive evaluations of dosimetric results and beam-on time using flattening filter-free (FFF) beams for all types of breast irradiations are still lacking. The purpose of this study was to investigate if FFF RT can maintain equal or better dose coverage than standard flattened-beam RT while reducing doses to organs at risk (OARs) and beam-on time for various types of breast cancer irradiations.Methods and Materials:FFF volumetric-modulated arc therapy (FFF-VMAT) and standard VMAT (STD-VMAT) treatment plans were created for 15 whole-breast irradiation (WBI) patients with 50 Gy/25 fractions, 13 partial-breast irradiation (PBI) patients with 38.5 Gy/10 fractions, and 9 postmastectomy irradiation (PMI) patients with 50 Gy/25 fractions. Planning target volume (PTV) coverage and dose to OARs were evaluated.Results:Both techniques produced clinically acceptable plans for all three types of irradiations. For WBI, FFF-VMAT plans exhibited similar PTV and OARs evaluation metrics as STD-VMAT. For PBI, FFF-VMAT plans showed significantly lower mean and maximum doses for ipsilateral and contralateral lungs, contralateral breast, and heart. For PMI, FFF-VMAT plans showed significantly lower mean dose and V5 for contralateral breast but significantly higher Dmean, Dmax, and V20 for ipsilateral lung and significantly higher Dmean, V22.5, and V30 for heart. FFF-VMAT techniques significantly reduced beam-on time than STD-VMAT for all cases.Conclusion:This work has shown that FFF beams are most beneficial for small-field irradiation such as PBI, and breast cancer patients could potentially benefit from the shortened beam-on time.

Novel Clinically Weight-Optimized Dynamic Conformal Arcs (WO-DCA) for Liver SBRT: A Comparison with Volumetric Modulated Arc Therapy (VMAT).

PurposeTo evaluate the feasibility of shortening the duration of liver stereotactic radiotherapy (SBRT) without jeopardizing dosimetry or conformity by utilizing weight-optimized dynamic conformal arcs (WO-DCA) as opposed to volumetric modulated arc therapy (VMAT) for tumors away from critical structures.MethodsNineteen patients with liver metastasis were included, previously treated with 50 Gy in 4 fractions with VMAT technique using two partial coplanar arcs of 6 MV beams delivered in high-definition multi-leaf collimator (HD-MLC). Two coplanar partial WO-DCA were generated on Pinnacle treatment planning system (TPS) for each patient; and MLC aperture around the planning target volume (PTV) was automatically generated at different margins for both arcs and maintained dynamically around the target during arc rotation. Weight of the two arcs using optimization method was adjusted between the arcs to maximize tumor coverage and protect organs at risk (OAR) based on the RTOG‐0438 protocol.ResultsThe WO-DCA plans successfully “agreed” with the standard VMAT for OAR (liver, spinal cord, stomach, duodenum, small bowel, and heart) and PTV (Dmean, D98%, D2%, CI, and GI), with superior mean quality assurance (QA) pass rate (97.06 vs 93.00 for VMAT; P < 0.001 and t = 8.87). Similarly, the WO-DCA technique additionally reduced the beam‐on time (3.26 vs 4.43; P < 0.001) and monitor unit (1860 vs 2705 for VMAT; P < 0.001) values significantly.ConclusionThe WO‐DCA plans might minimize small‐field dosimetry errors and defeat patient‐specific VMAT QA requirements due to the omission of MLC beam modulation through the target volume. The WO‐DCA plans may additionally enable faster treatment delivery times and lower OAR without sacrificing target doses in SBRT of liver tumors away from critical structures.

Effects of MR imaging time reduction on substitute CT generation for prostate MRI-only treatment planning.

The introduction of MRI linear accelerators (MR-linacs) and the increased use of MR imaging in radiotherapy, requires improved approaches to MRI-only radiotherapy. MRI provides excellent soft tissue visualisation but does not provide any electron density information required for radiotherapy dose calculation, instead MRI is registered to CT images to enable dose calculations. MRI-only radiotherapy eliminates registration errors and reduces patient discomfort, workload and cost. Electron density requirements may be addressed in different ways, from manually applying bulk density corrections, to more computationally intensive methods to produce substitute CT datasets (sCT), requiring additional sequences, increasing overall imaging time. Reducing MR imaging time would reduce potential artefacts from intrafraction motion and patient discomfort. The aim of this study was to assess the impact of reducing MR imaging time on a hybrid atlas-voxel sCT conversion for prostate MRI-only treatment planning, considering both anatomical and dosimetric parameters. 10 volunteers were scanned on a Siemens Skyra 3T MRI. Sequences included the 3D T2-weighted (T2-w) SPACE sequence used for sCT conversion as previously validated against CT, along with variations to this sequence in repetition time (TR), turbo factor, and combinations of these to reduce the imaging time. All scans were converted to sCT and were compared to the sCT from the original SPACE sequence, evaluating for anatomical changes and dosimetric differences for a standard prostate VMAT plan. Compared to the previously validated T2-w SPACE sequence, scan times were reduced by up to 80%. The external volume and bony anatomy were compared, with all but one sequence meeting a DICE coefficient of 0.9 or better, with the largest variations occurring at the edges of the external body volume. The generated sCT agreed with the gold standard sCT within an isocentre dose of 1% and a gamma pass rate of 99% for a 1%/1mm gamma tolerance for all but one sequence. This study demonstrates that the MR imaging sequence time was able to be reduced by approximately 80% with similar dosimetric results.

Comparison of Rapid to Standard Volumetric Modulated Arc Therapy for Palliative Radiotherapy in Lung Cancer Patients.

Patients with incurable lung cancer often present with debilitating symptoms that require urgent palliative radiotherapy. Volumetric modulated arc therapy (VMAT) provides several dosimetric advantages compared to basic non-conformal techniques, but involves complex planning resulting in a slower turn-around time for treatment. A simplified planning technique known as ‘rapid VMAT’ was developed with an aim to deliver palliative treatment to patients within 48 hours. The purpose of this study was to prospectively compare the dosimetric quality of rapid VMAT plans to standard VMAT plans. Fourteen consecutive rapid VMAT cases were re-planned de novo as per standard VMAT planning guidelines. Planning target volume (PTV) and organs at risk (OARs) were then compared. PTV coverage and dose to OARs including the spinal canal, lung, heart, and esophagus were similar between rapid and standard VMAT. Each plan was ready for treatment within 48 hours of the CT simulation. This study describes an expedited process for which palliative radiotherapy can be delivered to lung tumors with a similar robust quality that is provided for curative intent VMAT radiotherapy plans.

Cost-effectiveness analysis of advanced radiotherapy techniques for post-mastectomy breast cancer patients

BackgroundPrior cost-effectiveness studies of post-mastectomy radiotherapy (PMRT) only compared conventional radiotherapy versus no radiotherapy and only considered tumor control. The goal of this study was to perform cost-effectiveness analyses of standard of care (SOC) and advanced PMRT techniques including intensity-modulated radiotherapy (IMRT), standard volumetric modulated arc therapy (STD-VMAT), non-coplanar VMAT (NC-VMAT), multiple arc VMAT (MA-VMAT), Tomotherapy (TOMO), mixed beam therapy (MIXED), and intensity-modulated proton therapy (IMPT).MethodsUsing a Markov model, we estimated the cost-effectiveness of various techniques over 15 years. A cohort of women (55-year-old) was simulated in the model, and radiogenic side effects were considered. Transition probabilities, utilities, and costs for each health state were obtained from literature and Medicare data. Model outcomes include quality-adjusted life-years (QALYs) and incremental cost-effectiveness ratio (ICER).ResultsFor the patient cohort, STD-VMAT has an ICER of $32,617/QALY relative to SOC; TOMO is dominated by STD-VMAT; IMRT has an ICER of $19,081/QALY relative to STD-VMAT; NC-VMAT, MA-VMAT, MIXED are dominated by IMRT; IMPT has an ICER of $151,741/QALY relative to IMRT. One-way analysis shows that the probability of cardiac toxicity has the most significant impact on the model outcomes. The probability sensitivity analyses show that all advanced PMRT techniques are more cost-effective than SOC at a willingness-to-pay (WTP) threshold of $100,000/QALY, while almost none of the advanced techniques is more cost-effective than SOC at a WTP threshold of $50,000/QALY.ConclusionAdvanced PMRT techniques are more cost-effective for breast cancer patients at a WTP threshold of $100,000/QALY, and IMRT might be a cost-effective option for PMRT patients.

An in silico comparative planning study investigating the predicted benefit of dysphagia-optimised volumetric arc therapy (Do-VMAT), relative to standard VMAT (S-VMAT), to reduce radiotherapy-associated dysphagia (RAD) in oropharyngeal cancers (OPC).

e18540 Background: RAD is a significant treatment-related functional morbidity in OPC and is associated with irradiation of the pharyngeal constrictor muscle (PCM). This study investigated whether a novel PCM-sparing Do-VMAT technique reduced the risk of RAD, compared to S-VMAT. Methods: Twenty OPC patients who previously underwent radiotherapy (RT) to bilateral neck were selected for this retrospective study. For each patient, S-VMAT and Do-VMAT plans were created. In both arms, the mucosal and nodal tumour received 65 Gy while the remaining oropharynx and at-risk nodal levels received 54 Gy; delivered in 30 fractions over 6 weeks. For Do-VMAT, a mandatory mean dose constraint of &lt; 50 Gy to the volume of the combined superior (SPCM) and middle PCM (MPCM) together with an optimal dose constraint of &lt; 20 Gy to the volume of inferior PCM (IPCM), lying outside the high-dose clinical target volume, were additionally defined and optimised to minimise PCM dose. Normal tissue complication probabilities for physician-scored RAD at 6 months (RAD6M) were determined as per the model by Christianen et al in which the 2 variables were mean dose to SPCM and supraglottic larynx (SGL). Statistical analysis was performed using paired t tests. Results: All plans had adequate target volume coverage, and dose to critical organs were within acceptable parameters. Dose to the parotid and submandibular glands were similar between the 2 RT techniques. The mean integral dose was 110.8 Gy-litre (standard deviation 17.1) with S-VMAT and 103.4 Gy-litre (15.4) with Do-VMAT, p &lt; 0.001. With Do-VMAT, the average of the mean dose to PCM, SPCM, MPCM, IPCM, and SGL were 9.3 Gy (1.4), 4.7 Gy (1.7), 7.4 Gy (3.6), 22.7 Gy (4.1), and 2.6 Gy (2.3) respectively lower than S-VMAT (p &lt; 0.001 for all). The mean predicted risk for RAD6M was 31.0 % (4.4) with S-VMAT, and 23.9 % (3.8) with Do-VMAT, p &lt; 0.001. The absolute reduction in estimated toxicity risk with Do-VMAT was &gt; 5 %, the minimally clinically important difference, in every patient. Conclusions: Do-VMAT demonstrated clinically relevant reductions in the predicted risk of RAD, compared to S-VMAT, by reducing dose to the PCM in OPC. The clinical benefits of Do-VMAT is being prospectively evaluated in DARS (CRUK/14/014), a randomised phase III study.

Comparison of conventional and advanced radiotherapy techniques for left-sided breast cancer after breast conserving surgery

Whole breast radiotherapy (WBRT) after breast conserving surgery is the standard treatment to prevent recurrence and metastasis of early stage breast cancer. This study aims to compare seven WBRT techniques including conventional tangential, field-in-field (FIF), hybrid intensity-modulated radiotherapy (IMRT), IMRT, standard volumetric modulated arc therapy (STD-VMAT), noncoplanar VMAT (NC-VMAT), and multiple arc VMAT (MA-VMAT). Fifteen patients who were previously diagnosed with left-sided early stage breast cancer and treated in our clinic were selected for this study. WBRT plans were created for these patients and were evaluated based on target coverage and normal tissue toxicities. All techniques produced clinically acceptable WBRT plans. STD-VMAT delivered the lowest mean dose (1.1 ± 0.3 Gy) and the lowest maximum dose (7.3 ± 4.9 Gy) to contralateral breast, and the second lowest lifetime attributable risk (LAR) (4.1 ± 1.4%) of secondary contralateral breast cancer. MA-VMAT delivered the lowest mean dose to lungs (4.9 ± 0.9 Gy) and heart (5.5 ± 1.2 Gy), exhibited the lowest LAR (1.7 ± 0.3%) of secondary lung cancer, normal tissue complication probability (NTCP) (1.2 ± 0.2%) of pneumonitis, risk of coronary events (RCE) (10.3 ± 2.7%), and LAR (3.9 ± 1.3%) of secondary contralateral breast cancer. NC-VMAT plans provided the most conformal target coverage, the lowest maximum lung dose (46.2 ± 4.1 Gy) and heart dose (41.1 ± 5.4 Gy), and the second lowest LAR (1.8 ± 0.4%) of secondary lung cancer and RCE (10.5 ± 2.8%). MA-VMAT and NC-VMAT could be the preferred techniques for early stage breast cancer patients after breast conserving surgery.

Toward the combined optimization of dynamic axes (CODA) for stereotactic radiotherapy and radiosurgery using fixed couch trajectories.

To develop a novel system for patient-specific combined optimization of couch, collimator, and gantry angles for use in volumetric modulated arc therapy (VMAT) treatment planning. The system was designed to produce highly compact dose distributions by extensively sampling the 4π space. Automated fixed couch trajectory planning was used to reduce normal tissue doses by avoiding beams-eye-view (BEV) overlap with organs-at-risk (OARs) and improve monitor unit (MU) efficiency through collimator angle optimization. By merging distinct BEV objective functions used to optimize the couch rotation angle and collimator angle, a three-dimensional (3D) cost space (the CODA cube) was constructed with axes of gantry, couch, and collimator rotation angles. At each voxel in this CODA cube, the cost of implementing this combination of axes positions in fixed couch trajectories was quantified. The CODA cube was sampled and explored using a modified constrained Bellman-Ford algorithm to suggest low-cost fixed candidate arcs on each plane of the space, from which 10-arcs are chosen throughout the 3D space using a k-means clustering algorithm. These fixed couch trajectories were then imported into the Eclipse treatment planning system (v.11) and inverse-optimized according to clinical standards. Eight artificial cranial targets were contoured in a test-patient anatomy, and seven treatment plans were generated from combinations of three and four targets. The CODA cube optimized plans were compared to standard 4-arc VMAT plans for cranial stereotactic radiotherapy/surgery that were optimized for the same sets of targets; maximum dose to each OAR, V12Gy to normal brain, conformity, and total MUs were compared. Both planning methods were inverse-optimized with identical dosimetric objectives. CODA plans resulted in a reduction in maximum dose to OARs of 20.6% (P<0.01), with maximum brainstem dose decreased by 2.63Gy (P=0.031) on average when compared to the standard arc arrangement. The mean reduction in total MU was 8.6% (P=0.156), the mean increase in the inverse of the van't Riet conformation number was 0.1%, (P=0.67) and the mean decrease in normal brain tissue receiving 12Gy or higher was 3.9% (P=0.16), when compared to the standard VMAT arc configuration (n=7). The optimization of couch, collimator, and gantry angles simultaneously using a 3D optimization space achieved improvement on multiple clinical metrics when compared to the standard VMAT arc configuration. A statistically significant sparing to OAR maximum doses was seen. Combining these optimizations may yield superior results to independent optimization.

Single isocenter stereotactic radiosurgery for patients with multiple brain metastases: dosimetric comparison of VMAT and a dedicated DCAT planning tool

BackgroundIn this dosimetric study, a dedicated planning tool for single isocenter stereotactic radiosurgery for multiple brain metastases using dynamic conformal arc therapy (DCAT) was compared to standard volumetric modulated arc therapy (VMAT).MethodsTwenty patients with a total of 66 lesions who were treated with the DCAT tool were included in this study. Single fraction doses of 15–20 Gy were prescribed to each lesion. Patients were re-planned using non-coplanar VMAT. Number of monitor units as well as V4Gy, V5Gy and V8Gy were extracted for every plan. Using a density-based clustering algorithm, V10Gy and V12Gy and the volume receiving half of the prescribed dose were extracted for every lesion. Gradient indices and conformity indices were calculated. The correlation of the target sphericity, a measure of how closely the shape of the target PTV resembles a sphere, to the difference in V10Gy and V12Gy between the two techniques was assessed using Spearman’s correlation coefficient.ResultsThe automated DCAT planning tool performed significantly better in terms of all investigated metrics (p < 0.05), in particular healthy brain sparing (V10Gy: median 3.2 cm3 vs. 4.9 cm3), gradient indices (median 5.99 vs. 7.17) and number of monitor units (median 4569 vs. 5840 MU). Differences in conformity indices were minimal (median 0.75 vs. 0.73) but still significant (p < 0.05). A moderate correlation between PTV sphericity and the difference of V10Gy and V12Gy between the two techniques was found (Spearman’s rho = 0.27 and 0.30 for V10Gy and V12Gy, respectively, p < 0.05).ConclusionsThe dedicated DCAT planning tool performed better than VMAT in terms of healthy brain sparing and treatment efficiency, in particular for nearly spherical lesions. In contrast, VMAT can be superior in cases with irregularly shaped lesions.

Evaluation of Delta 4 system in patient specific QA for VMAT technique: Retrospective lung VMAT cases

The objective of this study was to evaluate the Delta 4 system for lung VMAT technique. The standard fields and VMAT plans were generated into the homogeneous and heterogeneous phantoms and were evaluated using 2%/2 mm criterion to validate the Delta 4 system. For retrospective study, five lung VMAT plans were evaluated using 2%/2 mm criterion. The point dose was measured to ensure the accuracy of beam delivery. Moreover, the 1.0 mm MLC position error was introduced into the VMAT plan of homogeneous phantom and was evaluated using 2%/2 mm and 1%/1 mm criteria to evaluate the small error detection. The range of percentage gamma passing rate for standard fields and VMAT in both phantoms was 94.6% to 99.8%. For five retrospective lung VMAT plans, the range of percentage gamma passing rate was 96.6% to 99.5%. The point dose differences were within ± 3%. For 1.0 mm MLC position error, the percentage gamma passing rate for 2%/2 mm and 1%/1 mm were 99.1% and 83.6%, respectively. In conclusion, the Delta 4 system can be used in patient specific QA for lung VMAT cases using 2%/2 mm criterion. The error magnitude 1.0 mm can be detected by 1%/1 mm criterion.