Improvement of VMAT plan quality for head and neck cancer with high resolution fluences generated by couch shift between arcs

PurposeTo investigate the changes in quality of the volumetric modulated arc therapy (VMAT) plans with couch-shift between arcs by half of a multi-leaf collimator (MLC) leaf width.MethodsA total of 22 patients with head-and-neck cancer were retrospectively selected. Since the smallest MLC leaf width was 5 mm in this study, the couch was shifted by 2.5 mm in the longitudinal-direction between arcs to increase the resolution of fluence map. A total of three types of VMAT plans were generated for each patient; the three types of plans were a two-full-arc plan without couch-shift (NS plan), a two-half-arc-pair plan with couch-shift (HAS plan), and a two-full-arc pair plan with couch-shift (FAS plan). Changes in the dose-volumetric parameters were investigated.ResultsThe FAS plan showed the best plan quality for the target volumes and organs at risk compared to the NS and HAS plans. However, the magnitudes of differences among the three types of plans were minimal, and every plan was clinically acceptable. The average integral doses of the NS, HAS, and FAS plans were 160,549 ± 37,600 Gy-cc, 147,828 ± 33,343 Gy-cc, and 156,030 ± 36,263 Gy-cc, respectively. The average monitor unit of the NS, HAS, and FAS plans were 717 ± 120 MU, 648 ± 100 MU, and 763 ± 158 MU, respectively.ConclusionsThe HAS plan was better than the others in terms of normal tissue sparing and plan efficiency. By shifting the couch by half of the MLC leaf width in the longitudinal direction between arcs, the VMAT plan quality could be improved.     

Incorrect dosimetric leaf separation in IMRT and VMAT treatment planning: Clinical impact and correlation with pretreatment quality assurance

PurposeDynamic treatment planning algorithms use a dosimetric leaf separation (DLS) parameter to model the multi-leaf collimator (MLC) characteristics. Here, we quantify the dosimetric impact of an incorrect DLS parameter and investigate whether common pretreatment quality assurance (QA) methods can detect this effect.Methods16 treatment plans with intensity modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) technique for multiple treatment sites were calculated with a correct and incorrect setting of the DLS, corresponding to a MLC gap difference of 0.5 mm. Pretreatment verification QA was performed with a bi-planar diode array phantom and the electronic portal imaging device (EPID). Measurements were compared to the correct and incorrect planned doses using gamma evaluation with both global (G) and local (L) normalization. Correlation, specificity and sensitivity between the dose volume histogram (DVH) points for the planning target volume (PTV) and the gamma passing rates were calculated.ResultsThe change in PTV and organs at risk DVH parameters were 0.4–4.1%. Good correlation (>0.83) between the PTV_mean dose deviation and measured gamma passing rates was observed. Optimal gamma settings with 3%L/3 mm (per beam and composite plan) and 3%G/2 mm (composite plan) for the diode array phantom and 2%G/2 mm (composite plan) for the EPID system were found. Global normalization and per beam ROC analysis of the diode array phantom showed an area under the curve <0.6.ConclusionsA DLS error can worsen pretreatment QA using gamma analysis with reasonable credibility for the composite plan. A low detectability was demonstrated for a 3%G/3 mm per beam gamma setting.     

Radiotherapy for the treatment of pituitary adenomas: A dosimetric comparison of three planning techniques

AimOur goal was to compare conformal 3D (C3D) radiotherapy (RT), modulated intensity RT (IMRT), and volumetric modulated arc therapy (VMAT) planning techniques in treating pituitary adenomas.BackgroundRT is important for managing pituitary adenomas. Treatment planning advances allow for higher radiation dosing with less risk of affecting organs at risk (OAR).Materials and methodsWe conducted a 5-year retrospective review of patients with pituitary adenoma treated with external beam radiation therapy (C3D with flattening filter, flattening filter-free [FFF], IMRT, and VMAT). We compared dose-volume histogram data. For OARs, we recorded D2%, maximum, and mean doses. For planning target volume (PTV), we registered V95%, V107%, D95%, D98%, D50%, D2%, minimum dose, conformity index (CI), and homogeneity index (HI).ResultsFifty-eight patients with pituitary adenoma were included. Target-volume coverage was acceptable for all techniques. The HI values were 0.06, IMRT; 0.07, VMAT; 0.08, C3D; and 0.09, C3D FFF (p < 0.0001). VMAT and IMRT provided the best target volume conformity (CI, 0.64 and 0.74, respectively; p < 0.0001). VMAT yielded the lowest doses to the optic pathway, lens, and cochlea. The position of the neck in extreme flexion showed that it helps in planning mainly with VMAT by allowing only one arc to be used and achieving the desired conformity, decreasing the treatment time, while allowing greater protection to the organs of risk using C3D, C3DFFF.ConclusionsOur results confirmed that EBRT in pituitary adenomas using IMRT, VMAT, C3D, C3FFF provide adequate coverage to the target. VMAT with a single arc or incomplete arc had a better compliance with desired dosimetric goals, such as target coverage and normal structures dose constraints, as well as shorter treatment time. Neck extreme flexion may have benefits in treatment planning for better preservation of organs at risk. C3D with extreme neck flexion is an appropriate treatment option when other treatment techniques are not available.     

Integrated scoring approach to assess radiotherapy plan quality for breast cancer treatment

BackgroundProposal of an integrated scoring approach assessing the quality of different treatment techniques in a radiotherapy planning comparison. This scoring method incorporates all dosimetric indices of planning target volumes (PTVs) as well as organs at risk (OARs) and provides a single quantitative measure to select an ideal plan.Materials and methodsThe radiotherapy planning techniques compared were field-in-field (FinF), intensity modulated radiation therapy (IMRT), volumetric modulated arc therapy (VMAT), hybrid IMRT (H-IMRT), and hybrid VMAT (H-VMAT). These plans were generated for twenty-five locally advanced left-sided breast cancer patients. The PTVs were prescribed a hypofractionation dose of 40.5 Gy in 15 fractions. The integrated score for each planning technique was calculated using the proposed formula.ResultsAn integrated score value that is close to zero indicates a superior plan. The integrated score that incorporates all dosimetric indices (PTVs and OARs) were 1.37, 1.64, 1.72, 1.18, and 1.24 for FinF, IMRT, VMAT, H-IMRT, and H-VMAT plans, respectively.ConclusionThe proposed integrated scoring approach is scientific to select a better plan and flexible to incorporate the patient-specific clinical demands. This simple tool is useful to quantify the treatment techniques and able to differentiate the acceptable and unacceptable plans.     

The effect of beam interruption during VMAT delivery on the delivered dose distribution

PurposeThe aim of this study is to investigate the effect of beam interruptions during delivery of volumetric modulated arc therapy (VMAT) on delivered dose distributions.MethodsTen prostate and ten head and neck (H&N) VMAT plans were retrospectively selected. Each VMAT plan was delivered using Trilogy™ without beam interruption, and with 4 and 8 intentional beam interruptions per a single arc. Two-dimensional global and local gamma evaluations with a diode array were performed with gamma criteria of 3%/3 mm, 2%/2 mm, 1%/2 mm and 2%/1 mm for each VMAT plan with and without beam interruptions. The VMAT plans were reconstructed with log files recorded during delivery and the dose-volumetric parameters were calculated for each reconstructed plan. The differences among dose-volumetric parameters due to the beam interruptions were calculated.ResultsThe changes in global gamma passing rates with various gamma criteria were less than 1.6% on average, while the changes in local gamma passing rates were less than 5.3% on average. The dose-volumetric parameter changes for the target volumes of prostate and H&N VMAT plans due to beam interruptions were less than 0.72% and 1.5% on average, respectively.ConclusionThe delivered dose distributions with up to 8 beam interruptions per an arc were clinically acceptable, showing minimal changes in both gamma passing rates and dose-volumetric parameters.     

Influence of multi-leaf collimator leaf width in radiosurgery via volumetric modulated arc therapy and 3D dynamic conformal arc therapy

PurposeTo study the influence of Multileaf Collimator (MLC) leaf width in radiosurgery treatment planning for Volumetric Modulated Arc Therapy (VMAT) and 3D Dynamic Conformal Arc Therapy (3D-DCA).Material and methods16 patients with solitary brain metastases treated with radiosurgery via the non-coplanar VMAT were replanned for the 3D-DCA. For each planning technique two MLC leaf width sizes were utilized, i.e. 5 mm and 2.5 mm. These treatment plans were compared using dosimetric indices (conformity, gradient and mean dose for brain tissue) and the normal tissue complication probability (NTCP).ResultsAn improvement in planning quality for VMAT was observed versus 3D-DCA for any MLC leaf width, mainly with regards to dose conformity and to a lesser extent regards dose gradient. No significant difference was observed for any of both techniques using smaller leaf width. However, dose gradient was improved in favor of the 2.5 mm MLC for either of both techniques (15% VMAT and 10% 3D-DCA); being noticeable for lesions smaller than 10 cm³. Nonetheless, the NTCP index was not significantly affected by variations in the dose gradient index.ConclusionsThis, our present study, suggests that the use of an MLC leaf width of 2.5 mm via the noncoplanar VMAT and 3D-DCA techniques provides improvement in terms of dose gradient for small volumes, over those results obtained with an MLC leaf width of 5 mm. The 3D-DCA does also benefit from MLC leaf widths of a smaller size, mainly in terms of conformity.     

Volumetric modulated arc therapy for synchronous bilateral whole breast irradiation – A case study

PurposeThe treatment planning of bilateral breast irradiation (BBI) is a challenging task. The overlapping of tangential fields is usually unavoidable without compromising the target coverage. The purpose of this study was to investigate the technical feasibility and benefits of a single isocentre volumetric modulated arc therapy (VMAT) in BBI.Methods and materialsTwo women with bilateral breast cancer were included in this case study. The first patient (Pat#1) underwent a bilateral breast-conserving surgery and sentinel lymph node biopsy. The second patient (Pat#2) underwent a bilateral ablation and axillary lymph node dissection. Planning target volumes (PTV) and organs at risk were delineated on CT images. VMAT plans were created with four (two for both sides, Pat#1) or two (one for each breast, Pat#2) separate VMAT fields. Subsequently, traditional tangential field plans were generated for each patient and the dosimetric parameters were compared.ResultsThe treatment times of the patients with VMAT were less than 15 min with daily CBCT imaging. When compared to the standard tangential field technique, the VMAT plans improved the PTV dose coverage and dose homogeneity with improved sparing of lungs and heart. With traditional field arrangement, the overlapping of the tangential fields was inevitable without significantly compromising the target coverage, whereas with VMAT the hotspots were avoided. The patients were treated with the VMAT technique and no acute skin toxicity was observed with either of the patients.ConclusionsA single isocentre VMAT technique has been implemented clinically for BBI. With the VMAT techniques, the dose delivery was quick and the hotspots in the field overlapping areas were avoided. The PTV dose coverage was superior in VMAT plans when compared with conventional tangential technique plans.     

Dosimetric accuracy of dual isocenter irradiation in low magnetic field resonance guided radiotherapy system for extended abdominal tumours

PurposeDue to limited field size of Magnetic Resonance Linear Accelerators (MR-Linac), some treatments could require a dual-isocenter planning approach to achieve a complete target coverage and thus exploit the benefits of the online adaptation. This study evaluates the dosimetric accuracy of the dual-isocenter intensity modulated radiation therapy (IMRT) delivery technique for MR-Linac.Material and MethodsDual-isocenter multi leaf collimator (MLC) and couch accuracy tests have been performed to evaluate the delivery accuracy of the system. A mono-isocenter plan delivered in clinical practice has then been retrospectively re-planned with dual-isocenter technique. The dual-isocenter plan has been re-calculated and delivered on a 3-dimensional (3D) ArcCHECK phantom and 2-dimensional (2D) films to assess its dosimetric accuracy in terms of gamma analysis. Clinical and planning target volume (CTV and PTV respectively) coverage robustness was then investigated after the introduction of ± 2 mm and ± 5 mm positioning errors by shifting the couch.ResultsMLC and couch accuracy tests confirmed the system accuracy in delivering a dual-isocenter irradiation.2D/3D gamma analysis results occurred always to be above 95% if considered a gamma criteria 1%/2 mm and 1%/1 mm respectively for the 2D and 3D analysis.The mean variations for CTV D98% and PTV V95% were 0.2% and 1.1% respectively when positioning error was introduced separately in each direction, while the maximum observed variations were 0.9% (CTV) and 3.7% (PTV).ConclusionThe dosimetric accuracy of dual-isocenter irradiation has been verified for MR-Linac, achieving accurate and robust treatment strategy and improving dose conformality also in presence of targets whose extension exceeds the nominal maximum field size.     

Static and rotational intensity modulated techniques for head-neck cancer radiotherapy: A planning comparison

PurposeTo compare helical Tomotherapy (HT), two volumetric-modulated arc techniques and conventional fixed-field intensity modulated techniques (S-IMRT) for head-neck (HN) cancers.Methods and materialsEighteen HN patients were considered. Four treatment plans were generated for each patient: HT, S-IMRT optimised with Eclipse treatment planning system and two volumetric techniques using Elekta–Oncentra approach (VMAT) and Varian-RapidArc (RA), using two full arcs. All techniques were optimised to simultaneously deliver 66Gy to PTV1 (GTV and enlarged nodes) and 54Gy to PTV2 (subclinical and electively treated nodes). Comparisons were assessed on several dosimetric parameters and, secondarily, on planned MUs and delivery time.ResultsConcerning PTV coverage, significantly better results were found for HT and RA. HT significantly improved the target coverage both compared to S-IMRT and VMAT. No significant differences were found between S-IMRT and volumetric techniques in terms of dose homogeneity. For OARs, all the techniques were able to satisfy all hard constraints; significantly better results were found for HT, especially in the intermediate dose range (15–30 Gy). S-IMRT reached a significantly better OARs sparing with respect to VMAT and RA. No significant differences were found for body mean dose, excepting higher values of V5–V10 for HT. A reduction of planned MUs and delivery treatment time was found with volumetric techniques.ConclusionsThe objectives of satisfying target coverage and sparing of critical structures were reached with all techniques. S-IMRT techniques were found more advantageous compared to RA and VMAT for OARs sparing. HT reached the best overall treatment plan quality.     

Dosimetric evaluation study of IMRT and VMAT techniques for prostate cancer based on different multileaf collimator designs

The hypofractionated radiotherapy modality was established to reduce treatment durations and enhance therapeutic efficiency, as compared to conventional fractionation treatment. However, this modality is challenging because of rigid dosimetric constraints. This study aimed to assess the impact of multi-leaf collimator (MLC) widths (10 mm and 5 mm) on plan quality during the treatment of prostate cancer. Additionally, this study aimed to investigate the impact of the MLC mode of energy on the Agility flattening filter (FF), MLC Agility-free flattening filter (FFF), and MLCi2 for patients receiving hypofractionated radiotherapy. Two radiotherapy techniques; Intensity Modulated Radiotherapy (IMRT) and Volumetric Modulated Arc Radiotherapy (VMAT), were used in this research. In the present study, computed tomography simulations of ten patients (six plans per patient) with localized prostate adenocarcinoma were analyzed. Various dosimetric parameters were assessed, including monitor units, treatment delivery times, conformity, and homogeneity indices. To evaluate the plan quality, dose-volume histograms (DVHs) were estimated for each technique. The results demonstrated that the determined dosimetric parameters of planning target volume (PTV)p (such as D mean, conformity, and homogeneity index) showed greater improvement with MLC Agility FF and MLC Agility FFF than with MLCi2. Additionally, the treatment delivery time was reduced in the MLC Agility FF (by 31%) and MLC Agility FFF (by 10.8%) groups compared to the MLCi2 group. It is concluded that for both the VMAT and IMRT techniques, the smaller width (5 mm) MLCs revealed better planning target volume coverage, improved the dosimetric parameters for PTV, reduced the treatment time, and met the constraints for OARs. It is therefore recommended to use 5 mm MLCs for hypofractionated prostate cancer treatment due to better target coverage and better protection of OARs.

     

Quantification of residual dose estimation error on log file-based patient dose calculation

PurposeThe log file-based patient dose estimation includes a residual dose estimation error caused by leaf miscalibration, which cannot be reflected on the estimated dose. The purpose of this study is to determine this residual dose estimation error.Methods and materialsModified log files for seven head-and-neck and prostate volumetric modulated arc therapy (VMAT) plans simulating leaf miscalibration were generated by shifting both leaf banks (systematic leaf gap errors: ±2.0, ±1.0, and ±0.5 mm in opposite directions and systematic leaf shifts: ±1.0 mm in the same direction) using MATLAB-based (MathWorks, Natick, MA) in-house software. The generated modified and non-modified log files were imported back into the treatment planning system and recalculated. Subsequently, the generalized equivalent uniform dose (gEUD) was quantified for the definition of the planning target volume (PTV) and organs at risks.ResultsFor MLC leaves calibrated within ±0.5 mm, the quantified residual dose estimation errors that obtained from the slope of the linear regression of gEUD changes between non- and modified log file doses per leaf gap are in head-and-neck plans 1.32 ± 0.27% and 0.82 ± 0.17 Gy for PTV and spinal cord, respectively, and in prostate plans 1.22 ± 0.36%, 0.95 ± 0.14 Gy, and 0.45 ± 0.08 Gy for PTV, rectum, and bladder, respectively.ConclusionsIn this work, we determine the residual dose estimation errors for VMAT delivery using the log file-based patient dose calculation according to the MLC calibration accuracy.     

Impact of shoulder deformation on volumetric modulated arc therapy doses for head and neck cancer

PurposeWhen using volumetric modulated arc therapy (VMAT) for head and neck cancer, setup errors regarding the shoulders can create loss of target coverage or increased organ-at-risk doses. This study created variations of realistic shoulder deformations to understand the associated VMAT dosimetric effects and investigated water-equivalent thickness (WET) differences using in-house software.MethodsTen patients with head and neck cancer with lower neck involvement were retrospectively and randomly enrolled. Their retrospective analysis comprised treatment planning using RayStation 5.0 (RaySearch Laboratories, Stockholm, Sweden), shoulder deformation of 5–15 mm in three-dimensional axes using the ImSimQA package (Oncology Systems Limited, Shrewsbury, Shropshire, UK), and evaluation of the clinical impact of the dose distribution after recalculating the dose distribution using computed tomography images of deformed shoulders and deforming the dose distribution. Additionally, our in-house software program was used to measure WET differences for shoulder deformation.ResultsWET differences were greater in the superoinferior (SI) direction than in the other directions (the WET difference was >20 mm for 15-mm SI deformation). D99%, D98%, and D95% for all clinical target volumes were within 3%. Local dose differences of more than ±10% were found for normal tissues at the level of the shoulder for 15-mm movement in the SI direction.ConclusionsShoulder deformation of >6 mm could cause large dose variations delivered to the targeted tissue at the level of the shoulder. Thus, to ensure delivery of appropriate treatment coverage to the targeted tissue, shoulder deformation should be taken into consideration during the planning stage.     

Characterization of robust optimization for VMAT plan for liver cancer

PurposeWe investigated the feasibility of robust optimization for volumetric modulated arc therapy (VMAT) stereotactic body radiation therapy (SBRT) for liver cancer in comparison with planning target volume (PTV)-based optimized plans. Treatment plan quality, robustness, complexity, and accuracy of dose delivery were assessed.MethodsTen liver cancer patients were selected for this study. PTV-based optimized plans with an 8-mm PTV margin and robust optimized plans with an 8-mm setup uncertainty were generated. Plan perturbed doses were evaluated using a setup error of 8 mm in all directions from the isocenter. The dosimetric comparison parameters were clinical target volume (CTV) doses (D_98%, D_50%, and D_2%), liver doses, and monitor unit (MU). Plan complexity was evaluated using the modulation complexity score for VMAT (MCSv).ResultsThere was no significant difference between the two optimizations with respect to CTV doses and MUs. Robust optimized plans had a higher liver dose than did PTV-based optimized plans. Plan perturbed dose evaluations showed that doses to the CTV for the robust optimized plans had small variations. Robust optimized plans were less complex than PTV-based optimized plans. Robust optimized plans had statistically significant fewer leaf position errors than did PTV-based optimized plans.ConclusionsComparison of treatment plan quality, robustness, and plan complexity of both optimizations showed that robust optimization could be feasibile for VMAT of liver cancer.     

Monte Carlo dose verification of VMAT treatment plans using Elekta Agility 160-leaf MLC

In this study, we verified volumetric modulated arc therapy (VMAT) plans in an Elekta Synergy system with an integrated Agility 160-leaf multileaf collimator (MLC) by comparing them with Monte Carlo (MC)-calculated dose distributions using the AAPM TG-119 structure sets. The head configuration of the linear accelerator with the integrated MLC was simulated with the EGSnrc/BEAMnrc code. Firstly, the dosimetric properties of the MLC were evaluated with the MC technique and film measurements. Next, VMAT plans were created with the Pinnacle³ treatment planning system (TPS) for four regions in the AAPM TG-119 structures. They were then verified by comparing them with MC-calculated dose distributions using dose volume histograms (DVHs) and three-dimensional (3D) gamma analysis. The MC simulations for the Agility MLC dosimetric properties were in acceptable agreement with measurements. TPS-VMAT plans using TG-119 structure sets agreed with MC dose distributions within 2% in the comparison of D₉₅ in planning target volumes (PTVs) evaluated from DVHs. In contrast, higher dose regions such as D₂₀, D₁₀, and D₅ in PTVs for TPS tended to be smaller than MC values. This tendency was particularly noticeable for mock head and neck with complicated structures. In 3D gamma analysis, the passing rates with 3%/3mm criteria in PTVs were ≥99%, except for mock head and neck (89.5%). All passing rates for organs at risk (OARs) were in acceptable agreement of >96%. It is useful to verify dose distributions of PTVs and OARs in TPS-VMAT plans by using MC dose calculations and 3D gamma analysis.     

Gamma-index method sensitivity for gauging plan delivery accuracy of volumetric modulated arc therapy

PurposeThe aim of this study was to investigate the sensitivity of the gamma-index method according to various gamma criteria for volumetric modulated arc therapy (VMAT).MethodsTwenty head and neck (HN) and twenty prostate VMAT plans were retrospectively selected for this study. Both global and local 2D gamma evaluations were performed with criteria of 3%/3 mm, 2%/2 mm, 1%/2 mm and 2%/1 mm. In this study, the global and local gamma-index calculated the differences in doses relative to the maximum dose and the dose at the current measurement point, respectively. Using log files acquired during delivery, the differences in parameters at every control point between the VMAT plans and the log files were acquired. The differences in dose–volumetric parameters between reconstructed VMAT plans using the log files and the original VMAT plans were calculated. The Spearman's rank correlation coefficients (r_s) were calculated between the passing rates and those differences.ResultsConsiderable correlations with statistical significances were observed between global 1%/2 mm, local 1%/2 mm and local 2%/1 mm and the MLC position differences (r_s = −0.712, −0.628 and −0.581). The numbers of r_s values with statistical significance between the passing rates and the changes in dose–volumetric parameters were largest in global 2%/2 mm (n = 16), global 2%/1 mm (n = 15) and local 2%/1 mm (n = 13) criteria.ConclusionLocal gamma-index method with 2%/1 mm generally showed higher sensitivity to detect deviations between a VMAT plan and the delivery of the VMAT plan.     

Examination of the dose distribution of volumetric modulated arc radiotherapy using a high-definition multi-leaf collimator for breast cancer patients with irradiated regional lymph nodes

BackgroundA high-definition multi-leaf collimator (HD-MLC) with 5- and 10-mm fine MLCs is useful for radiotherapy. However, it is difficult to irradiate the mammary gland and supraclavicular region using a HD-MLC because of the narrow field of volumetric modulated arc radiotherapy (VMAT). Therefore, we aimed to evaluate the dose distribution of the VMAT dose using a HD-MLC in 15 patients with left breast cancer undergoing postoperative irradiation of breast and regional lymph nodes, including the internal mammary node.Materials and methodsThe following four plans were generated: three-arc VMAT using HD-MLC (HD-VMAT), two tangential arcs and one-arc VMAT using HD-MLC (tHD-VMAT), three-dimensional conformal radiotherapy (3DCRT) using HD-MLC, and two-arc VMAT using the Millennium 120-leaf MLC (M-VMAT). We assessed the doses to the target volume and organs at risk.ResultsThe target dose distributions were higher for HD-VMAT than 3DCRT. There were no significant differences in the heart mean dose (D_mean) or lung volume receiving 20 Gy (V20 Gy) between HD-VMAT and 3DCRT. The heart D_mean and lung V20 Gy of tHD-VMAT were higher than those of HD-VMAT, and the heart D_mean of M-VMAT was higher than that of HD-VMAT. However, the target doses of tHD-VMAT, M-VMAT, and HD-VMAT were equivalent.ConclusionsIn cases of the mammary gland and regional lymph node irradiation, including the internal mammary node in patients with left breast cancer, HD-VMAT was not inferior to M-VMAT and provided a better dose distribution to the target volume and organs at risk compared with 3DCRT and tHD-VMAT.     

Dosimetric impact of statistical uncertainty on Monte Carlo dose calculation algorithm in volumetric modulated arc therapy using Monaco TPS for three different clinical cases

AimTo study the dosimetric impact of statistical uncertainty (SU) per plan on Monte Carlo (MC) calculation in Monaco? treatment planning system (TPS) during volumetric modulated arc therapy (VMAT) for three different clinical cases.BackgroundDuring MC calculation SU is an important factor to decide dose calculation accuracy and calculation time. It is necessary to evaluate optimal acceptance of SU for quality plan with reduced calculation time.Materials and methodsThree different clinical cases as the lung, larynx, and prostate treated using VMAT technique were chosen. Plans were generated with Monaco? V5.11 TPS with 2% statistical uncertainty. By keeping all other parameters constant, plans were recalculated by varying SU, 0.5%, 1%, 2%, 3%, 4%, and 5%. For plan evaluation, conformity index (CI), homogeneity index (HI), dose coverage to PTV, organ at risk (OAR) dose, normal tissue receiving dose ≥5 Gy and ≥10 Gy, integral dose (NTID), calculation time, gamma pass rate, calculation reproducibility and energy dependency were analyzed.ResultsCI and HI improve as SU increases from 0.5% to 5%. No significant dose difference was observed in dose coverage to PTV, OAR doses, normal tissue receiving dose ≥5 Gy and ≥10 Gy and NTID. Increase of SU showed decrease in calculation time, gamma pass rate and increase in PTV max dose. No dose difference was seen in calculation reproducibility and dependent on energy.ConclusionFor VMAT plans, SU can be accepted from 1% to 3% per plan with reduced calculation time without compromising plan quality and deliverability by accepting variations in point dose within the target.     

Feasibility of intensity-modulated radiotherapy to treat gastric cancer

AimTo present a proposed gastric cancer intensity-modulated radiotherapy (IMRT) treatment planning protocol for an institution that have not introduced volumetric modulated arc therapy in clinical practice. A secondary aim was to determine the impact of 2DkV set-up corrections on target coverage and organ at risk (OAR).Methods and MaterialsTwenty consecutive patients were treated with a specially-designed non-coplanar 7-field IMRT technique. The isocenter-shift method was used to estimate the impact of 2DkV-based set-up corrections on the original base plan (BP) coverage. An alternative plan was simulated (SP) by taking into account isocenter shifts. The SP and BP were compared using dose-volume histogram (DVH) plots calculated for the internal target volume (ITV) and OARs.ResultsBoth plans delivered a similar mean dose to the ITV (100.32 vs. 100.40%), with no significant differences between the plans in internal target coverage (5.37 vs. 4.96%). Similarly, no significant differences were observed between the maximal dose to the spinal cord (67.70 and 67.09%, respectively) and volume received 50% of the prescribed dose of: the liver (62.11 vs. 59.84%), the right (17.62 vs. 18.58%) and left kidney (29.40 vs. 30.48%). Set-up margins (SM) were computed as 7.80 mm, 10.17 mm and 6.71 mm in the left-right, cranio-caudal and anterior-posterior directions, respectively.ConclusionPresented IMRT protocol (OAR dose constraints with selected SM verified by 2DkV verification) for stomach treatment provided optimal dose distribution for the target and the critical organs. Comparison of DVH for the base and the modified plan (which considered set-up uncertainties) showed no significant differences.     

A comparative analysis of Acuros XB and the analytical anisotropic algorithm for volumetric modulation arc therapy

BackgroundThis study aimed to verify the dosimetric impact of Acuros XB (AXB) (AXB, Varian Medical Systems Palo Alto CA, USA), a two model-based algorithm, in comparison with Anisotropic Analytical Algorithm (AAA ) calculations for prostate, head and neck and lung cancer treatment by volumetric modulated arc therapy (VMAT ), without primary modification to AA. At present, the well-known and validated AA algorithm is clinically used in our department for VMAT treatments of different pathologies. AXB could replace it without extra measurements. The treatment result and accuracy of the dose delivered depend on the dose calculation algorithm.Materials and methodNinety-five complex VMAT plans for different pathologies were generated using the Eclipse version 15.0.4 treatment planning system (TPS). The dose distributions were calculated using AA and AXB (dose-to-water, AXB_w and dose-to-medium, AXB_m), with the same plan parameters for all VMAT plans. The dosimetric parameters were calculated for each planning target volume (PTV) and involved organs at risk (OA R). The patient specific quality assurance of all VMAT plans has been verified by Octavius^®-4D phantom for different algorithms.ResultsThe relative differences among AA, AXB_w and AXB_m, with respect to prostate, head and neck were less than 1% for PTV D_95%. However, PTV D_95% calculated by AA tended to be overestimated, with a relative dose difference of 3.23% in the case of lung treatment. The absolute mean values of the relative differences were 1.1 ± 1.2% and 2.0 ± 1.2%, when comparing between AXB_w and AA, AXB_m and AA, respectively. The gamma pass rate was observed to exceed 97.4% and 99.4% for the measured and calculated doses in most cases of the volumetric 3D analysis for AA and AXB_m, respectively.ConclusionThis study suggests that the dose calculated to medium using AXB_m algorithm is better than AAA and it could be used clinically. Switching the dose calculation algorithm from AA to AXB does not require extra measurements.     

Analysis of the planned,delivered dose distributions and quality assurance for helical tomotherapy and volumetric modulated arc therapy in locally advanced non-small cell lung cancer

BackgroundWith full access to both helical tomotherapy (HT) and volumetric modulated arc therapy (VMAT), we compared locally advanced non-small cell lung cancer (LA-NSCLC) treatment plans and verified the plans using patient-specific pretreatment quality assurance (PSQA).Materials and methodsFor each of the seventeen patients included in the study, two treatment plans (i.e. HT and VMAT) were created. Optimized plans were evaluated following the ICRU 83 criteria. Planned quality indexes and dosimetric parameters were compared. Lastly, all plans were subjected to PSQA assessment by determining the gamma passing rate (GPR).ResultsAll dosimetry results obtained from the planning target volume passed the ICRU 83 criteria. With regard to similar homogeneity indices, VMAT produced better conformity number values than HT (0.78 vs. 0.64), but differences in the values were insignificant. Furthermore, VMAT was associated with a significantly shorter mean treatment time (1.91 minutes vs. 6.66 minutes). For PSQA assessment, both techniques resulted in adequate GPR values (> 90% at the 3%/3 mm criteria).ConclusionBoth HT and VMAT techniques led to the generation of clinically satisfactory and reliable radiotherapy plans. However, the VMAT plan was associated with a non-significantly better degree of conformity and a significantly shorter treatment time. Thus, VMAT was determined to be a better choice for LA-NSCLC.