In-house spread sheet based monitor unit verification program for volumetric modulated arc therapy

Independent monitor unit verification calculation (MUVC) has been recommended by several authors for intensity modulated radiotherapy (IMRT) as a patient specific quality assurance tool. Aim of the present work is to develop an in-house excel spread sheet based MUVC program for volumetric modulated arc therapy (VMAT) using Clarkson's integration technique. Total scatter factor (S_c,p) and tissue maximum ratio (TMR) for circular fields obtained from Treatment planning system (TPS) were used for the calculation. Multileaf collimator (MLC) interleaf leakage, MLC round edge transmission and tongue and groove effect were accounted. MUVC calculation was performed for 58 patients both for patient anatomy and for homogenous cylindrical phantom. Radiological path lengths were used as water equivalent depths (WED) for calculations using patient anatomy. Monitor unit (MU) discrepancies between −2.60% and 0.28% with mean deviation of −0.92% ± 0.75% were obtained for homogenous cylindrical phantom calculations. MUVC for patient anatomy resulted in large variations between −19.02% and 0.67% for 14 plans where isocenter was at a region below −350 HU. But For 44 plans where the isocenter was at a region above −350 HU, variations between −3.44% and 0.48% were obtained with mean deviation of −1.73% ± 1.12%. For VMAT patient specific quality assurance, the independent MUVC algorithm can be used as an easy and quick auxiliary to measurement based verification for plans with isocenter at a region above −350 HU.     

Performance of the eclipse monitor unit objective tool utilizing volumetric modulated arc therapy for rectal cancer

AimTo assess the performance of the monitor unit (MU) Objective tool in Eclipse treatment planning system (TPS) utilizing volumetric modulated arc therapy (VMAT) for rectal cancer.BackgroundEclipse VMAT planning module includes a tool to control the number of MUs delivered: the MU Objective tool. This tool could be utilized to reduce the total number of MUs in rectal cancer treatments.Materials and methods20 rectal cancer patients were retrospectively studied using VMAT and the MU Objective tool. The baseline plan for each patient was selected as the one with no usage of the MU Objective tool. The number of MUs of this plan was set to be the reference number of MUs (MU_ref). Five plans were re-optimized for each patient only varying the Max MU parameter. The selected values were 30%, 60%, 90%, 120% and 150% of MU_ref for each patient. Differences with respect to the baseline plan were evaluated regarding MU number and parameters for PTVs coverage evaluation, PTVs homogeneity and OARs doses assessment. A two-tailed, paired-samples t-test was used to quantify these differences.ResultsAverage relative differences in MU number obtained was 10% for Max MU values of 30% and 60% of MU_ref, respectively (p < 0.03). PTVs coverage and homogeneity were not compromised and discrepancies obtained with respect to baseline plans were not significant. Furthermore, maximum OARs doses deviations were also not significant.ConclusionsA 10% reduction in the MU number could be obtained without an alteration of PTV coverage and OARs doses for rectal cancer.     

Dosimetric comparison of dynamic conformal arc integrated with segment shape optimization and variable dose rate versus volumetric modulated arc therapy for liver SBRT

AimThe aim is a dosimetric comparison of dynamic conformal arc integrated with the segment shape optimization and variable dose rate (DCA_SSO_VDR) versus VMAT for liver SBRT and interaction of various treatment plan quality indices with PTV and degree of modulation (DoM) for both techniques.BackgroundThe DCA is the state-of-the-art technique but overall inferior to VMAT, and the DCA_SSO_VDR technique was not studied for liver SBRT.Materials and methodsTwenty-five patients of liver SBRT treated using the VMAT technique were selected. DCA_SSO_VDR treatment plans were also generated for all patients in Monaco TPS using the same objective constraint template and treatment planning parameters as used for the VMAT technique. For comparison purpose, organs at risk (OARs) doses and treatment plans quality indices, such as maximum dose of PTV (D_max%), mean dose of PTV (D_mean%), maximum dose at 2 cm in any direction from the PTV (D_2cm%), total monitor units (MU’s), gradient index R_50%, degree of modulation (DoM), conformity index (CI), homogeneity index (HI), and healthy tissue mean dose (HTMD) were compared.ResultsSignificant dosimetric differences were observed in several OARs doses and lowered in VMAT plans. The D_2cm%, R_50%, CI, HI and HTMD are dosimetrically inferior in DCA_SSO_VDR plans. The higher DoM results in poor dose gradient and better dose gradient for DCA_SSO_VDR and VMAT treatment plans, respectively.ConclusionsFor liver SBRT, DCA_SSO_VDR treatment plans are neither dosimetrically superior nor better alternative to the VMAT delivery technique. A reduction of 69.75% MU was observed in DCA_SSO_VDR treatment plans. For the large size of PTV and high DoM, DCA_SSO_VDR treatment plans result in poorer quality.     

Dosimetric validation of a redundant independent calculation software for VMAT fields

A redundant independent dosimetric calculation (RIDC) prior to treatment has become a basic part of the QA process for 3D conventional radiotherapy, and is strongly recommended in several international publications. On the other hand, the rapid growth in the number of intensity modulated treatments has led to a significant increase in the workflow associated with QA treatments. Diamond (“K&S Associates”) is RIDC software which is capable of calculating VMAT (Volumetric Modulated Arc Therapy) fields. Modeling, validation and commissioning are necessary steps thereby making it a useful tool for VMAT QA. In this paper, a procedure for the validation of the calculation algorithm is demonstrated. A set 3D conventional field was verified in two ways: firstly, a comparison was made between Diamond calculations and experimental measures obtaining an average deviation of ?0.1 ± 0.7%(1SD), and secondly, a comparison made between Diamond and the treatment planning system (TPS) Eclipse, obtaining an average deviation of 0.4 ± 0.8%(1SD). For both steps, a plastic slab phantom was used. VMAT validation was carried out by analyzing 59 VMAT plans in two ways: first, Diamond calculation versus experimental measurement with an average deviation of ?0.2 ± 1.7%(1SD), and second, Diamond calculation versus TPS calculation with an average deviation of 0.0 ± 1.6%(1SD). In this phase a homogeneous cylindrical phantom was used. These results led us to consider this calculation algorithm validated for use in VMAT verifications.     

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.     

A Deep Learning-based correction to EPID dosimetry for attenuation and scatter in the Unity MR-Linac system

PurposeEPID dosimetry in the Unity MR-Linac system allows for reconstruction of absolute dose distributions within the patient geometry. Dose reconstruction is accurate for the parts of the beam arriving at the EPID through the MRI central unattenuated region, free of gradient coils, resulting in a maximum field size of ~10 × 22 cm² at isocentre. The purpose of this study is to develop a Deep Learning-based method to improve the accuracy of 2D EPID reconstructed dose distributions outside this central region, accounting for the effects of the extra attenuation and scatter.MethodsA U-Net was trained to correct EPID dose images calculated at the isocenter inside a cylindrical phantom using the corresponding TPS dose images as ground truth for training. The model was evaluated using a 5-fold cross validation procedure. The clinical validity of the U-Net corrected dose images (the so-called DEEPID dose images) was assessed with in vivo verification data of 45 large rectum IMRT fields. The sensitivity of DEEPID to leaf bank position errors (±1.5 mm) and ±5% MU delivery errors was also tested.ResultsCompared to the TPS, in vivo 2D DEEPID dose images showed an average γ-pass rate of 90.2% (72.6%–99.4%) outside the central unattenuated region. Without DEEPID correction, this number was 44.5% (4.0%–78.4%). DEEPID correctly detected the introduced delivery errors.ConclusionsDEEPID allows for accurate dose reconstruction using the entire EPID image, thus enabling dosimetric verification for field sizes up to ~19 × 22 cm² at isocentre. The method can be used to detect clinically relevant errors.     

Comparison between two treatment planning systems for volumetric modulated arc therapy optimization for prostate cancer

PurposeTo investigate the performances of two commercial treatment planning systems (TPS) for Volumetric Modulated Arc Therapy (VMAT) optimization regarding prostate cancer. The TPS were compared in terms of dose distributions, treatment delivery parameters and quality control results.Materials and methodsFor ten patients, two VMAT plans were generated: one with Monaco TPS (Elekta) and one with Pinnacle TPS (Philips Medical Systems). The total prescribed dose was 78 Gy delivered in one 360° arc with a Synergy^® linear accelerator equipped with a MLCi2^®.ResultsVMAT with Monaco provided better homogeneity and conformity indexes but lower mean dose to PTVs than Pinnacle. For the bladder wall (p = 0.019), the femoral heads (p = 0.017), and healthy tissues (p = 0.005), significantly lower mean doses were found using Monaco. For the rectal wall, VMAT with Pinnacle provided a significantly (p = 0.047) lower mean dose, and lower dose into 50% of the volume (p = 0.047) compared to Monaco. Despite a greater number of monitor units (factor 1.5) for Monaco TPS, the total treatment time was equivalent to that of Pinnacle. The treatment delivery parameter analysis showed larger mean MLC area for Pinnacle and lower mean dose rate compared to Monaco. The quality control results gave a high passing rate (>97.4%) for the gamma index for both TPS but Monaco provided slightly better results.ConclusionFor prostate cancer patients, VMAT treatment plans obtained with Monaco and Pinnacle offered clinically acceptable dose distributions. Further investigations are in progress to confirm the performances of the two TPS for irradiating more complex volumes.     

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.     

Multi-institutional comparison of secondary check of treatment planning using computer-based independent dose calculation for non-C-arm linear accelerators

PurposeThis report covers the first multi-institutional study of independent monitor unit (MU)/dose calculation verification for the CyberKnife, Vero4DRT, and TomoTherapy radiotherapy delivery systems.MethodsA total of 973 clinical treatment plans were collected from 12 institutions. Commercial software employing the Clarkson algorithm was used for verification after a measurement validation study, and the doses from the treatment planning systems (TPSs) and verification programs were compared on the basis of the mean value ± two standard deviations. The impact of heterogeneous conditions was assessed in two types of sites: non-lung and lung.ResultsThe dose difference for all locations was 0.5 ± 7.2%. There was a statistically significant difference (P < 0.01) in dose difference between non-lung (−0.3 ± 4.4%) and lung sites (3.5 ± 6.7%). Inter-institutional comparisons showed that various systematic differences were associated with the proportion of different treatment sites and heterogeneity correction.ConclusionsThis multi-institutional comparison should help to determine the departmental action levels for CyberKnife, Vero4DRT, and TomoTherapy, as patient populations and treatment sites may vary between the modalities. An action level of ±5% could be considered for intensity-modulated radiation therapy (IMRT), non-IMRT, and volumetric modulated arc radiotherapy using these modalities in homogenous and heterogeneous conditions with a large treatment field applied to a large region of homogeneous media. There were larger systematic differences in heterogeneous conditions with a small treatment field because of differences in heterogeneity correction with the different dose calculation algorithms of the primary TPS and verification program.     

Dosimetric comparison of normal breathing and deep inspiration breath hold technique for synchronous bilateral breast cancer using 6MV flattened beam and flattening filter free beam

BackgroundThe present study was to investigate the usefulness of deep inspiration breath hold (DIBH) in bilateral breast patients using 6MV flattened beam (FB) and flattening filter free beam (FFFB).Materials and methodsTwenty bilateral breast cancer patients were simulated, using left breast patients treated with DIBH technique. CT scans were performed in the normal breathing (NB) and DIBH method. Three-dimensional conformal radiotherapy (3DCRT) and volumetric arc therapy (VMAT) plans were generated.ResultsIn our study the best organ at risk (OAR) sparing is achieved in the 3DCRT DIBH plan with adequate PTV coverage (V₉₅ ≥ 47.5 Gy) as compared to 6MV FB and FFFB VMAT DIBH plans. The DIBH scan plan reduces the heart mean dose significantly at the rate of 49% in 3DCRT (p = 0.00) and 22% in VMAT (p = 0.010). Similarly, the DIBH scan plan produces lesser common lung mean dose of 18% in 3DCRT (p = 0.011) and 8% in VMAT (0.007) as compared to the NB scan. The conformity index is much better in VMAT FB (1.04 ± 0.04 vs. 1.04 ± 0.05), p =1.00 and VMAT FFFB (1.04 ± 0.05 vs. 1 ± 0.24, p = 0.345) plans as compared to 3DCRT (1.63 ± 0.2 vs. 1.47 ± 0.28, p = 0.002). The homogeneity index of all the plans is less than 0.15. The global dmax is more in VMAT FFFB DIBH plan (113.7%). The maximum MU noted in the NB scan plan (478 vs. 477MU, 1366 vs. 1299 MU and 1853 vs. 1788 MU for 3DCRT, VMAT FB and VMAT FFFB technique as compared to DIBH scan.ConclusionWe recommend that the use of DIBH techniques for bilateral breast cancer patients significantly reduces the radiation doses to OARs in both 3DCRT and VMAT plans.     

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.     

Dosimetric study of a hybrid plan technique for external beam radiotherapy in patients with cervical cancer

The aim of this study was to investigate the effect of a hybrid technique which results from combining intensity modulated radiotherapy (IMRT) and volumetric modulated arc therapy (VMAT) for the treatment of cervical cancer patients. Plans made with the hybrid technique and pure IMRT and VMAT were retrospectively compared in 20 patients with cervical cancer at different stages. All plans were made using the same contours based on the original computed tomography (CT) scans. Conformity (CI) and homogeneity (HI) indices of the planning target volumes (PTVs) were calculated for each technique in order to evaluate plan quality. All techniques were compared in terms of dose to organs at risk (OARs), number of monitor units (MUs) and treatment time. It turned out that plans made with the hybrid technique had improved dose conformity and homogeneity compared to plans made only with IMRT and VMAT (p < 0.001). Regarding the OARs, the maximum dose (D_max) delivered to the bladder, rectum and femoral heads was lower for the hybrid plans compared to the IMRT and VMAT plans (p < 0.001). The volumes irradiated to doses of 50 Gy (V_50Gy) for rectum, bladder and bowel were lower for the hybrid plans (p < 0.001, p = 0.002). Furthermore, the treatment time and MU values for the hybrid plans were found to be between of the values for the IMRT and VMAT plans. It is concluded that, as compared to IMRT and VMAT plans, the hybrid plan technique allowed a better conformity and homogeneity for the dose distribution in the PTV and a dose reduction to the OARs.

     

Evaluation of three VMAT-TMI planning methods to find an appropriate balance between plan complexity and the resulting dose distribution

PurposeEvaluation of different planning methods of treatment plan preparation for volumetric modulated arc therapy during total marrow irradiation (VMAT-TMI).MethodThree different planning methods were evaluated to establish the most appropriate VMAT-TMI technique, based on organ at risk (OAR) dose reduction, conformity and plan simplicity. The methods were: (M₁) the sub-plan method, (M₂) use of eight arcs optimised simultaneously and (M₃) M₂ with monitor unit reduction. Friedman ANOVA comparison, with Nemenyi's procedures, was used in the statistical analysis of the results.ResultsThe dosimetric results obtained for the planning target volume and for most OARs do not differ statistically between methods. The M₃ method was characterized by the lowest numbers of monitor units (3259 MU vs. 4450 MU for M₁ and 4216 MU for M₂) and, in general, the lowest complexity. The variability of the monitor units from control points was almost half for M₃ than M₁ and M₂ (i.e. 0.33 MU vs. 0.61 MU for M₁ and 0.58 for M₂). Analysing the relationship between the dose distributions obtained for the plans and their complexity, the best result was observed for the M₃ method.ConclusionThe use of eight simultaneously optimised arcs with MU reduction allows to obtain VMAT-TMI plans that are characterized by the lowest complexity, with dose distributions comparable to the plans generated by other methods.     

Software simulation of tumour motion dose effects during flattened and unflattened ITV-based VMAT lung SBRT

PurposeRestricted studies comparing different dose rate parameters are available while ITV-based VMAT lung SBRT planning leads to perform the analysis of the most suitable parameters of the external beams used. The special emphasis was placed on the impact of dose rate on dose distribution variations in target volumes due to interplay effects.MethodsFour VMAT plans were calculated for 15 lung tumours using 6 MV photon beam quality (flattening filter FF vs. flattening filter free FFF beams) and maximum dose rate of 600 MU/min, 1000 MU/min and 1400 MU/min. Three kinds of motion simulations were performed finally giving 180 plans with perturbed dose distributions.Results6FFF-1400 MUs/min plans were characterized by the shortest beam on time (1.8 ± 0.2 min). Analysing the performed motion simulation results, the mean dose (Dmean) is not a sensitive parameter to related interplay effects. Looking for local maximum and local minimum doses, some discrepancies were found, but their significance was presented for individual patients, not for the whole cohort. The same was observed for other verified dose metrics.ConclusionsGenerally, the evaluation of VMAT robustness between FF and FFF concepts against interplay effect showed a negligible effect of simulated motion influence on tumour coverage among different photon beam quality parameters. Due to the lack of FFF beams, smaller radiotherapy centres are able to perform ITV-based VMAT lung SBRT treatment in a safe way. Radiotherapy department having FFF beams could perform safe, fast and efficient ITV-based VMAT lung SBRT without a concern about significance of interplay effects.     

Dosimetric comparison of analytic anisotropic algorithm and Acuros XB algorithm in VMAT plans for high-grade glioma

PurposeTo investigate the dosimetric impact between the anisotropic analytical algorithm (AAA) and the Acuros XB (AXB) algorithm in volumetric-modulated arc therapy (VMAT) plans for high-grade glioma (HGG).MethodsWe used a heterogeneous phantom to quantify the agreement between the measured and calculated doses from the AAA and from the AXB. We then analyzed 14 patients with HGG treated by VMAT, using the AAA. We newly created AXB plans for each corresponding AAA plan under the following conditions: (1) re-calculation for the same number of monitor units with an identical beam and leaf setup, and (2) re-optimization under the same conditions of dose constraints. The dose coverage for the planning target volume (PTV) was evaluated by dividing the coverage into the skull, air, and soft-tissue regions.ResultsCompared to the results obtained with the AAA, the AXB results were in good agreement with the measured profiles. The dose differences in the PTV between the AAA and re-calculated AXB plans were large in the skull region contained in the target. The dose difference in the PTV in both types of plan was significantly correlated with the volume of the skull contained in the target (r = 0.71, p = 0.0042). A re-optimized AXB plan's dose difference was lower vs. the re-calculated AXB plan's.ConclusionsWe observed dose differences between the AAA and AXB plans, in particular in the cases in which the skull region of the target was large. Considering the phantom measurement results, the AXB algorithm should be used in VMAT plans for HGG.     

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.     

Simplification of head and neck volumetric modulated arc therapy patient-specific quality assurance,using a Delta4 PT

Background/AimIn many facilities, intensity-modulated radiation therapy (IMRT), and volumetric modulated arc therapy (VMAT) use intensity-modulated beams, formed by a multi-leaf collimator (MLC). In IMRT and VMAT, MLC and linear accelerator errors (both geometric and dose), can significantly affect the doses administered to patients. Therefore, IMRT and VMAT treatment plans must include the use of patient-specific quality assurance (QA) before treatment to confirm dose accuracy.Materials and methodsIn this study, we compared and analyzed the results of dose verification using a multi-dimensional dose verification system Delta4 PT, an ionization chamber dosimeter, and gafchromic film, using data from 52 patients undergoing head and neck VMAT as the test material.ResultBased on the results of the absolute dose verification for the ionization chamber dosimeter and Delta4 PT, taking an axial view, the upper limit of the 95% confidence interval was 3.13%, and the lower limit was −3.67%, indicating good agreement. These results mean that as long as absolute dose verification for the axial view does not deviate from this range, Delta4 PT can be used as an alternative to an ionization chamber dosimeter for absolute dose verification. When we then reviewed dose distribution verification, the pass rate for Delta4 PT was acceptable, and was less varied than that of gafchromic film.ConclusionThis results in that provided the pass rate result for Delta4 PT does not fall below 96%, it can be used as a substitute for gafchromic film in dose distribution verification. These results indicate that patient-specific QA could be simplified.     

Delta4 Discover transmission detector: A comprehensive characterization for in-vivo VMAT monitoring

ObjectiveTo investigate the dosimetric behaviour, influence on photon beam fluence and error detection capability of Delta⁴ Discover transmission detector.MethodsThe transmission detector (TRD) was characterized on a TrueBeam linear accelerator with 6 MV beams. Linearity, reproducibility and dose rate dependence were investigated. The effect on photon beam fluence was evaluated in terms of beam profiles, percentage depth dose, transmission factor and surface dose for different open field sizes. The transmission factor of the 10x10 cm² field was entered in the TPS’s configuration and its correct use in the dose calculation was verified recalculating 17 clinical IMRT/VMAT plans. Surface dose was measured for 20 IMRT fields. The capability to detect different delivery errors was investigated evaluating dose gamma index, MLC gamma index and leaf position of 15 manually modified VMAT plans.ResultsTRD showed a linear dependence on MU. No dose rate dependence was observed. Short-term and long-term reproducibility were within 0.1% and 0.5%. The presence of the TRD did not significantly affect PDDs and profiles. The transmission factor of the 10x10 cm² field size was 0.985 and 0.983, for FF and FFF beams respectively. The 17 recalculated plans met our clinical gamma-index passing rate, confirming the correct use of the transmission factor by the TPS. The surface dose differences for the open fields increase for shorter SSDs and greater field size. Differences in surface dose for the IMRT beams were less than 2%. Output variation ≥2%, collimator angle variations within 0.3°, gantry angle errors of 1°, jaw tracking and leaf position errors were detected.ConclusionsDelta⁴ Discover shows good linearity and reproducibility, is not dependent on dose rate and does not affect beam quality and dose profiles. It is also capable to detect dosimetric and geometric errors and therefore it is suitable for monitoring VMAT delivery.     

A comparison of a moderately hypofractionated IMRT planning technique used in a randomised UK external beam radiotherapy trial with an in-house technique for localised prostate cancer

AimTo compare the radiotherapy technique used in a randomised trial with VMAT and an in-house technique for prostate cancer.BackgroundTechniques are evolving with volumetric modulated arc therapy (VMAT) commonly used. The CHHiP trial used a 3 PTV forward planned IMRT technique (FP_CH). Our centre has adopted a simpler two PTV technique with locally calculated margins.Materials and methods25 patients treated with FP_CH to 60 Gy in 20 fractions were re-planned with VMAT (VMAT_CH) and a two PTV protocol (VMAT_60/52 and VMAT_60/48). Target coverage, conformity index (CI), homogeneity index (HI), monitor units (MU) and dose to the rectum, bladder, hips and penile bulb were compared.ResultsPTV coverage was high for all techniques. VMAT_CH plans had better CI than FP_CH (p ≤  0.05). VMAT_60/52/48 plans had better CI than VMAT_CH. FP_CH had better HI and fewer MU than VMAT (p ≤  0.05). More favourable rectum doses were found for VMAT _CH than FP_CH (V_48.6, V_52.8, V₅₇, p ≤  0.05) with less difference for bladder (p ≥  0.05). Comparing VMAT_CH to VMAT_60/52/48 showed little differences for the bladder and rectum but VMAT_CH had larger penile bulb doses (V_40.8, V_48.6, mean, D_2, p ≤  0.05). Femoral head doses (V_40.8) were similarly low for all techniques (p = ≥ 0.05).ConclusionVMAT produced more conformal plans with smaller rectum doses compared to FP_CH albeit worse HI and more MU. VMAT_60/52 and VMAT_60/48 plans had similar rectal and bladder doses to VMAT_CH but better CI and penile bulb doses which may reduce toxicity.     

Dosimetric comparison of RapidPlan and manually optimized plans in volumetric modulated arc therapy for prostate cancer

PurposeThis study evaluated whether RapidPlan based plans (RP plans) created by a single optimization, are usable in volumetric modulated arc therapy (VMAT) for patients with prostate cancer.MethodsWe used 51 previously administered VMAT plans to train a RP model. Thirty RP plans were created by a single optimization without planner intervention during optimization. Differences between RP plans and clinical manual optimization (CMO) plans created by an experienced planner for the same patients were analyzed (Wilcoxon tests) in terms of homogeneity index (HI), conformation number (CN), D_95%, and D_2% to planning target volume (PTV), mean dose, V_50Gy, V_70Gy, V_75Gy, and V_78Gy to rectum and bladder, monitor unit (MU), and multi-leaf collimator (MLC) sequence complexity.ResultsRP and CMO values for PTV D_95%, PTV D_2%, HI, and CN were significantly similar (p < 0.05 for all). RP mean dose, V_50Gy, and V_70Gy to rectum were superior or comparable to CMO values; RP V_75Gy and V_78Gy were higher than in CMO plans (p < 0.05). RP bladder dose-volume parameter values (except V_78Gy) were lower than in CMO plans (p < 0.05). MU values were RP: 730 ± 55 MU and CMO: 580 ± 37 MU (p < 0.05); and MLC sequence complexity scores were RP: 0.25 ± 0.02 and CMO: 0.35 ± 0.03 (p < 0.05).ConclusionsRP plans created by a single optimization were clinically acceptable in VMAT for patient with prostate cancer. Our simple model could reduce optimization time, independently of planner’s skill and knowledge.