Use of PTW-microDiamond for relative dosimetry of unflattened photon beams

PurposeThe increasing interest in SBRT treatments encourages the use of flattening filter free (FFF) beams. Aim of this work was to evaluate the performance of the PTW60019 microDiamond detector under 6 MV and 10MVFFF beams delivered with the EDGE accelerator (Varian Medical System, Palo Alto, USA). A flattened 6 MV beam was also considered for comparison.MethodsShort term stability, dose linearity and dose rate dependence were evaluated. Dose per pulse dependence was investigated in the range 0.2–2.2 mGy/pulse. MicroDiamond profiles and output factors (OFs) were compared to those obtained with other detectors for field sizes ranging from 40 × 40 cm² to 0.6 × 0.6 cm². In small fields, volume averaging effects were evaluated and the relevant correction factors were applied for each detector.ResultsMicroDiamond short term stability, dose linearity and dependence on monitor unit rate were less than 0.8% for all energies. Response variations with dose per pulse were found within 1.8%. MicroDiamond output factors (OF) values differed from those measured with the reference ion-chamber for less than 1% up to 40 × 40 cm² fields where silicon diodes overestimate the dose of ≈3%. For small fields (<3 × 3 cm²) microDiamond and the unshielded silicon diode were in good agreement.ConclusionsMicroDiamond showed optimal characteristics for relative dosimetry even under high dose rate beams. The effects due to dose per pulse dependence up to 2.2 mGy/pulse are negligible. Compared to other detectors, microDiamond provides accurate OF measurements in the whole range of field sizes. For fields <1 cm correction factors accounting for fluence perturbation and volume averaging could be required.     

Small field characterization of a Nanochamber prototype under flattening filter free photon beams

IntroductionNanochambers present some advantages in terms of energy independence and absolute dose measurement for small field dosimetry in the SBRT scenario. Characterization of a micro-chamber prototype was carried out both under flattened and flattening-filter-free (FFF) beams with particular focus on stem effect.MethodsThe study included characterization of leakage and stem effects, dose rate and dose per pulse dependence, measurement of profiles, and percentage depth doses (PDDs). Ion collection efficiency and polarity effects were measured and evaluated against field size and dose per pulse. The 6_MV, 6_MV_FFF and 10_MV FFF beams of a Varian EDGE were used. Output factors were measured for field sizes ranging from 0.8 × 0.8 cm² to 20 × 20 cm² and were compared with other detectors.ResultsThe 2 mm diameter of this chamber guarantees a high spatial resolution with low penumbra values. In orthogonal configuration a strong stem (and cable) effect was observed for small fields. Dose rate and dose per pulse dependence were <0.3% and 0.6% respectively for the whole range of considered values. The Nanochamber exhibits a field size (FS) dependence of the polarity correction >2%. The OF values were compared with other small field detectors showing a good agreement for field sizes >2 × 2 cm². The large field over-response was corrected applying k_pol(FS).ConclusionsNanochamber is an interesting option for small field measurements. The spherical shape of the active volume is an advantage in terms of reduced angular dependence. An interesting feature of the Nanochamber is its beam quality independence and, as a future development, the possibility to use it for small field absolute dosimetry.     

Determination of inflection points of CyberKnife dose profiles within acceptability criteria of deviations in measurements

AimThe aim of this study was to determine the Inflection Points (IPs) of flattening filter free (FFF) CyberKnife dose profiles for cone-based streotactic radiotherapy. In addition, dosimetric field sizes were determined.BackgroundThe increased need for treatment in the early stages of cancer necessitated the treatment of smaller tumors. However, efforts in that direction required the modeling accuracy of the beam. Removal of the flattening filter (FF) from the path of x-ray beam has provided the solution to those efforts, but required a different normalization approach for the beam to ensure the delivery of the dose accurately. As a solution, researchers proposed a normalization factor based on IPs.Materials and methodsMeasurements using microDiamond (PTW 60019), Diode SRS (PTW 60018) and Monte Carlo (MC) calculations of dose profiles were completed at SAD 80 cm and 5 cm depth for 15–60 mm cones. Performance analysis of detectors with respect to MC calculation was carried out. Gamma evaluation method was used to determine achievable acceptability criteria for FFF CyberKnife beams.ResultsAcceptability within (3%–0.5 mm) was found to be anachievable criterion for all dose profile measurements of the cone beams used in this study. To determine the IP, the first and second derivatives of the dose profile were determined via the cubic spline interpolation technique.ConclusionDerivatives of the interpolated profiles showed that locations of IPs and 50% isodose points coincide.     

Characterization of a new unshielded diode for small field dosimetry under flattening filter free beams

PurposeTo characterize the performance of a new unshielded silicon diode (Razor-IBA) for dose measurements in small flattening filter free beams.MethodsThe Razor has an active volume of 0.6 mm in diameter and 20 µm in length. The detector response stability in measured dose, dose rate, dose per pulse, and dark current were evaluated. The detector response in square fields (0.6–5.0 cm) was determined using PDD curves, axial beam profiles and output ratios. The performances were compared to that of the previously available SFD-IBA and PFD-IBA diodes.Results and discussionThe Razor short term stability relative to the SFD was much improved (<±0.1% after 1.2 kGy). The linearity was <±1% (0.05–30 Gy range) and the dose rate dependence was <±0.5% (4–24 Gy/min range). The dose per pulse dependence was <±0.7% (0.08–0.21 cGy/pulse range). The PDDs measured with Razor and PFD differed <1%. A larger dark current was observed with increase in dose (0.0025 pA/Gy) compared to the SFD (0.0002 pA/Gy). This characteristic is attributed to an increased concentration of recombination centers. The beam profile showed good agreement with the SFD. Penumbra differences were <±0.3 mm relative to PFD, with a slight overestimation of the tails (<1%), due to the absence of diode shielding. Output ratios were in good agreement for fields up to 5 × 5 cm² (<1%).ConclusionsThe Razor diode has the same spatial resolution and performance reliability as its predecessor (SFD), but exhibits the additional advantage of improved stability. These features make the Razor diode detector a good candidate for small field dosimetry.     

Use of IAEA’s phase-space files for virtual source model implementation: Extension to large fields

In a previous work, phase-space data files (phsp) provided by the International Atomic Energy Agency (IAEA) were used to develop a hybrid virtual source model (VSM) for clinical photon beams. Very good agreement with dosimetric measurements performed on linear accelerators was obtained for field sizes up to 15 × 15 cm². In the present work we extend the VSM to larger field sizes, for which phsp are not available. We incorporate a virtual flattening filter to our model, which can be determined from dose measurements for larger fields. In this way a fully functional VSM can be built, from publicly available IAEA’s phsps and standard dose measurements, for fields of any size and tailored to a particular linac.     

CyberKnife beam output factor measurements: A multi-site and multi-detector study

PurposeNew promising detectors are available for measuring small field size output factors (OFs). This study focused on a multicenter evaluation of two new generation detectors for OF measurements on CyberKnife systems.MethodsPTW-60019 microDiamond and W1 plastic scintillation detector (PSD) were used to measure OFs on eight CyberKnife units of various generations for 5–60 mm fixed cones. MicroDiamond and PSD OF were compared to routinely used silicon diodes data corrected applying published Monte Carlo (MC) factors. PSD data were corrected for Čerenkov Light Ratio (CLR). The uncertainties related to CLR determination were estimated.ResultsConsidering OF values averaged over all centers, the differences between MC corrected diode and the other two detectors were within 1.5%. MicroDiamond exhibited an over-response of 1.3% at 7.5 mm and a trend inversion at 5 mm with a difference of 0.2%. This behavior was consistent among the different units. OFs measured by PSD slightly under-responded compared to MC corrected diode for the smaller cones and the differences were within 1%. The observed CLR variability was 2.5% and the related variation in OF values was 1.9%.ConclusionThis study indicates that CyberKnife microDiamond OF require corrections below 2%. The results are enhanced by the consistency observed among different units. Scintillator shows a good agreement to MC corrected diode but CLR determination remains critical requiring further investigations. The results emphasized the value of a multi-center validation over a single center approach.     

“Edge-on” MOSkin detector for stereotactic beam measurement and verification

Dosimetry in small radiation field is challenging and complicated because of dose volume averaging and beam perturbations in a detector. We evaluated the suitability of the “Edge-on” MOSkin (MOSFET) detector in small radiation field measurement. We also tested the feasibility for dosimetric verification in stereotactic radiosurgery (SRS) and stereotactic radiotherapy (SRT). “Edge-on” MOSkin detector was calibrated and the reproducibility and linearity were determined. Lateral dose profiles and output factors were measured using the “Edge-on” MOSkin detector, ionization chamber, SRS diode and EBT2 film. Dosimetric verification was carried out on two SRS and five SRT plans. In dose profile measurements, the “Edge-on” MOSkin measurements concurred with EBT2 film measurements. It showed full width at half maximum of the dose profile with average difference of 0.11 mm and penumbral width with difference of ±0.2 mm for all SRS cones as compared to EBT2 film measurement. For output factor measurements, a 1.1% difference was observed between the “Edge-on” MOSkin detector and EBT2 film for 4 mm SRS cone. The “Edge-on” MOSkin detector provided reproducible measurements for dose verification in real-time. The measured doses concurred with the calculated dose for SRS (within 1%) and SRT (within 3%). A set of output correction factors for the “Edge-on” MOSkin detector for small radiation fields were derived from EBT2 film measurement and presented. This study showed that the “Edge-on” MOSkin detector is a suitable tool for dose verification in small radiation field.     

Experimental assessment of the influence of beam hardening filters on image quality and patient dose in volumetric 64-slice X-ray CT scanners

Beam hardening filters have long been employed in X-ray Computed Tomography (CT) to preferentially absorb soft and low-energy X-rays having no or little contribution to image formation, thus allowing the reduction of patient dose and beam hardening artefacts. In this work, we studied the influence of additional copper (Cu) and aluminium (Al) flat filters on patient dose and image quality and seek an optimum filter thickness for the GE LightSpeed VCT 64-slice CT scanner using experimental phantom measurements. Different thicknesses of Cu and Al filters (0.5–1.6 mm Cu, 0.5–4 mm Al) were installed on the scanner’s collimator. A planar phantom consisting of 13 slabs of Cu having different thicknesses was designed and scanned to assess the impact of beam filtration on contrast in the intensity domain (CT detector’s output). To assess image contrast and image noise, a cylindrical phantom consisting of a polyethylene cylinder having 16 holes filled with different concentrations of K₂HPO₄ solution mimicking different tissue types was used. The GE performance and the standard head CT dose index (CTDI) phantoms were also used to assess image resolution characterized by the modulation transfer function (MTF) and patient dose defined by the weighted CTDI. A 100 mm pencil ionization chamber was used for CTDI measurement. Finally, an optimum filter thickness was determined from an objective figure of merit (FOM) metric. The results show that the contrast is somewhat compromised with filter thickness in both the planar and cylindrical phantoms. The contrast of the K₂HPO₄ solutions in the cylindrical phantom was degraded by up to 10% for a 0.68 mm Cu filter and 6% for a 4.14 mm Al filter. It was shown that additional filters increase image noise which impaired the detectability of low density K₂HPO₄ solutions. It was found that with a 0.48 mm Cu filter the 50% MTF value is shifted by about 0.77 lp/cm compared to the case where the filter is not used. An added Cu filter with approximately 0.5 mm thickness accounts for 50% reduction in radiation-absorbed dose as measured by the weighted CTDI. The FOM results indicate that with an additional filter of 0.5 mm Cu or minimum 4 mm Al, a good compromise between image quality and patient dose is achieved for CT images acquired at tube voltages of 120 and 140 kVp. The results seem to indicate that an optimum filter for high kVp acquisitions, routinely used in cardiovascular imaging, should be 0.5 mm copper or 4 mm aluminium minimum.     

On-site audits to investigate the quality of radiation physics of radiation therapy institutions in the Republic of Korea

PurposeTo investigate and improve the domestic standard of radiation therapy in the Republic of Korea.MethodsOn-site audits were performed for 13 institutions in the Republic of Korea. Six items were investigated by on-site visits of each radiation therapy institution, including collimator, gantry, and couch rotation isocenter check; coincidence between light and radiation fields; photon beam flatness and symmetry; electron beam flatness and symmetry; physical wedge transmission factors; and photon beam and electron beam outputs.ResultsThe average deviations of mechanical collimator, gantry, and couch rotation isocenter were less than 1 mm. Those of radiation isocenter were also less than 1 mm. The average difference between light and radiation fields was 0.9 ± 0.6 mm for the field size of 20 cm × 20 cm. The average values of flatness and symmetry of the photon beams were 2.9% ± 0.6% and 1.1% ± 0.7%, respectively. Those of electron beams were 2.5% ± 0.7% and 0.6% ± 1.0%, respectively. Every institutions showed wedge transmission factor deviations less than 2% except one institution. The output deviations of both photon and electron beams were less than ±3% for every institution.ConclusionsThrough the on-site audit program, we could effectively detect an inappropriately operating linacs and provide some recommendations. The standard of radiation therapy in Korea is expected to improve through such on-site audits.     

Dosimetric shield evaluation with tungsten sheet in 4, 6, and 9 MeV electron beams

In electron radiotherapy, shielding material is required to attenuate beam and scatter. A newly introduced shielding material, tungsten functional paper (TFP), has been anticipated to become a very useful device that is lead-free, light, flexible, and easily processed, containing very fine tungsten powder at as much as 80% by weight. The purpose of this study was to investigate the dosimetric changes due to TFP shielding for electron beams. TFP (thickness 0–15 mm) was placed on water or a water-equivalent phantom. Percentage depth ionization and transmission were measured for 4, 6, and 9 MeV electron beams. Off-center ratio was also measured using film dosimetry at depth of dose maximum under similar conditions. Then, beam profiles and transmission with two shielding materials, TFP and lead, were evaluated. Reductions of 95% by using TFP at 0.5 cm depth occurred at 4, 9, and 15 mm with 4, 6, and 9 MeV electron beams, respectively. It is found that the dose tend to increase at the field edge shaped with TFP, which might be influenced by the thickness. TFP has several unique features and is very promising as a useful tool for radiation protection for electron beams, among others.     

Optimal planning parameters for simultaneous boost IMRT treatment of prostate cancer using a Beam Modulator™

ObjectiveTo determine the optimum energy and beam arrangement for prostate intensity-modulated radiation therapy (IMRT) delivery using an Elekta Beam Modulator? linear accelerator, in order to inform decisions when commissioning IMRT for prostate cancer.MethodsCMS XiO was used to create IMRT plans for a prostate patient. Arrangements with 3, 5, 7, 9 and 11 equally spaced fields, containing both a direct anterior and a direct posterior beam were used, with both 6 MV and 10 MV photons. The effects of varying the maximum number of iterations, leaf increment, number of intensity levels and minimum segment size were investigated. Treatment plans were compared using isodose distributions, conformity indices for targets and critical structures, target dose homogeneity, body dose and plan complexity.ResultsTarget dose conformity and homogeneity and sparing of critical structures improved with an increasing number of beams, although any improvements were small for plans containing more than five fields. Set-ups containing a direct posterior field provided superior conformality around the rectum to anterior beam arrangements. Mean non-target dose and total number of monitor units were higher with 6 MV for all beam arrangements. The dose distribution resulting from seven 6 MV beams was considered clinically equivalent to that with five 10 MV beams.ConclusionMethods have been developed to plan IMRT treatments using XiO for delivery with a Beam Modulator? that fulfil demanding dose criteria, using many different set-ups. This study suggests that 6 MV photons can produce prostate IMRT plans that are comparable to those using 10 MV. Work is ongoing to develop a complete class solution.     

Commissioning Monte Carlo algorithm for robotic radiosurgery using cylindrical 3D-array with variable density inserts

IntroductionTo commission the Monte Carlo (MC) algorithm based model of CyberKnife robotic stereotactic system (CK) and evaluate the feasibility of patient specific QA using the ArcCHECK cylindrical 3D-array (AC) with Multiplug inserts (MP).ResultsFour configurations were used for simple beam setup and two for patient QA, replacing water equivalent inserts by lung. For twelve collimators (5–60 mm) in simple setup, mean (SD) differences between MC and RayTracing algorithm (RT) of the number of points failing the 3%/1 mm gamma criteria were 1(1), 1(3), 1(2) and 1(2) for the four MP configurations. Tracking fiducials were placed within AC for patient QA. Single lung insert setup resulted in mean gamma-index 2%/2 mm of 90.5% (range [74.3–95.9]) and 82.3% ([66.8–94.5]) for MC and RT respectively, while 93.5% ([86.8–98.2]) and 86.2% ([68.7–95.4]) in presence of largest inhomogeneities, showing significant differences (p < 0.05).DiscussionAfter evaluating the potential effects, 1.12 g/cc PMMA and 0.09 g/cc lung material assignment showed the best results. Overall, MC-based model showed superior results compared to RT for simple and patient specific testing, using a 2%/2 mm criteria. Results are comparable with other reported commissionings for flattening filter free (FFF) delivery. Further improvement of MC calculation might be challenging as Multiplan has limited material library.ConclusionsThe AC with Multiplug allowed for comprehensive commissioning of CyberKnife MC algorithm and is useful for patient specific QA for stereotactic body radiation therapy. MC calculation accuracy might be limited due to Multiplan’s insufficient material library; still results are comparable with other reported commissioning measurements using FFF beams.     

Full Monte Carlo and measurement-based overall performance assessment of improved clinical implementation of eMC algorithm with emphasis on lower energy range

New version 13.6.23 of the electron Monte Carlo (eMC) algorithm in Varian Eclipse™ treatment planning system has a model for 4 MeV electron beam and some general improvements for dose calculation. This study provides the first overall accuracy assessment of this algorithm against full Monte Carlo (MC) simulations for electron beams from 4 MeV to 16 MeV with most emphasis on the lower energy range. Beams in a homogeneous water phantom and clinical treatment plans were investigated including measurements in the water phantom. Two different material sets were used with full MC: (1) the one applied in the eMC algorithm and (2) the one included in the Eclipse™ for other algorithms. The results of clinical treatment plans were also compared to those of the older eMC version 11.0.31. In the water phantom the dose differences against the full MC were mostly less than 3% with distance-to-agreement (DTA) values within 2 mm. Larger discrepancies were obtained in build-up regions, at depths near the maximum electron ranges and with small apertures. For the clinical treatment plans the overall dose differences were mostly within 3% or 2 mm with the first material set. Larger differences were observed for a large 4 MeV beam entering curved patient surface with extended SSD and also in regions of large dose gradients. Still the DTA values were within 3 mm. The discrepancies between the eMC and the full MC were generally larger for the second material set. The version 11.0.31 performed always inferiorly, when compared to the 13.6.23.     

High resolution ion chamber array delivery quality assurance for robotic radiosurgery: Commissioning and validation

PurposeHigh precision radiosurgery demands comprehensive delivery-quality-assurance techniques. The use of a liquid-filled ion-chamber-array for robotic-radiosurgery delivery-quality-assurance was investigated and validated using several test scenarios and routine patient plans.Methods and materialPreliminary evaluation consisted of beam profile validation and analysis of source–detector-distance and beam-incidence-angle response dependence. The delivery-quality-assurance analysis is performed in four steps: (1) Array-to-plan registration, (2) Evaluation with standard Gamma-Index criteria (local-dose-difference ⩽ 2%, distance-to-agreement ⩽ 2 mm, pass-rate ⩾ 90%), (3) Dose profile alignment and dose distribution shift until maximum pass-rate is found, and (4) Final evaluation with 1 mm distance-to-agreement criterion. Test scenarios consisted of intended phantom misalignments, dose miscalibrations, and undelivered Monitor Units. Preliminary method validation was performed on 55 clinical plans in five institutions.ResultsThe 1000SRS profile measurements showed sufficient agreement compared with a microDiamond detector for all collimator sizes. The relative response changes can be up to 2.2% per 10 cm source–detector-distance change, but remains within 1% for the clinically relevant source–detector-distance range. Planned and measured dose under different beam-incidence-angles showed deviations below 1% for angles between 0° and 80°. Small-intended errors were detected by 1 mm distance-to-agreement criterion while 2 mm criteria failed to reveal some of these deviations. All analyzed delivery-quality-assurance clinical patient plans were within our tight tolerance criteria.ConclusionWe demonstrated that a high-resolution liquid-filled ion-chamber-array can be suitable for robotic radiosurgery delivery-quality-assurance and that small errors can be detected with tight distance-to-agreement criterion. Further improvement may come from beam specific correction for incidence angle and source–detector-distance response.     

Impact of physiological breathing motion for breast cancer radiotherapy with proton beam scanning – An in silico study

This study investigates the impact of breathing motion on proton breast treatment plans. Twelve patients with CT datasets acquired during breath-hold-at-inhalation (BHI), breath-hold-at-exhalation (BHE) and in free-breathing (FB) were included in the study. Proton plans were designed for the left breast for BHI and subsequently recalculated for BHE or designed for FB and recalculated for the extreme breath-hold phases. The plans were compared from the point of view of their target coverage and doses to organs-at-risk. The median amplitude of breathing motion determined from the positions of the sternum was 4.7 mm (range 0.5–14.6 mm). Breathing motion led to a degradation of the dose coverage of the target (heterogeneity index increased from 4–7% to 8–11%), but the degraded values of the dosimetric parameters of interest fulfilled the clinical criteria for plan acceptance. Exhalation decreased the lung burden [average dose 3.1–4.5 Gy (RBE)], while inhalation increased it [average dose 5.8–6.8 Gy (RBE)]. The individual values depended on the field arrangement. Smaller differences were seen for the heart [average dose 0.1–0.2 Gy (RBE)] and the LAD [1.9–4.6 Gy (RBE)]. Weak correlations were generally found between changes in dosimetric parameters and respiratory motion. The differences between dosimetric parameters for various breathing phases were small and their expected clinical impact is consequently quite small. The results indicated that the dosimetric parameters of the plans corresponding to the extreme breathing phases are little affected by breathing motion, thus suggesting that this motion might have little impact for the chosen beam orientations with scanned proton beams.     

Surface dose measurements with GafChromic EBT film for 6 and 18 MV photon beams

The aim of this study was to determine the surface doses using GafChromic EBT films and compare them with plane-parallel ionization chamber measurements for 6 and 18 MV high energy photon beams. The measurements were made in a water equivalent solid phantom in the build-up region of the 6 and 18 MV photon beams at 100 cm SSD for various field sizes. Markus type plane-parallel ion chamber with fixed-separation between collecting electrodes was used to measure the percent depth doses. GafChromic EBT film measurements were performed both on the phantom surface and maximum dose depth at the same geometry with ion chamber measurements. The surface doses found using GafChromic EBT film were 15%, 20%, 29%and 39% ± 2% (1SD) for 6 MV photons, 6%, 11%, 23% and 32% ± 2% (1SD) for 18 MV photons at 5, 10, 20 and 30 cm² field sizes, respectively. GafChromic EBT film provides precise measurements for surface dose in the high energy photons. Agreement between film and plane-parallel chamber measurements was found to be within ±3% for 18 MV photon beams. There was 5% overestimate on the surface doses when compared with the plane-parallel chamber measurements for all field sizes in the 6 MV photon beams.     

Commissioning of the 4-D treatment delivery system for organ motion management in synchrotron-based scanning ion beams

PurposeThe aim of this work was the commissioning of delivery procedures for the treatment of moving targets in scanning pencil beam hadrontherapy.MethodsEBT3 films fixed to the Anzai Respiratory Phantom were exposed to carbon ion scanned homogeneous fields (E = 332 MeV/u). To evaluate the interplay effect, field size and flatness for 3 different scenarios were compared to static condition: gated irradiation or repainting alone and combination of both. Respiratory signal was provided by Anzai pressure sensor or optical tracking system (OTS). End-exhale phase and 1 s gating window were chosen (2.5 mm residual motion). Dose measurements were performed using a PinPoint ionization chamber inserted into the Brainlab ET Gating Phantom. A sub-set of tests was also performed using proton beams.ResultsThe combination of gating technique and repainting (N = 5) showed excellent results (6.1% vs 4.3% flatness, identical field size and dose deviation within 1.3%). Treatment delivery time was acceptable. Dose homogeneity for gated irradiation alone was poor. Both Anzai sensor and OTS appeared suitable for providing respiratory signal. Comparisons between protons and carbon ions showed that larger beam spot sizes represent more favorable condition for minimizing motion effect.ConclusionResults of measurements performed on different phantoms showed that the combination of gating and layered repainting is suitable to treat moving targets using scanning ion beams. Abdominal compression using thermoplastic masks, together with multi-field planning approach and multi-fractionation, have also been assessed as additional strategies to mitigate the effect of patient respiration in the clinical practice.     

Dosimetric comparison between two MLC systems commonly used for stereotactic radiosurgery and radiotherapy: A Monte Carlo and experimental study

In this work dosimetric parameters of two multi-leaf collimator (MLC) systems, namely the beam modulator (BM), which is the MLC commercial name for Elekta “Synergy S” linear accelerator and Radionics micro-MLC (MMLC), are compared using measurements and Monte Carlo simulations. Dosimetric parameters, such as percentage depth doses (PDDs), in-plane and cross-plane dose profiles, and penumbras for different depths and field sizes of the 6 MV photon beams were measured using ionization chamber and a water tank. The collimator leakages were measured using radiographic films. MMLC and BM were modeled using the EGSnrc-based BEAMnrc Monte Carlo code and above dosimetric parameters were calculated. The energy fluence spectra for the two MLCs were also determined using the BEAMnrc and BEAMDP. Dosimetric parameters of the two MLCs were similar, except for penumbras. Leaf-side and leaf-end 80–20% dose penumbras at 10 cm depth for a 10 × 10 cm² field size were 4.8 and 5.1 mm for MMLC and 5.3 mm and 6.3 mm for BM, respectively. Both Radionics MMLC and Elekta BM can be used effectively based on their dosimetric characteristics for stereotactic radiosurgery and radiotherapy, although the former showed slightly sharper dose penumbra especially in the leaf-end direction.     

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.     

Dosimetric characterization of small fields using a plastic scintillator detector: A large multicenter study

PurposeIn modern radiation therapy accurate small fields dosimetry is a challenge and its standardization is fundamental to harmonize delivered dose in different institutions. This study presents a multicenter characterization of MLC-defined small field for Elekta and Varian linear accelerators. Measurements were performed using the Exradin W1 plastic scintillator detector.Materials and methodsThe project enrolled 24 Italian centers. Each center performed Tissue Phantom Ratio (TPR), in-plane and cross-plane dose profiles of 0.8 × 0.8 cm² field, and Output Factor (OF) measurements for square field sizes ranging from 0.8 to 10 cm. Set-up conditions were 10 cm depth in water phantom at SSD 90 cm. Measurements were performed using two twin Exradin W1 plastic scintillator detectors (PSD) correcting for the Cerenkov effect as proposed by the manufacturer.ResultsData analysis from 12 Varian and 12 Elekta centers was performed. Measurements of 7 centers were not included due to cable problems. TPR measurements showed standard deviations (SD) < 1%; SD < 0.4 mm for the profile penumbra was obtained, while FWHM measurements showed SD < 0.5 mm. OF measurements showed SD < 1.5% for field size greater than 2 × 2 cm². Median OFs values were in agreement with the recent bibliography.ConclusionsHigh degree of consistency was registered for all the considered parameters. This work confirmed the importance of multicenter dosimetric intercomparison. W1 PSD could be considered as a good candidate for small field measurements.