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Purpose EPID-based in vivo dosimetry (IVD) has been implemented for stereotactic body radiotherapy treatments of non-small cell lung cancer to check both isocenter dose and the treatment reproducibility comparing EPID portal images. Methods 15 patients with lung tumors of small dimensions and treated with volumetric modulated arc therapy were enrolled for this initial experience. IVD tests supplied ratios R between in vivo reconstructed and planned isocenter doses. Moreover a γ-like analysis between daily EPID portal images and a reference one, in terms of percentage of points with γ-value smaller than 1, P γ< 1 , and mean γ-values, γ mean , using a local 3%–3 mm criteria, was adopted to check the treatment reproducibility. Tolerance levels of 5% for R ratio, P γ< 1 higher than 90% and γ mean lower than 0.67 were adopted. Results A total of 160 EPID images, two images for each therapy session, were acquired during the treatment of the 15 patients. The overall mean of the R ratios was equal to 1.005 ± 0.014 (1 SD), with 96.9% of tests within ± 5%. The 2 D image γ-like analysis showed an overall γ mean of 0.39 ± 0.12 with 96.1% of tests within the tolerance level, and an average P γ<1 value equal to 96.4 ± 3.6% with 95.4% of tests with P γ< 1 > 90%. Paradigmatic discrepancies were observed in three patients: a set-up error and a patient morphological change were identified thanks to CBCT image analysis whereas the third discrepancy was not fully justified. Conclusions This procedure can provide improved patient safety as well as a first step to integrate IVD and CBCT dose recalculation. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic resonance imaging requested for a potentially serious indication, provided a unique opportunity to explore the intervillous circulation of placentas from pregnancies complicated by Intra Uterine Growth Restriction (IUGR) and to compare them to normal cases. This allowed an innovative characterization of in vivo utero-placental blood flow, correlating a compromised intervillous circulation in IUGR to the deterioration of fetal condition. MR imaging was requested to rule out suspected posterior placental adhesive disorders in 26 patients. Twelve patients had fetuses appropriate for gestational age, while in 14 patients fetuses were affected by severe IUGR. Multiphasic dynamic contrast-enhanced sagittal sequences were acquired and a quantitative analysis of signal intensity and enhancement kinetics was performed for both the entire placenta and for selected regions. Images disclosed a homogeneous perfusion overall the placenta in normal cases, while IUGR placentas displayed a slow intervillous blood flow, along with many patchy unperfused areas. Intermittent stops worsen the perfusion dynamics of the intervillous mostly in IUGR cases with an elevated ductus venosus pulsatility index. In conclusion, we proved that in IUGR placenta maternal placental blood flow is extremely compromised and that superimposed dynamic phenomena concur to worsen the intervillous circulation leading to an end-stage fetal decompensation. [ABSTRACT FROM AUTHOR]

Purpose MR-guided radiotherapy is a new promising technique. Our department is equipped with a MRIdian 60Co system (Viewray) and we are implementing the TPS of the first 6 MV MR-Linac available for clinical practice (ViewRay). The aim of this study is to assess the impact of the MRIdian Linac through a planning analysis for rectal cancer treatments. Methods Four plans (MRIdian Linac, MRIdian 60Co, RapidArc and sliding windows IMRT) were calculated for 10 patients affected by rectal cancer. ROIs were contoured on Eclipse TPS (version 11.0, Varian). The body, the bowel bag and the bladder were the OaR considered. The prescribed dose for PTV2 was 45 Gy and 55 Gy for PTV1 through SIB. RapidArc (6–15 MV) and 5 beams (6–15 MV) sliding windows IMRT treatment plans were calculated on Eclipse using the Analytical Anisotropic Algorithm. 60Co step and shoot IMRT and 7 beams 6 MV step and shoot IMRT were perfomed with the MRIdian systems, equipped with a GPU-accelerated Monte Carlo Algorithm. The PTV V95 and OaRs QUANTEC dose constraints on the DVHs were considered for the QA of the plans. Results Both MRIdian 60Co and MRIdian Linac showed a better PTV1 coverage. Comparable mean doses for the bladder were registered, while a higher bowel V45 was observed for MRIdian 60Co. Low dose body V5 was higher for the MRIdian 60Co plans whilst the MRIdian Linac low doses resulted to be comparable to the conventional Linac. Conclusions MRIdian allowed reaching dosimetrical goals comparable to RapidArc and IMRT gold standards. The MRIdian Linac resulted in smaller low doses distribution, comparable to the conventional IMRT technique. The availability of an on-board MR imaging system might lead to the reduction of the target margin. This aspect will be investigated when the system will become operative. [ABSTRACT FROM AUTHOR]

Abstract Purpose Optical fibre sensor technology offer numerous advantages over conventional dosimeters, including the use in the MRI environment, as they are non-magnetic and do not cause interface on the image. This study aims to dosimetrically characterize a novel inorganic scintillator detector (DoseWireTM, DoseVue) to evaluate its feasibility for a clinical application in MRI guided radiotherapy. Methods Experimental setup was realized with a water equivalent slab phantom: the detector was placed orthogonal to the beam source (SAD = 105 cm, SSD = 100 cm), 10 cm thickness slabs were placed below the detector to provide backscattered radiation. The detector holder was realized facing with 1,5 mm spacing two additional slabs of 1 mm between the 10 cm and 5 cm slabs. MRidian system (ViewRay, three cobalt-60 sources with 0.35 T MRI technology), was used for the detector irradiation. Dose linearity was performed with 10.5 × 10.5 cm2 field delivering 0.1 Gy, 0.5 Gy, 1 Gy, 2 Gy, 5 Gy and 7.5 Gy. Detector resolution was assessed increasing the delivered dose of the dose linearity test of 0.1 Gy, for each dose levels already delivered. Detector repeatability was estimated repeating the 0.5 Gy delivering set four times with two different Cobalt sources. Finally, field size dependency (2.1 × 2.1 cm2, 6.3 × 6.3 cm2, 10.5 × 10.5 cm2, 14.7 × 14.7 cm2, 21 × 21 cm2) was investigated delivering 0.5 Gy. Results Dose linearity and resolution results are shown in Fig. 1. All the other results, are summarized in Table 1. The detector can resolve 0.1 Gy and it shows a linear dependency within the dose. Repeatability was within 99–100%. Conclusions A preliminary characterization of the detector has been successfully performed. For its physical properties, the detector response is not influenced by magnetic field, so DoseWire can be a promising detector for in vivo dosimetry in MR-Guided Radiotherapy. Further investigations have been already scheduled, to validate the detector for clinical use, like surface and transmission measurements, evaluation of the electron return effect, quality assurance tests and comparison with a reference ionization chamber. [ABSTRACT FROM AUTHOR]

Abstract Purpose Aim of this study is to investigate the MRIdian (ViewRay) TPS accuracy in calculating the dose distribution in presence of magnetic field at tissue-lung interface, where the electron return effect (ERE) plays a primary role. Methods The investigation was realised using Gafchromic EBT3 films. A dose calibration step was planned on MRIdian, exposing thirteen 3 × 3 cm2 film pieces perpendicularly to the beam axis at different Co-60 source exposure times (15–460 s). Films were arranged in a slab water phantom (SAD = 105 cm; SSD = 100 cm) and analysed one day after irradiation. An absolute dose measurement was performed by using a 125 cc ion chamber to estimate correlation between exposure time and absorbed dose. To evaluate dose accuracy at lung-tissue interface, two EBT3 films were inserted at the interfaces of a non-homogeneous sandwich phantom composed by a 5 cm lung-equivalent slab (ρ = 0.39 g/cm3) embedded inside two water-equivalent slabs (see Fig. 1). Three treatment plans were calculated and delivered: • Plan A: one 4.2 × 4.2 cm2 beam delivered at 0 degree • Plan B: three 6.3 × 6.3 cm2 beams delivered at 330,0 and 30 degree • Plan C: an IMRT complex plan constituted by 9 equidistant beams The dose distributions measured by EBT3 films were compared to those calculated by TPS in terms of 1%/1 mm, 3%/1 mm, 3%/3 mm and 25 cGy/1 mm gamma analysis. The dosimetric workflow was also validated by delivering Plan A on a totally homogeneous phantom. Results Table 1 contains the γ < 1 values obtained for the different plans delivered. All plans show a 3%/3 mm gamma-passing rate higher than 90% (tolerance criteria recommended for IMRT by ESTRO Booklet 7). When tighter spatial criteria were considered (delta distance = 1 mm), a decrease of passing rate is observed, whose entity is correlated to the plan complexity. Conclusions MRIdian TPS is able to calculate with high accuracy dose distribution in presence of magnetic field at the tissue-lung interface, correctly modelling the ERE contribution. [ABSTRACT FROM AUTHOR]

Purpose MR-guided adaptive Radiotherapy (MRgART) is considered a promising resource for pancreatic cancer, as it allows to online modify the dose distribution according to daily anatomy. This in silico study aims to compare the dosimetric performance of a 6 MV MR-Linac (MRIdian Linac, ViewRay) with those obtained using a standard 6 MV SBRT-Linac (Edge, Varian). Methods Ten patients affected by locally advanced pancreatic cancer were considered. GTV, stomach, duodenum, liver, bowel bag, spinal cord and kidneys were contoured on the simulation CT scan and CTV was considered equal to GTV. PTV was generated from CTV adding an isotropic 3 mm margin and excluding areas of overlap with organ at risks (OARs). IMRT plans with 20 equidistant beams were performed for the MR-Linac, while dual arc VMAT plans were optimised for the SBRT-Linac. Prescription dose was 40 Gy in 5 fractions. All plans had to respect the following OARs constraints: V33Gy < 1 cc and V25Gy < 20 cc for duodenum, stomach and bowel bag, V12Gy < 50% volume for liver and kidneys, V25Gy < 0.5 cc for spinal cord [1]. The dosimetric comparison was performed on target coverage (D98CTV and V95PTV), dose homogeneity (HI), conformity index (CI) and dose diffusion (V10Gy,V20Gy). Statistical significance was estimated using the Wilcoxon-Mann-Whitney test for paired samples. Results All results are summarised in Table 1 and Fig. 1: nine plans were considered clinically acceptable (V95PTV > 95%). Plans calculated with standard Linac show less low dose diffusion and higher CTV homogeneity, probably due to the different optimisation software, that is more sophisticated but also slower in processing (optimisation mean time ̃25 min) in comparison to the MR-Linac one (∼2 min). Conclusions Dosimetric performance between MR and standard Linac is comparable in SBRT pancreatic treatment. Considering the MRgART need to optimise the treatment plan while the patient is on the couch, MR Linac allows to elaborate clinically and dosimetrically satisfying SBRT plans for pancreatic cancer. [ABSTRACT FROM AUTHOR]

The umbilical cord is a complex structure containing three vessels, one straight vein and two coiled arteries, encased by the Wharton Jelly (WJ) a spongy structure made of collagen and hydrated macromolecules. Fetal blood reaches the placenta through the arteries and flows back to the fetus through the vein. The role of the WJ in maintaining cord circulation proficiency and the ultimate reason for arterial coiling still lack of reasonable mechanistic interpretations. We performed biaxial tension tests and evidenced significant differences in the mechanical properties of the core and peripheral WJ. The core region, located between the arteries and the vein, resulted rather stiffer close to the fetus. Finite element modelling and optimization based inverse method were used to create 2D and 3D models of the cord and to simulate stress distribution in different hemodynamic conditions, compressive loads and arterial coiling. We recorded a facilitated stress transmission from the arteries to the vein through the soft core of periplacental WJ. This condition generates a pressure gradient that boosts the venous backflow circulation towards the fetus. Peripheral WJ allows arteries to act as pressure buffering chambers during the cardiac diastole and helps to dissipate compressive forces away from vessels. Altered WJ biomechanics may represent the structural basis of cord vulnerability in many high-risk clinical conditions. [ABSTRACT FROM AUTHOR]

Purpose The aim of this work was to compare two approaches of cone-beam computed tomography (CBCT) image intensity-correction for dose recalculation of volumetric arc radiotherapy (VMAT) plans. Methods and materials CBCT-based dose calculation accuracy was assessed for pelvic, lung, and head and neck (H&N) treatment sites. The dose distributions of 10 patients for each body regions were re-calculated by RayStation (RS) and Eclipse treatment planning systems (TPS). In particular CBCT RS were calibrated by patient specific density assignment method supplied by RS TPS, while CBCT ROI , obtained applying ROI-based lookup tables for each body region were used in Eclipse TPS. Dose Volume Histogram (DVH) statistics and 3D γ -analysis (3%, 3 mm) of the dose maps and dose-volume statistics were determined to compare the CBCT dose distributions with those of the planning CTs in absence of morphological changes. Results The results indicate a comparable performance of both intensity correction techniques in the scope of VMAT. The differences in the investigated DVH parameters with respect to the pCT were mostly below 2% for both methods, however the γ -index analysis, being more sensitive to local dose changes, pointed out slight inaccuracies in the CBCT ROI dose distributions with respect to CBCT RS. In particular γ -index pass-rates were found increased for CBCT RS with respect to CBCT ROI in pelvic (98% against 95%) in lung (97% against 93%) and in H&N (99% against 95%). Conclusion Both the CBCT intensity correction approaches are attractive options for CBCT dose recalculation and in vivo dose reconstructions [1,2]. No added value was found by using a patient-specific HU table, showing similar results as the group based HU tables in terms of DVH indices while the γ -analysis showed better results for CBCT RS. [ABSTRACT FROM AUTHOR]

Abstract Purpose Aim of this study is to dosimetric evaluate the gating accuracy of an MRI-guided radiotherapy system (MRIdian, ViewRay), combining a 0.35 T scanner with 3 Co-60 gamma sources. Methods An MRI compatible phantom consisting in a piston able to slide inside a cylinder executing a periodic movement was used (CIRS Dynamic Thorax Phantom). An ion chamber (0.125 cc) was inserted inside the piston to perform absolute dose measurement. The extremity of the ion chamber housing is inserted in a 3 × 3 × 2 cm3 parallelepiped marker that allows its reliable MR visualization. A 3D MR image of the phantom in static position was acquired by using a True Fast Imaging with Steady state Precession (True FISP) sequence. A small target (18 cc) surrounding the ion chamber volume was contoured and a homogeneous dose of 2.16 Gy was prescribed (see Fig. 1). The tracking boundary was obtained expanding the target volume of 5 mm in all directions. The same treatment plan was delivered moving the piston at different speeds (6,8,10 mm/s) and for different length path (20,24,36,48 mm). The dose value measured maintaining the piston steady was considered as reference. The gating process was carried out acquiring 4 images/s on a sagittal plan (using a 2D True FISP sequence) and stopping the beam each time the target volume was outside the boundary. Results Table 1 summarises results obtained. The dose values measured at the different dynamic situations resulted to be very close to the dose reference: in 4 on 5 cases the dose difference resulted less than 0.5%. Even the worst result, obtained when the piston was executing large elongations in short lapses, shows an acceptable value (dose difference equal to 1.58%). Conclusions The system showed encouraging performances in tracking SBRT target volumes with dose differences always less than 2% at different motion length and speed. [ABSTRACT FROM AUTHOR]