VMAT - Volumetric Modulated Arc Therapy

Volumetrična modulirana ločna terapija (angl. Volumetric Modulated Arc Therapy, VMAT) je ena izmed najsodobnejših obsevalnih tehnik, ki jo od marca 2011 uporabljamo tudi v Sektorju radioterapije na Onkološkem inštitutu v Ljubljani. Temeljna značilnost VMAT je, da linearni pospeševalnik seva fotonski snop ves čas kroženja glave obsevalnika okoli bolnikovega telesa. Pri tem sočasno prihaja do spreminjanja 3 parametrov: oblike obsevalnega polja, hitrosti vrtenja roke oz. glave obsevalnika ter hitrosti doze izsevanega fotonskega snopa. Nasprotno pa pri intenzitetno moduliranem obsevanju (angl. Intensity Modulated RadioTherapy, IMRT) uporabljamo statična obsevalna polja in je hitrost doze ves čas nespremenjena. Najpomembnejši prednosti VMAT pred IMRT sta krajše obsevanje in s tem manjša verjetnost, da se bo bolnik (ali tarča, tj. tumor v njem) na mizi obsevalnika premaknil, ter manjša dozna obremenitev zdravih tkiv v okolici tarče, pri čemer sta konformnost porazdelitve doze v območju tarče in stopnja zaščite zdravih organov in tkiv v okolici najmanj primerljiva s tisto, ki jo dosežemo z IMRT, v nekaterih primerih (obsevanje območja medenice) pa celo izboljšani. V prispevku predstavljamo VMAT: njene značilnosti, potek, indikacije za izbiro in nevarnosti, s katerimi se srečamo pri tako natančnem obsevanju.Volumetric Modulated Arc Therapy (VMAT) Volumetric Modulated Arc Therapy (VMAT) is one of the most advanced radiation therapy techniques, which has been used by the Division of Radiation Oncology at the Institute of Oncology in Ljubljana since March 2011. The fundamental characteristic of VMAT is that the linear accelerator delivers the photon beam for the entire duration of the treatment head’s rotation around the patient’s body. While doing so, it simultaneously changes 3 parameters: the shape of the radiation field, the treatment head’s rotation speed and the dose speed of the delivered photon beam. On the contrary, the Intensity Modulated Radiotherapy (IMRT) utilises static radiation fields and delivers the dose at a constant speed. Compared to IMRT, the most important strengths of VMAT are shorter irradiation time, which makes it less likely that the patient (or target - the tumour) will move on the table of the irradiation device, and a lower dose burden to healthy tissues around the target so that the conformity of the dose distribution in the target region and the level of protection of the healthy organs and tissues surrounding it are comparable to those provided by IMRT or, in some cases (irradiation of the pelvic region), even more improved. The article presents VMAT: its features, course of treatment, indications for the selection and the hazards associated with such an accurate radiation therapy

VMAT - volumetrična modulirana ločna terapija

Volumetric Modulated Arc Therapy (VMAT) Volumetric Modulated Arc Therapy (VMAT) is one of the most advanced radiation therapy techniques, which has been used by the Division of Radiation Oncology at the Institute of Oncology in Ljubljana since March 2011. The fundamental characteristic of VMAT is that the linear accelerator delivers the photon beam for the entire duration of the treatment head’s rotation around the patient’s body. While doing so, it simultaneously changes 3 parameters: the shape of the radiation field, the treatment head’s rotation speed and the dose speed of the delivered photon beam. On the contrary, the Intensity Modulated Radiotherapy (IMRT) utilises static radiation fields and delivers the dose at a constant speed. Compared to IMRT, the most important strengths of VMAT are shorter irradiation time, which makes it less likely that the patient (or target - the tumour) will move on the table of the irradiation device, and a lower dose burden to healthy tissues around the target so that the conformity of the dose distribution in the target region and the level of protection of the healthy organs and tissues surrounding it are comparable to those provided by IMRT or, in some cases (irradiation of the pelvic region), even more improved. The article presents VMAT: its features, course of treatment, indications for the selection and the hazards associated with such an accurate radiation therapy.Volumetrična modulirana ločna terapija (angl. Volumetric Modulated Arc Therapy, VMAT) je ena izmed najsodobnejših obsevalnih tehnik, ki jo od marca 2011 uporabljamo tudi v Sektorju radioterapije na Onkološkem inštitutu v Ljubljani. Temeljna značilnost VMAT je, da linearni pospeševalnik seva fotonski snop ves čas kroženja glave obsevalnika okoli bolnikovega telesa. Pri tem sočasno prihaja do spreminjanja 3 parametrov: oblike obsevalnega polja, hitrosti vrtenja roke oz. glave obsevalnika ter hitrosti doze izsevanega fotonskega snopa. Nasprotno pa pri intenzitetno moduliranem obsevanju (angl. Intensity Modulated RadioTherapy, IMRT) uporabljamo statična obsevalna polja in je hitrost doze ves čas nespremenjena. Najpomembnejši prednosti VMAT pred IMRT sta krajše obsevanje in s tem manjša verjetnost, da se bo bolnik (ali tarča, tj. tumor v njem) na mizi obsevalnika premaknil, ter manjša dozna obremenitev zdravih tkiv v okolici tarče, pri čemer sta konformnost porazdelitve doze v območju tarče in stopnja zaščite zdravih organov in tkiv v okolici najmanj primerljiva s tisto, ki jo dosežemo z IMRT, v nekaterih primerih (obsevanje območja medenice) pa celo izboljšani. V prispevku predstavljamo VMAT: njene značilnosti, potek, indikacije za izbiro in nevarnosti, s katerimi se srečamo pri tako natančnem obsevanju

Obsevanje bolnikov z vstavljenim srčnim spodbujevalnikom ali defibrilatorjem

Ionizing radiation can damage an inserted electronic device, such as a pacemaker (PM) or implantable cardioverter defibrillator (ICD), if it lies in the irradiation field. Recent findings reported a possible failure of an electronic device, which lies outside the irradiation field due to scattered radiation or the production of secondary neutrons, especially when using higher photon beam energies. For this reason, new recommendations for irradiation of patients with inserted pacemakers or cardioverter-defibrillators were made.Znano je, da ionizirajoče sevanje lahko okvari vstavljeno elektronsko napravo, kot je srčni spodbujevalnik (angl. pacemaker, PM) ali kardioverter-defibrilator (angl. implantable cardioverter defibrillator, ICD), če leži v obsevalnem polju. Nedavna odkritja pa navajajo tudi možno okvaro elektronske naprave, ki leži izven obsevalnega polja, zaradi sipanega sevanja oz. nastanka sekundarnih nevtronov, predvsem pri uporabi višjih energij fotonskega snopa.  Iz tega razloga so bila izdelana nova priporočila obsevanja bolnikov z vstavljenimi srčnimi spodbujevalniki ali  kardioverterji-defibrilatorji

Law of corresponding states for osmotic swelling of vesicles

As solute molecules permeate into a vesicle due to a concentration difference across its membrane, the vesicle swells through osmosis. The swelling can be divided into two stages: (a) an "ironing" stage, where the volume-to-area ratio of the vesicle increases without a significant change in its area; (b) a stretching stage, where the vesicle grows while remaining essentially spherical, until it ruptures. We show that the crossover between these two stages can be represented as a broadened continuous phase transition. Consequently, the swelling curves for different vesicles and different permeating solutes can be rescaled into a single, theoretically predicted, universal curve. Such a data collapse is demonstrated for giant unilamellar POPC vesicles, osmotically swollen due to the permeation of urea, glycerol, or ethylene glycol. We thereby gain a sensitive measurement of the solutes' membrane permeability coefficients, finding a concentration-independent coefficient for urea, while those of glycerol and ethylene glycol are found to increase with solute concentration. In addition, we use the width of the transition, as extracted from the data collapse, to infer the number of independent bending modes that affect the thermodynamics of the vesicle in the transition region.Comment: 10 page

The prolate-to-oblate shape transition of phospholipid vesicles in response to frequency variation of an AC electric field can be explained by the dielectric anisotropy of a phospholipid bilayer

The external electric field deforms flaccid phospholipid vesicles into spheroidal bodies, with the rotational axis aligned with its direction. Deformation is frequency dependent: in the low frequency range (~ 1 kHz), the deformation is typically prolate, while increasing the frequency to the 10 kHz range changes the deformation to oblate. We attempt to explain this behaviour with a theoretical model, based on the minimization of the total free energy of the vesicle. The energy terms taken into account include the membrane bending energy and the energy of the electric field. The latter is calculated from the electric field via the Maxwell stress tensor, where the membrane is modelled as anisotropic lossy dielectric. Vesicle deformation in response to varying frequency is calculated numerically. Using a series expansion, we also derive a simplified expression for the deformation, which retains the frequency dependence of the exact expression and may provide a better substitute for the series expansion used by Winterhalter and Helfrich, which was found to be valid only in the limit of low frequencies. The model with the anisotropic membrane permittivity imposes two constraints on the values of material constants: tangential component of dielectric permittivity tensor of the phospholipid membrane must exceed its radial component by approximately a factor of 3; and the membrane conductivity has to be relatively high, approximately one tenth of the conductivity of the external aqueous medium.Comment: 17 pages, 6 figures; accepted for publication in J. Phys.: Condens. Matte

Electroformation in a flow chamber with solution exchange as a means of preparation of flaccid giant vesicles

A recently described technique (Estes and Mayer, Biochim. Biophys. Acta 1712 (2005) 152--160) for the preparation of giant unilamellar vesicles (GUVs) in solutions with high ionic strength is examined. By observing a series of osmotic swellings followed by vesicle bursts upon a micropipette transfer of a single POPC GUV from a sucrose solution into an isoosmolar glycerol solution, a value for the permeability of POPC membrane for glycerol, P = (2.09+/-0.82) x 10^{-8} m/s, has been obtained. Based on this result, an alternative mechanism is proposed for the observed exchange of vesicle interior. With modifications, the method of Estes and Mayer is then applied to preparation of flaccid GUVs.Comment: 13 pages, 10 figures, accepted for publication in Colloids and Surfaces B: Biointerface

HaN-Seg

Purpose: For the cancer in the head and neck (HaN), radiotherapy (RT) represents an important treatment modality. Segmentation of organs-at-risk (OARs) is the starting point of RT planning, however, existing approaches are focused on either computed tomography (CT) or magnetic resonance (MR) images, while multimodal segmentation has not been thoroughly explored yet. We present a dataset of CT and MR images of the same patients with curated reference HaN OAR segmentations for an objective evaluation of segmentation methods. Acquisition and validation methods: The cohort consists of HaN images of 56 patients that underwent both CT and T1-weighted MR imaging for image-guided RT. For each patient, reference segmentations of up to 30 OARs were obtained by experts performing manual pixel-wise image annotation. By maintaining the distribution of patient age and gender, and annotation type, the patients were randomly split into training Set 1 (42 cases or 75%) and test Set 2 (14 cases or 25%). Baseline auto-segmentation results are also provided by training the publicly available deep nnU-Net architecture on Set 1, and evaluating its performance on Set 2. Data format and usage notes: The data are publicly available through an open-access repository under the name HaN-Seg: The Head and Neck Organ-at-Risk CT & MR Segmentation Dataset. Images and reference segmentations are stored in the NRRD file format, where the OAR filenames correspond to the nomenclature recommended by the American Association of Physicists in Medicine, and OAR and demographics information is stored in separate comma-separated value files. Potential applications: The HaN-Seg: The Head and Neck Organ-at-Risk CT & MR Segmentation Challenge is launched in parallel with the dataset release to promote the development of automated techniques for OAR segmentation in the HaN. Other potential applications include out-of -challenge algorithm development and benchmarking, as well as external validation of the developed algorithms

Reducing the dosimetric impact of positional errors in field junctions for craniospinal irradiation using VMAT

AimTo improve treatment plan robustness with respect to small shifts in patient position during the VMAT treatment by ensuring a linear ramp-like dose profile in treatment field overlap regions.BackgroundCraniospinal irradiation (CSI) is considered technically challenging because the target size exceeds the maximal field size, which necessitates using abutted or overlapping treatment fields. Volumetric modulated arc therapy (VMAT) is increasingly being examined for CSI, as it offers both better dose homogeneity and better dose conformance while also offering a possibility to create field junctions which are more robust towards small shifts in patient position during the treatment.Materials and methodsA VMAT treatment plan with three isocenters was made for a test case patient. Three groups of overlapping arc field pairs were used; one for the cranial and two for the spinal part. In order to assure a ramp-like dose profile in the field overlap region, the upper spinal part was optimised first, with dose prescription explicitly enforcing a ramp-like dose profile. The cranial and lower spinal part were done afterwards, taking into account the dose contribution of the upper spinal fields.ResultsUsing simple geometrical reasoning, we demonstrated that hot- and cold spots which arise from small displacement of one treatment field relative to the other treatment field can be reduced by taking two precautions: (a) widening the field overlap region, and (b) reducing the field gradient across the overlap region. The function with the smallest maximal gradient is a linear ramp. We present a treatment planning technique which yields the desired dose profile of the two contributing fields, and minimises dosimetric dependence on minor positional errors in patient set-up

Comparison of conventional and hippocampus-sparing radiotherapy in nasopharyngeal carcinoma: In silico study and systematic review

Background and purpose: Radiation-induced damage to the hippocampi can cause cognitive decline. International recommendations for nasopharyngeal cancer (NPC) radiotherapy (RT) lack specific guidelines for protecting the hippocampi. Our study evaluates if hippocampi-sparing (HS) RT in NPC ensures target coverage and meets recommended dose limits for other at-risk organs. Materials and methods: In a systematic literature review, we compared hippocampal D40% in conventional and HS RT plans. In an in silico dosimetric study, conventional and HS-VMAT plans were created for each patient, following international recommendations for OAR delineation, dose prioritization and acceptance criteria. We assessed the impact on neurocognitive function using a previously published normal tissue complication probability (NTCP) model. Results: In four previous studies (n = 79), researchers reduced D40% hippocampal radiation doses in HS plans compared to conventional RT on average from 24.9 Gy to 12.6 Gy.Among 12 NPC patients included in this in silico study, statistically significant differences between HS and conventional VMAT plans were observed in hippocampal EQD2 Dmax (23.8 vs. 46.4 Gy), Dmin (3.8 vs. 4.6 Gy), Dmean (8.1 vs. 15.1 Gy), and D40% (8.3 vs. 15.8 Gy). PTV coverage and OAR doses were similar, with less homogeneous PTV coverage in HS plans (p = 0.038). This translated to a lower probability of memory decline in HS plans (interquartile range 15.8–29.6 %) compared to conventional plans (33.8–81.1 %) based on the NTCP model (p = 0.002). Conclusion: Sparing the hippocampus in NPC RT is safe and feasible. Given the life expectancy of many NPC patients, their cognitive well-being must be paramount in radiotherapy planning