Homogeneous Dose Distribution Research Articles

Dosimetric study for breathing-induced motion effects in an abdominal pancreas phantom for carbon ion mini-beam radiotherapy.

Particle mini-beam therapy exhibits promise in sparing healthy tissue through spatial fractionation, particularly notable for heavy ions, further enhancing the already favorable differential biological effectiveness at both target and entrance regions. However, breathing-induced organ motion affects particle mini-beam irradiation schemes since the organ displacements exceed the mini-beam structure dimensions, decreasing the advantages of spatialfractionation. In this study, the impact of breathing-induced organ motion on the dose distribution was examined at the target and organs at risk(OARs) during carbon ion mini-beam irradiation for pancreaticcancer. As a first step, the carbon ion mini-beam pattern was characterized with Monte Carlo simulations. To analyze the impact of breathing-induced organ motion on the dose distribution of a virtual pancreas tumor as target and related OARs, the anthropomorphic Pancreas Phantom for Ion beam Therapy (PPIeT) was irradiated with carbon ions. A mini-beam collimator was used to deliver a spatially fractionated dose distribution. During irradiation, varying breathing motion amplitudes were induced, ranging from 5to 15mm. Post-irradiation, the 2D dose pattern was analyzed, focusing on the full width at half maximum (FWHM), center-to-center distance (ctc), and the peak-to-valley dose ratio (PVDR). The mini-beam pattern was visible within OARs, while in the virtual pancreas tumor a more homogeneous dose distribution was achieved. Applied motion affected the mini-beam pattern within the kidney, one of the OARs, reducing the PVDR from 3.78 0.12 to 1.478 0.070 for the 15mm motion amplitude. In the immobile OARs including the spine and the skin at the back, the PVDR did not change within 3.4% comparing reference and motionconditions. This study provides an initial understanding of how breathing-induced organ motion affects spatial fractionation during carbon ion irradiation, using an anthropomorphic phantom. A decrease in the PVDR was observed in the right kidney when breathing-induced motion was applied, potentially increasing the risk of damage to OARs. Therefore, further studies are needed to explore the clinical viability of mini-beam radiotherapy with carbon ions when irradiating abdominalregions.

Safety and Efficacy of Stereotactic Body Radiation Therapy for Ultra-central Thoracic Tumors: A Single Center Retrospective Review

Purpose/Objective(s)We sought to evaluate the toxicity and efficacy of stereotactic body radiotherapy (SBRT) for ultra-central thoracic tumours at our institution. Materials/MethodsPatients with ultra-central lung tumours or nodes, defined as having the planning target volume (PTV) overlapping or abutting the central bronchial tree and/or esophagus, treated at our institution with SBRT between 2009 and 2019 were retrospectively reviewed. All SBRT plans were generated with the goal of creating homogenous dose distributions. The primary endpoint was incidence of SBRT-related grade ≥3 toxicity, defined using the Common Terminology Criteria for Adverse Events (V5.0). Secondary endpoints included local failure (LF), progression-free survival (PFS) and overall survival (OS). Competing risk analysis was used to estimate incidence and identify predictors of severe toxicity and LF, while the Kaplan-Meier method was used to estimate PFS and OS. Results154 patients receiving 162 ultra-central courses of SBRT were included. The most common prescription was 50Gy in 5 fractions (42%), with doses ranging from 30-55Gy in 5 fractions (BED10 range 48-115Gy). The incidence of severe toxicity was 9.4% at 3 years. The most common severe toxicity was pneumonitis (n=4). There was 1 possible treatment related death from pneumonitis/pneumonia. Predictors of severe toxicity included increased PTV size, decreased PTV V95%, lung V5Gy, and lung V20Gy. The incidence of LF was 14% at 3 years. Predictors of LF included younger age, and greater volume of overlap between the PTV and esophagus. Median PFS was 8.8 months, while median OS was 44.0 months. ConclusionIn the largest case series of ultra-central thoracic SBRT to date, homogenously prescribed SBRT was associated with relatively low rates of severe toxicity and LF. Predictors of toxicity should be interpreted in the context of the heterogeneity in toxicities observed.

TLD measurement to investigate the bolus effect on skin dose in total body irradiation with helical tomotherapy

The aim of this study is to investigate the skin dose of total body irradiation (TBI) with Helical Tomotherapy and the need to use a bolus to achieve an adequate skin dose. To simulate the RANDO phantom geometrically, arms and legs created from rice were added to the RANDO phantom and it was immobilized in a vacuum bed. Computed tomography (CT) images of the lower and upper parts of the phantom with a slice thickness of 3 mm were acquired with and without bolus. Fifteen thermoluminescent dosimeters (TLDs) were placed on the skin line of the RANDO phantom. Treatment plans were prepared, and the phantom was irradiated both with and without bolus by the Helical Tomotherapy technique. Results of the measurements and treatment planning system (TPS) doses were compared by the non-parametric Wilcoxon signed-rank test. All procedures were repeated three times and results were evaluated. At the end of the study, it was proven that even when no bolus is used, the skin receives an adequate dose in TBI with Helical Tomotherapy while achieving homogeneous dose distribution and saving critical organs at the same time.

Using the photon isoeffective dose formalism to compare and combine BNCT and CIRT in a head and neck tumour

Boron Neutron Capture Therapy (BNCT) is a radiotherapy technique based on the enrichment of tumour cells with suitable 10-boron concentration and on subsequent neutron irradiation. Low-energy neutron irradiation produces a localized deposition of radiation dose caused by boron neutron capture reactions. Boron is vehiculated into tumour cells via proper borated formulations, able to accumulate in the malignancy more than in normal tissues. The neutron capture releases two high-LET charged particles (i.e., an alpha particle and a lithium ion), losing their energy in a distance comparable to the average dimension of one cell. Thus BNCT is selective at the cell level and characterized by high biological effectiveness. As the radiation field is due to the interaction of neutrons with the components of biological tissues and with boron, the dosimetry requires a formalism to express the absorbed dose into photon-equivalent units. This work analyzes a clinical case of an adenoid cystic carcinoma treated with carbon-ion radiotherapy (CIRT), located close to optic nerve and deep-seated as a practical example of how to apply the formalism of BNCT photon isoeffective dose and how to evaluate the BNCT dose distribution against CIRT. The example allows presenting different dosimetrical and radiobiological quantities and drawing conclusions on the potential of BNCT stemming on the clinical result of the CIRT. The patient received CIRT with a dose constraint on the optic nerve, affecting the peripheral part of the Planning Target Volume (PTV). After the treatment, the tumour recurred in this low-dose region. BNCT was simulated for the primary tumour, with the goal to calculate the dose distribution in isoeffective units and a Tumour Control Probability (TCP) to be compared with the one of the original treatment. BNCT was then evaluated for the recurrence in the underdosed region which was not optimally covered by charged particles due to the proximity of the optic nerve. Finally, a combined treatment consisting in BNCT and carbon ion therapy was considered to show the consistency and the potential of the model. For the primary tumour, the photon isoeffective dose distribution due to BNCT was evaluated and the resulted TCP was higher than that obtained for the CIRT. The formalism produced values that are consistent with those of carbon-ion. For the recurrence, BNCT dosimetry produces a similar TCP than that of primary tumour. A combined treatment was finally simulated, showing a TCP comparable to the BNCT-alone with overall dosimetric advantage in the most peripheral parts of the treatment volume. Isoeffective dose formalism is a robust tool to analyze BNCT dosimetry and to compare it with the photon-equivalent dose calculated for carbon-ion treatment. This study introduces for the first time the possibility to combine the dosimetry obtained by two different treatment modalities, showing the potential of exploiting the cellular targeting of BNCT combined with the precision of charged particles in delivering an homogeneous dose distribution in deep-seated tumours.

A dosimetric study comparing Cyberknife and LINAC-based stereotactic radiotherapy or radiosurgery treatments

BackgroundWhile Gamma Knife and CyberKnife are commonly used for Stereotactic radiosurgery (SRS), their availability is limited. In contrast, linac-based SRS using linear accelerators (linacs) is more accessible and widely used. PurposeOur study aims to evaluate Linac-based SRS with the Varian RapidArc technique for treating brain metastases. The primary objective is to compare the quality of treatment plans between RapidArc and CyberKnife modalities. Materials & methodsTen patients with a single brain metastasis were treated using the CyberKnife machine with prescribed doses ranging from 18 to 25 Gy. Treatment plans utilized Multiplan software for CyberKnife and Eclipse for RapidArc plans. The plans were evaluated based on dose metrics for the target and critical structures, treatment time, and brain tissue volume. ResultsBoth techniques provided acceptable Gross tumor volume coverage and organs at risk sparing. CyberKnife delivered a higher integral dose to the Gross tumor volume and brain tissues compared to RapidArc (p < 0.05). The conformity index was slightly better in RapidArc (1.14 ± 0.25) compared to CyberKnife (1.22 ± 0.12), but the difference was not statistically significant (p > 0.05). However, CyberKnife exhibited a better dose gradient index (4.52 ± 0.99) compared to RapidArc (8.6 ± 7.28). The treatment time in RapidArc was significantly lower compared to CyberKnife, p < 0.05. RapidArc demonstrated lower minimum and maximum doses to some organs at risk, although some differences were not statistically significant. For normal brain tissues, V12Gy was significantly lower with RapidArc (3.27 ± 2.92) compared to CyberKnife (4.09 cc ± 3.27). ConclusionThe RapidArc technique is capable of achieving similar treatment quality as CyberKnife systems for SRS treatments. It offers improved target coverage, higher conformity, and homogeneous dose distribution within the Planning target volume if homogeneity is desirable in some intuitions or cases. In addition, it can also achieve low integral doses to organs at risk and brain tissues.

Safety and Efficacy of Stereotactic Body Radiotherapy for Ultra-Central Thoracic Tumors

Stereotactic body radiotherapy (SBRT) is increasingly utilized in the management of ultra-central thoracic tumors, although concerns regarding significant toxicity remain. We sought to evaluate the toxicity and efficacy of SBRT to these tumors at our institution. Patients with ultra-central lung tumors or nodes treated at our institution with SBRT between 2009 and 2019 were retrospectively reviewed. Ultra-central was defined as having the planning target volume (PTV) overlapping or abutting the central bronchial tree and/or esophagus. All SBRT plans were generated with homogenous dose distributions using target coverage objectives of ITV V100% >99%, PTV V95% >99%, and an ideal PTV Dmax <105% (strict <120%). All plans were reviewed in quality assurance rounds by a team of dosimetrists, physicists, and radiation oncologists. The primary endpoint was incidence of severe toxicity (ST), defined as SBRT-related grade ≥3 toxicities, graded using the Common Terminology Criteria for Adverse Events V5.0. Secondary endpoints included local failure (LF), progression-free survival (PFS) and overall survival (OS). Competing risk analysis was used to estimate incidence and predictors of ST and LF, with death as a competing risk. Kaplan-Meier method was used to estimate PFS and OS. A total of 154 patients who received 162 ultra-central courses of SBRT were included, with a median follow-up of 21.5 months. Treatment intent was most commonly for oligoprogression (46%), oligometastasis (30%), followed by curative (20%). The most frequent tumor histologies were NSCLC (41%) and RCC (26%). SBRT prescription doses ranged from 30-55 Gy in 5 fractions (BED10 range 48-115 Gy). The most common prescription was 50 Gy in 5 fractions (42%). The cumulative incidence of ST was 8.9% at 3-years. The most common ST was pneumonitis (n = 4). Notable toxicities included bronchopleural fistula (n = 2, grade 3 and 4), bronchial stricture (n = 1, grade 3), and esophagitis leading to bleeding (n = 1, grade 4). There were no esophageal strictures or perforations, and no bronchial bleeds. There was 1 possible treatment related death from pneumonitis/pneumonia. Predictors of any ST included increased lung V5 Gy, decreased PTV V95%, and not having prior radiation therapy to the chest. The cumulative incidence of LF was 4.8%, 11% and 14% at 1-, 2-, and 3-years respectively. Predictors of LF included younger age, and greater volume of overlap between the PTV and esophagus. Median PFS was 8.4 months, while median OS was 3.7 years. In one of the largest case series of ultra-central thoracic SBRT reported to date, homogenously prescribed SBRT plans were associated with relatively low rates of ST and LF across a variety of treatment indications. Predictors of ST should be interpreted recognizing the heterogeneity in toxicities observed. Identified predictors of both ST and LF can contribute to future work to optimize the therapeutic ratio in treatment of ultra-central thoracic tumors.

Dose Coverage Variation Caused by Setup Uncertainties in VMAT-TBI Treatment

Volumetric Modulated Arc Therapy Total Body Irradiation (VMAT TBI) offers several advantages over conventional TBI techniques, including reduced cost of the treatment room (a normal-sized vault versus a large shielded vault), increased patient comfort during treatment, lung sparing without the need for physical blocks, and a more homogeneous dose distribution. In VMAT-TBI treatment, plans at multiple isocenters are utilized to cover the entire body, and patients are positioned with the aid of image guidance. However, aligning the patient's entire body during setup can be challenging due to its pliability, and the setup time is heavily dependent on the tolerance allowed by the image guidance. Therefore, studying the variation in dose coverage caused by setup uncertainties in VMAT-TBI treatments can help optimize the clinical workflow and determine the optimal tolerance for patient positioning. New plans were generated to simulate the uncertainties that occur during treatment setup for each patient. These plans were created by shifting the original VMAT TBI plans at the head, chest, abdomen (or pelvis) isocenters by 5mm and 1cm in the left-right (LR), inferior-superior (IS), and anterior-posterior (AP) directions, respectively. Dose DicomRT files were exported, and the dose change due to the shifts was analyzed. The statistical quantification of the percentage of the body that experienced a dose change of over 2%, 5%, and 10% of the prescription due to the shifts from the original plans was calculated for all patients. Histograms were generated, showing the percentage of body getting dose change of 1-2%, 2-3%, 3-5%, 5-7%, 7-10%, 10-15%. The table below displays the percent volume receiving a dose change of 2%, 5%, and 10% of the prescription for a 5mm shift. Among the shift directions, the dose change is most sensitive in the IS direction, with similar impact observed the in LR and AP directions. Among different sites, the chest experiences the largest dose change, followed by the pelvis. For a 5mm shift in the IS direction, the average percent volume receiving a dose change of 2%, 5%, and 10% in the chest is 9.25%, 2.64%, and 0.27%, respectively. For a 1cm shift, the numbers are 12.23%, 6.75%, and 1.29%. In the pelvis (abdomen), these values are 9.03%, 1.67%, and 0.17% for a 5mm shift and 13.28%, 6.1%, and 0.85% for a 1cm shift. For head plans, the values are 2.72%, 0.9%, and 0.14% for a 5mm shift and 3.77%, 1.66%, and 0.53% for a 1cm shift. Accurate alignment in the chest region is crucial in VMAT TBI treatment. Efforts should be made to minimize shifts over 1cm in the IS direction.

Development and Quality Assurance of Multileaf Collimator (MLC) Auto-Feathering Junctions for Multi-Isocenter Supine Volumetric Modulated Arc Therapy (VMAT) Craniospinal Axis Irradiation on Halcyon.

Currently, there is alack of methods and tools that efficiently evaluate the auto-feathering junctions created by multileaf collimator (MLCs) for supine volumetric modulated arc therapy (VMAT) craniospinalirradiation (CSI) plans. We have investigated the feasibility of stitching together multi-isocenter fluence maps to then analyze the feathered junctions for patient-specific quality assurance (QA). Furthermore, we investigated the capability of Halcyon for the treatment of CSI patients.Three patients, who previously underwent VMAT CSI treatment on TrueBeam (6-MV flattening filter-free (FFF)) for 36 Gy in 20 fractionswerereplanned for Halcyon. A multi-isocenter approach with only translational superior-inferior shifts was used for both platforms. Each isocenter consists of two full arcs with anterior avoidance sectors, ±5° collimator rotations between arcs, and 5-8 cm of overlapping MLC auto-feathering junctions. All plans were QA'd via electronicportalimaging device (EPID) portal dosimetry and analyzed with a gamma criteria of 3%/3 mm. A variety of plan quality metrics were analyzed to evaluate dose distributions to the target, doses to organs at risk (OARs), and integral dose to the patient. A MATLAB script was developed to stitch the calculated and measured fluence maps in order to perform patient-specific QA for the composite fluence.The Halcyon plans provided highly conformal and homogenous dose distributions to the entire CSI target, superior to the clinical TrueBeam plans, while sparing critical organs with significantly lower values of V10Gy and V18Gy by up to 2% and 2.5%, respectively.Qualitative depictions of vertical dose profiles from the stitched DICOM of the entire CSI target for both planned and delivered fluence maps demonstrated equivalency, with slightly lower average pass rates with Halcyon (97%) compared to TrueBeam(99.9%). This approach to stitch multiple measured versus calculated EPID fluence maps has shown to be a feasible and accurate method and will be helpful for comprehensive VMAT CSI QA on both platforms. Further implementation of this script will be used in examining dosimetric impacts of daily patient positioning errors at MLC auto-feathering junctions.

Evaluation of intensity-modulated electron FLASH radiotherapy in a clinical setting using veterinary cases.

The increased normal tissue tolerance for FLASH radiotherapy (FLASH-RT), as compared to conventional radiotherapy, was first observed in ultra-high dose rate electron beams. Initial clinical trials in companion animals have revealed a high risk of developing osteoradionecrosis following high-dose single-fraction electron FLASH-RT, which may be related to inhomogeneities in the dose distribution. In the current study, we aim to evaluate the possibilities of intensity-modulated electron FLASH-RT in a clinical setting to ensure a homogeneous dose distribution in future veterinary and human clinical trials. Our beam model in the treatment planning system electronRT (.decimal, LLC, Sanford, FL, USA) was based on a 10-MeV electron beam from a clinical linear accelerator used to treat veterinary patients with FLASH-RT in a clinical setting. In electronRT, the beam can be intensity-modulated using tungsten island blocks in the electron block cutout, and range-modulated using a customized bolus with variable thickness. Modulations were first validated in a heterogeneous phantom by comparing measured and calculated dose distributions. To evaluate the impact of intensity modulation in superficial single-fraction FLASH-RT, a treatment planning study was conducted, including eight canine cancer patient cases with simulated tumors in the head-and-neck region. For each case, treatment plans with and without intensity modulation were created for a uniform bolus and a range-modulating bolus. Treatment plans were evaluated using a target dose homogeneity index (HI), a conformity index (CI), the near-maximum dose outside the target ( ), and the near-minimum dose to the target ( ). By adding intensity modulation to plans with a uniform bolus, the HI could be improved (p=0.017). The combination of a range-modulating bolus and intensity modulation provided a further significant improvement of the HI as compared to using intensity modulation in combination with a uniform bolus (p=0.036). The range-modulating bolus also improved the CI compared to using a uniform bolus, both with an open beam (p=0.046) and with intensity modulation (p=0.018), as well as increased the (p=0.036 with open beam and p=0.05 with intensity modulation) and reduced the median (not significant). By using intensity-modulated electron FLASH-RT in combination with range-modulating bolus, the target dose homogeneity and conformity in canine patients with simulated tumors in complex areas in the head-and-neck region could be improved. By utilizing this technique, we hope to decrease the dose outside the target volume and avoid hot spots in future clinical electron FLASH-RT studies, thereby reducing the risk of radiation-induced toxicity.

Proposal of a New Conformal Factor and Normal Tissue Penalty Factor for Radiosurgery Treatments.

The quality of a treatment plan is evaluated by the conformality of the prescribed isodose around the target and the homogeneity of dose distribution inside the target. Presently, to check the target volume conformality, a number of published conformity indices are in use. Most of these indices are based on the target volume coverage by prescribed isodose, with respect to the total volume of the target. Some take into account the normal tissue covered by the prescribed isodose and suitably weigh the target coverage to evaluate conformity. In this study, for the irradiation of normal tissue by the prescription isodose, a normal tissue penalty factor is proposed and for the target conformality, a new conformal factor is proposed by applying this normal tissue penalty factor to the target coverage. The proposed conformal factor is evaluated for a few sample analytical cases and the results are compared with those obtained using the published conformity indices.

Comparison of helical and TomoDirect techniques with simultaneous integrated boost in early breast cancer patients.

The aim of the study was to perform dosimetric comparisons of helical (H) and TomoDirect (TD) plans for whole-breast irradiation (WBI) with simultaneous integrated boost (SIB) in early-stage breast cancer patients undergoing breast conserving surgery. Fifty patients, 25 with left-side and 25 with right-side tumors, were determined for a treatment planning system for a total dose of 50.4Gy in 1.8Gy per fraction to WBI, with a SIB of 2.3Gy per fraction delivered to the tumor bed. The planning target volume (PTV) doses and the conformity (CI) and homogeneity indices (HI) for PTVbreast and PTVboost, as well as organ-at-risk (OAR) doses and treatment times, were compared between the H and TD plans. All plans met the PTV coverage criteria for the H plan, except for mean V107 of PTVbreast for TD plan. The H plan yielded better homogeneity and conformity of dose distribution compared to the TD plan. The ipsilateral mean lung doses were not significantly different between the two plans. The TD plans is advantageous for mean doses to the heart, contralateral breast and lung, spinal cord, and esophagus than the H plans. In both the H and TD plans, the right-sided breast patients had lower heart dose parameters than the left-sided breast patients. The TD plan is superior to the H plan in sparing the contralateral breast and lung by decreasing low-dose volumes. While the OAR dose advantages of TD are appealing, shorter treatment times or improved dose homogeneity and conformity for target volume may be advantageous for H plan.

Inhomogeneously distributed ferroptosis with a high peak-to-valley ratio may improve the antitumor immune response.

Combined radiotherapy (RT) and mild hyperthermia have been used clinically for decades to increase local control. Both modalities tend to achieve a homogeneous dose distribution within treatment targets to induce immunogenic cell death. However, marked, and long-lasting abscopal effects have not usually been observed. We proposed a hypothesis to emphasize the importance of the peak-to-valley ratio of the dose distribution inside the tumor to induce immunogenic ferrroptosis in peak area while avoid nonimmunogenic ferroptosis in valley area. Although inhomogeneous distributed energy absorption has been noted in many anticancer medical fields, the idea of sedulously created dose inhomogeneity related to antitumor immunity has not been discussed. To scale up the peak-to-valley ratio, we proposed possible implications by the combination of nanoparticles (NP) with conventional RT or hyperthermia, or the use of a high modulation depth of extremely low frequency hyperthermia or high resolution spatially fractionated radiotherapy (SFRT) to enhance the antitumor immune reactions.

Radio-immune response modelling for spatially fractionated radiotherapy.

Radiation-induced cell death is a complex process influenced by physical, chemical and biological phenomena. Although consensus on the nature and the mechanism of the bystander effect were not yet made, the immune process presumably plays an important role in many aspects of the radiotherapy including the bystander effect. A mathematical model of immune response during and after radiation therapy is presented. Immune response of host body and immune suppression of tumor cells are modelled with four compartments in this study; viable tumor cells, T cell lymphocytes, immune triggering cells, and doomed cells.&#xD;The growth of tumor was analyzed in two distinctive modes of tumor status (immune limited and immune escape) and its bifurcation condition. Tumors in the immune limited mode can grow only up to a finite size, named as terminal tumor volume analytically calculated from the model. The dynamics of the tumor growth in the immune escape mode is much more complex than the tumors in the immune limited mode especially when the status of tumor is close to the bifurcation condition. Radiation can kill tumor cells not only by radiation damage but also by boosting immune reaction. The model demonstrated that the highly heterogeneous dose distribution in spatially fractionated radiotherapy (SFRT) can make a drastic difference in tumor cell killing compared to the homogeneous dose distribution. SFRT can not only enhance but also moderate the cell killing depending on the immune response triggered by many factors such as dose prescription parameters, tumor volume at the time of treatment and tumor characteristics. The model was applied to the lifted data of 67NR tumors on mice and a sarcoma patient treated multiple times over 1200 days for the treatment of tumor recurrence as a demonstration.&#xD.

Comparison of h-BN material with metal implants in radiotherapy applications: Characterization and dose distribution measurements in LINAC

Patients with metal implants having high atomic numbers (Zeff) can be encountered in radiotherapy applications. These heterogeneous structures cause dose perturbations at the bone-tissue interface. These perturbations are undesired due to the lack of accurate dose calculation and contouring process through a higher image quality. In this study, the effect of 6–18 MV photon irradiation on hexagonal boron nitride (h-BN) in comparison with titanium and stainless-steel implants was proposed as an implant material due to its biocompatibility and low Zeff. First, h-BN material was characterized and its mass attenuation coefficient was determined. Then, the percent depth doses (PDD) were obtained for the implant materials. Next, the dose distributions were evaluated with both TPS using CT images and TLDs. A dose perturbation was not observed at the interface for h-BN material different from the metal implants as seen from the PDD distributions. The change in PDD varied between 1 and 3% for h-BN with respect to RW3 slab phantom, which was 4–7 times lower than that of metal implants, depending on the material thickness. No metal artifact was seen at h-BN implants compared to metal implants. Homogenous dose distribution was also seen from the isodose lines on the CT images, while dose perturbations were more pronounced in metal implants. In addition, a more severe dose reduction at the metal implant/bone tissue interface was observed at 6 MV photon beam energy compared to 18 MV. In contrast, no dose change was observed for the h-BN material at each photon beam energy. Hence, an accurate dose calculation can be done for h-BN as a result of these advantages. The results showed that, due to its biocompatible property, h-BN material can be evaluated as an implant material in radiotherapy applications if its mechanical properties were further improved.

Comparative dosimetric and radiobiological assessment of left-sided whole breast and regional nodes with advanced radiotherapy techniques.

The aim of this study was to analyze the dosimetric and radiobiologic differences of the left-sided whole breast and regional nodes in intensity-modulated radiotherapy (IMRT), volume-modulated arc therapy (VMAT), and helical tomotherapy (HT). The IMRT, VMAT, and HT plans in this study were generated for thirty-five left-sided breast cancer patients after breast-conserving surgery (BCS). The planning target volume (PTV) included the whole breast and supraclavicular nodes. PTV coverage, homogeneity index (HI), conformity index (CI), dose to organs at risk (OARs), secondary cancer complication probability (SCCP), and excess absolute risk (EAR) were used to evaluate the plans. Compared to IMRT, the VMAT and HT plans resulted in higher PTV coverage and homogeneity. The VMAT and HT plans also delivered a lower mean dose to the ipsilateral lung (9.19±1.36 Gy, 9.48±1.17 Gy vs. 11.31±1.42 Gy) and heart (3.99±0.86 Gy, 4.48±0.62 Gy vs. 5.53±1.02 Gy) and reduced the V5Gy, V10Gy, V20Gy, V30Gy, and V40Gy of the ipsilateral lung and heart. The SCCP and EAR for the ipsilateral lung were reduced by 3.67%, 3.09% in VMAT, and 22.18%, 19.21% in HT, respectively. While were increased for the contralateral lung and breast. This study showed that VMAT plans provide a more homogeneous dose distribution to the PTV, minimizing exposure to ipsilateral structures and significantly reducing SCCP and EAR, and slightly increasing dose to contralateral structures. Overall, the VMAT plan can be considered a beneficial technique for BCS patients whose PTV includes the whole breast and regional nodes.

Influence of Homogeneous Dose Distribution Inside the Cancer Target on Local Control in Glioblastomas

Purpose: Assessment of the effect of homogeneous distribution inside the radiation target on local control in glioblastoma.&#x0D; Material and methods: In patients under the age of 50, fractionation with the prescribed dose of 2 and 3 Gy was alternately used: the first patient was prescribed a regimen with a dose of 3 Gy, the next – with a dose of 2 Gy (a study involving a pairwise selection strategy). The same approach is used for the older age group (50 years and more). A total of 141 patients had morphologically confirmed glioblastoma (grade 4). 10 patients who had not completed a full course of radiation therapy were excluded. Additionally, 27 patients with GBM were added, treated in the experimental study of National Medical Research Radiological Centre without methodological features. In total, 158 patients were included in the study. 74 patients were treated with the prescribed dose of 2 Gy, 84 had a dose of 3 Gy.&#x0D; Results: The median homogeneity index in our study was 7.5 (95% Cl: 6,7-8,3). The minimum HI indicator was at the level of 2.5, the maximum – 36.9. We managed to identify the most informative level of the homogeneity index according to the criterion of local control. The median disease-free survival in the group with HI&gt;=8.5 was 7.9 months (n=62; 95 % Cl: 1.6-14.2), in the group with a lower level of homogeneity - 5.4 months (n=96; p=0.024). The significance of the highlighted HI range depended on the fractionation mode used. The median relapse–free survival with fractionation of 3 Gy in the group with HI&gt;=8.5 was 12.7 months (n=38), in the group with a lower level of homogeneity - 8.3 months (n=46; p=0.020). In the group with fractionation regime of 2 Gy: 6.4 and 3.8 months, respectively (n=24/50; p=0.336). The median overall survival in the group with HI&gt;=8.5 was 27.2 months, in the group with a lower level of homogeneity – 21.2 months (p=0.904).&#x0D; Conclusion: The level of homogeneity has no significance according to the criterion of overall survival. Despite the fact that this predictor is of value only in scientific terms, the fact of its influence on local control allows to continue research in this direction. This primarily concerns the fractionation regimen with a prescribed dose of 3 Gy.

Challenges in commissioning the "TSET" technique: A new approach towards monitor unit calculation and beam profile measurements.

Total skin electron beam therapy, commonly known as TSET, is a good choice of treatment for patients suffering from mycosis fungoides. The aim of this study was to introduce a new approach to the beam profile measurement using diodes and to calculate the monitor units required for the TSET treatment by the use of a simple setup of output measurement. Dosimetric measurements required for the treatment were taken to establish the Stanford technique in the department, and the measured data was compared with the published data. High-energy Linear Accelerator Clinac-DHX, Varian medical system, Palo Alto, CA, was commissioned for TSET. The output of the machine was measured by the use of a Parallel-Plate Chamber (PPC40) as per the TRS 398 recommendation. Diode dosimeters (EDD2 and EDD5) were used for beam profile measurements due to easy setup and to reduce the measurement time. Homogeneous dose distribution within a field size of 80 cm x160 cm was observed with the variation of -5.0% on the horizontal axis and -5.4% on the vertical axis. The calculated monitor unit to deliver 200 cGy per fraction per field at the source to surface (SSD) of 416 cm was 489 MU. The technique described for the output measurements is simple and accurate. Results of the absorbed dose and MU measured were within good agreement compared to the published literature.

Dose Distribution of High Dose-Rate and Low Dose-Rate Prostate Brachytherapy at Different Intervals—Impact of a Hydrogel Spacer and Prostate Volume

The study aimed to compare the dose distribution in permanent low-dose-rate brachytherapy (LDR-BT) and high-dose-rate brachytherapy (HDR-BT), specifically focusing on the impact of a spacer and prostate volume. The relative dose distribution of 102 LDR-BT patients (prescription dose 145 Gy) at different intervals was compared with the dose distribution of 105 HDR-BT patients (232 HDR-BT fractions with prescription doses of 9 Gy, n = 151, or 11.5 Gy, n = 81). A hydrogel spacer (10 mL) was only injected before HDR-BT. For the analysis of dose coverage outside the prostate, a 5 mm margin was added to the prostate volume (PV+). Prostate V100 and D90 of HDR-BT and LDR-BT at different intervals were comparable. HDR-BT was characterized by a considerably more homogenous dose distribution and lower doses to the urethra. The minimum dose in 90% of PV+ was higher for larger prostates. As a consequence of the hydrogel spacer in HDR-BT patients, the intraoperative dose at the rectum was considerably lower, especially in smaller prostates. However, prostate volume dose coverage was not improved. The dosimetric results well explain clinical differences between these techniques reported in the literature review, specifically comparable tumor control, higher acute urinary toxicity rates in LDR-BT in comparison to HDR-BT, decreased rectal toxicity after spacer placement, and improved tumor control after HDR-BT in larger prostate volumes.

Dosimetric comparison of target volume coverage and sparing of organs at risk (OARs) between helical intensity modulated radiotherapy (H-IMRT) versus volumetric arc therapy (VMAT) for mid and lower thoracic oesophageal carcinoma.

469 Background: To evaluate the effectiveness of H-IMRT and VMAT in covering target volume while adequately sparing the OARs for patients with mid and distal oesophageal carcinoma, on the basis of dosimetric analysis. Methods: The target areas and organs at risk in 30 patients with locally advanced carcinoma oesophagus undergoing neo-adjuvant chemo-radiotherapy were specified and transmitted to Eclipse, Version 13.6, Varian Medical Systems) &amp; Accuray Treatment Planning System (Accuray Precision, Version 2.1.4, Accuray Medical System). Two plans (H-IMRT and VMAT) were generated for each patient for a total dose of 41.4 Gy delivered to the PTV in 23 fractions, and the Homogeneity Index (HI), Conformity Index (CI) and the dose distribution to the OARs (spinal cord, heart, lungs, kidneys and liver) were compared using dose volume histograms. Results: H-IMRT resulted in a more homogeneous dose distribution to the target (HI- 0.059) as compared to VMAT (HI- 0.07) [p-0.004]. The Conformity Indices showed no significant difference between the two techniques (H-IMRT – 0.989, VMAT – 0.987) [p-0.66]. VMAT resulted in a significantly less dose to the spinal cord (23.598 Gy vs .25.657 Gy) [p-0.021]. With VMAT plans, the heart mean dose (18.101 Gy vs. 20.031) [p- 0.00007], heart V40 (4.902% vs. 6.143) [p-0.002,] the averaged lung V20 [p-0.00005] and the mean dose to the left kidney (3.84 Gy vs. 4.721 Gy) [p- 0.010] were significantly less as compared to H-IMRT No statistically significant difference in both techniques with respect to heart V30, Mean Lung Dose (MLD), and mean dose to the righ and left kidney was observed Conclusions: VMAT proved to be better at sparing of OARs whilst providing almost the same Conformity as compared to a H-IMRT. H-IMRT has statistically better dose homogeneity but, it tends to deposit a slightly higher dose to the OARs. Whether the aforementioned differences in the dosimetric parameters translate into clinical benefits has to be evaluated by clinical outcome studies.

Dosimetric comparison of 3DCRT and IMRT in radical chemoradiotherapy of squamous cell carcinoma esophagus.

Radical chemoradiation is the standard of treatment for locally advanced squamous cell carcinoma of esophagus and for patients with operable disease, but who are medically unfit or unwilling for surgery. As the esophagus is a central organ, the planning target volume (PTV) is central, lies close to the spinal cord and heart, and is surrounded by the lung, which is a radiosensitive organ. Irradiation of these critical structures is reduced by the use of three-dimensional conformal radiation therapy (3DCRT). Intensity-modulated radiation therapy (IMRT) has the potential to improve the uniformity of dose distribution to the tumor and reduce the dose received by surrounding normal tissues. 1. To compare the dose distribution, conformity, and homogeneity indices in radical radiotherapy of squamous cell carcinoma of esophagus using 3DCRT and IMRT techniques 2. To compare the doses received by critical structures such as heart, lung, spinal cord, and liver. All cases of squamous cell carcinoma esophagus treated with radical chemoradiation to a dose of 50 Gy in 25 fractions using 3DCRT technique from January 2018 to July 2019 were included. IMRT plans were generated for these cases.The parameters that represent dose distribution to the target volume and the dose received by the organs at risk were obtained from the dose-volume histogram. The difference in the mean values of the parameters between the two techniques was calculated. The statistical significance of the difference was determined using Student's t-test and Wilcoxon signed-rank test. The volume of PTV receiving 105% and 107% of prescribed dose was significantly lower with IMRT (3.540% and 0.008%, respectively) compared to 3DCRT (7.654% and 0.623%). The homogeneity index was better with IMRT (0.088 vs. 0.107) than 3DCRT. Conformity index was found to be better with IMRT (1.149 vs. 1.573). Mean heart dose (18.216 vs. 24.591 Gy) and the volume of heart receiving 30 Gy were reduced with IMRT. The volume of lung receiving 20 Gy and the volume receiving 5 Gy were not significantly different between 3DCRT and IMRT. Maximum dose to spinal cord was similar with 3DCRT and IMRT. IMRT avoids areas of excessive irradiation within the PTV. IMRT improves dose conformity to the target volume and homogeneity of dose distribution within the PTV. The cardiac dose is significantly reduced with IMRT. The mean lung dose remains similar to 3DCRT. There is no significant increase in the volume of lung receiving low-dose radiation with IMRT.