Patient exposure dose in interventional cardiology per clinical and technical complexity levels. Part 1: results of the VERIDIC project

Background Patients can be exposed to high skin doses during complex interventional cardiology (IC) procedures. Purpose To identify which clinical and technical parameters affect patient exposure and peak skin dose (PSD) and to establish dose reference levels (DRL) per clinical complexity level in IC procedures. Material and Methods Validation and Estimation of Radiation skin Dose in Interventional Cardiology (VERIDIC) project analyzed prospectively collected patient data from eight European countries and 12 hospitals where percutaneous coronary intervention (PCI), chronic total occlusion PCI (CTO), and transcatheter aortic valve implantation (TAVI) procedures were performed. A total of 62 clinical complexity parameters and 31 technical parameters were collected, univariate regressions were performed to identify those parameters affecting patient exposure and define DRL accordingly. Results Patient exposure as well as clinical and technical parameters were collected for a total of 534 PCI, 219 CTO, and 209 TAVI. For PCI procedures, body mass index (BMI), number of stents ≥2, and total stent length >28 mm were the most prominent clinical parameters, which increased the PSD value. For CTO, these were total stent length >57 mm, BMI, and previous anterograde or retrograde technique that failed in the same session. For TAVI, these were male sex, BMI, and number of diseased vessels. DRL values for Kerma-area product ( PKA), air kerma at patient entrance reference point ( Ka,r), fluoroscopy time (FT), and PSD were stratified, respectively, for 14 clinical parameters in PCI, 10 in CTO, and four in TAVI. Conclusion Prior knowledge of the key factors influencing the PSD will help optimize patient radiation protection in IC.

Establishing a priori and a posteriori predictive models to assess patients’ peak skin dose in interventional cardiology. Part 2: results of the VERIDIC project

Background Optimizing patient exposure in interventional cardiology is key to avoid skin injuries. Purpose To establish predictive models of peak skin dose (PSD) during percutaneous coronary intervention (PCI), chronic total occlusion percutaneous coronary intervention (CTO), and transcatheter aortic valve implantation (TAVI) procedures. Material and Methods A total of 534 PCI, 219 CTO, and 209 TAVI were collected from 12 hospitals in eight European countries. Independent associations between PSD and clinical and technical dose determinants were examined for those procedures using multivariate statistical analysis. A priori and a posteriori predictive models were built using stepwise multiple linear regressions. A fourfold cross-validation was performed, and models’ performance was evaluated using the root mean square error (RMSE), mean absolute percentage error (MAPE), coefficient of determination (R²), and linear correlation coefficient (r). Results Multivariate analysis proved technical parameters to overweight clinical complexity indices with PSD mainly affected by fluoroscopy time, tube voltage, tube current, distance to detector, and tube angulation for PCI. For CTO, these were body mass index, tube voltage, and fluoroscopy contribution. For TAVI, these parameters were sex, fluoroscopy time, tube voltage, and cine acquisitions. When benchmarking the predictive models, the correlation coefficients were r = 0.45 for the a priori model and r = 0.89 for the a posteriori model for PCI. These were 0.44 and 0.67, respectively, for the CTO a priori and a posteriori models, and 0.58 and 0.74, respectively, for the TAVI a priori and a posteriori models. Conclusion A priori predictive models can help operators estimate the PSD before performing the intervention while a posteriori models are more accurate estimates and can be useful in the absence of skin dose mapping solutions

Clinically Delivered Treatment for Glioma Patients on Hybrid Magnetic Resonance Imaging (MRI)-Linear Accelerator (MR-Linac) versus Cone Beam CT (CBCT)-Guided Linac

Magnetic Resonance Imaging (MRI)-Linear Accelerator (MR-Linac) radiotherapy is an innovative technology that requires special consideration for secondary electron interactions within the magnetic field, which can alter dose deposition at air-tissue interfaces. Thirty-seven consecutive glioma patients had treatment planning completed and approved prior to radiotherapy initiation using commercial treatment planning systems (TPS): a Monte Carlo-based or convolution-based TPS for MR-Linac or Cone Beam CT (CBCT)-guided Linac, respectively. In vivo skin dose was measured using an Optically Stimulated Luminescent Dosimeter (OSLD) and correlated with TPS skin dose. We found that Monte Carlo-based MR-Linac plans and convolution-based CBCT-Linac plans had similar dosimetric parameters for target volumes and organs-at-risk. However, MR-Linac plans had 1.52 Gy higher mean dose to air cavities (p<0.0001) and 1.10 Gy higher mean dose to skin (p<0.0001). In vivo skin dose was 14.5% greater for MR-Linac (p=0.0027), and were more accurately predicted by Monte Carlo-based calculation (ρ=0.95, p<0.0001) vs. convolution-based (ρ=0.80, p=0.0096). This is the first prospective dosimetric comparison of glioma patients clinically treated on both MR-Linac and CBCT-guided Linac. Skin doses were significantly greater with MR-Linac and correlated with in vivo measurements. Future MR-Linac planning processes are being designed to account for skin dosimetry and treatment delivery.

Dosimetric Evaluation of Photons Versus Protons in Postmastectomy Planning for Ultrahypofractionated Breast Radiotherapy

Background: Ultrahypofractionation can shorten the irradiation period. This study is the first dosimetric investigation comparing ultrahypofractionation using volumetric arc radiation therapy (VMAT) and intensity-modulated proton radiation therapy (IMPT) techniques in postmastectomy treatment planning. Materials and methods: Twenty postmastectomy patients (10-left and 10-right sided) were replanned with both VMAT and IMPT techniques. There were 4 scenarios: left chest wall, left chest wall including regional nodes, right chest wall, and right chest wall including regional nodes. The prescribed dose was 26 Gy (RBE) in 5 fractions. For VMAT, a 1-cm bolus was added for 2 in 5 fractions. For IMPT, robust optimization was performed on the CTV structure with a 3-mm setup uncertainty and a 3.5% range uncertainty. This study aimed to compare the dosimetric parameters of the PTV, ipsilateral lung, contralateral lung, heart, skin, esophageal, and thyroid doses. Results: The PTV-D95 was kept above 24.7 Gy in both VMAT and IMPT plans. The ipsilateral lung mean dose of the IMPT plans was comparable to that of the VMAT plans. In three of four scenarios, the V5 of the ipsilateral lung in IMPT plans was lower than in VMAT plans. The Dmean and V5 of heart dose were reduced by a factor of 4 in the IMPT plans of the left side. For the right side, the Dmean of the heart was less than 1 Gy for IMPT, while the VMAT delivered approximately 3 Gy. The IMPT plans showed a significantly higher skin dose owing to the lack of a skin-sparing effect in the proton beam. The IMPT plans provided lower esophageal and thyroid mean dose. Conclusion: Despite the higher skin dose with the proton plan, IMPT significantly reduced the dose to adjacent organs at risk, which might translate into the reduction of late toxicities when compared with the photon plan. Key words: proton therapy, ultrahypofractionation, postmastectomy, breast irradiation

Skin dose contamination conversion coefficients. Benchmark with three simulation codes

Handling of radioactive material by operators can lead to contamination at the surface of the skin in case of an accident. The quantification of the dose received by the skin due to a contamination scenario is performed by means of dedicated dose coefficients as it is the case for other radiation protection dose quantities described in the literature. However, most available coefficients do not match realistic scenarios according to state-of-the-art of science and technology. Therefore, this work deals with dedicated dose conversion factors for skin contamination. Since there is an increasing demand on dose coefficients in general, these specific coefficients can be used for various calculations in radiation protection. In this work a method to evaluate such coefficients for the skin contamination dose related to photons, electrons, positrons, alpha and neutron particles is proposed. The coefficients are generated using Monte-Carlo simulations with three well established calculation codes (FLUKA, MCNP, and GEANT4). The results of the various codes are compared against each other for benchmarking purposes. The new dose coefficients allow the computation of the skin received dose, in the case of skin contamination scenario of an individual, taking into account the decay radiation of the radionuclides of interest. To benchmark the quantity derived here, comparisons of radionuclide contamination doses to the skin using the VARSKIN code available in the literature are performed with the results of this work.

Dosimetric Evaluation of a Set of Specifically Designed Grids for Treating Subcutaneous Superficial Tumor with 6 and 18 MeV Electron Beam External Radiotherapy

Background: Conventional electron beam radiotherapy used for treating superficial cancer tumors suffers from the disadvantage of low skin sparing effect. Furthermore, increasing electron energy for treating deeper-seated tumors leads to significant increase of skin dose. To overcome this, various grids are recommended for electron beam radiotherapy of subcutaneous tumors. However, appropriate grids are required to be designed for decreasing skin dose while delivering uniform high doses to deep-seated superficial tumors. Our goal was to design, examine and propose appropriate grid(s) for optimum electron beam radiotherapy of subcutaneous tumors with the best skin sparing with 6 and 18 MeV energies.Materials and Methods: Relevant dosimetric characteristics were determined and analyzed for five grids manufactured from dry lead having various cavity diameters (1.5, 2.0, 2.5, 3.0, 3.5 cm) and shielded areas (0.3, 0.4, 0.5, 0.6, 0.7 cm) among the cavities but the same fraction of cavity/open (68%) and shielded/closed (38%) areas under the grid plates. Isodose distributions and dose profiles resulted from the grids were investigated using EDR2 films and MATLAB software. Results: The grids with 2 and 2.5 cm diameter cavities and 0.4 and 0.5 cm shielded areas were the most appropriate grids for 6 and 18 MeV radiotherapy, respectively. With these grids, the 100% PDDs (percentage depth doses) located at 1.25 and 2.5 cm for an open filed (without the grids) were moved down to 1.87 and 5.4 cm for 6 and 18 MeV energies, respectively. Furthermore, the proposed grids provided the least peak to valley dose variations hence the most uniform doses delivered at their relevant depths of treatment. Conclusions: To decrease the skin dose in 6 and 18 MeV electron beam radiotherapy of superficial subcutaneous tumors, various home-made grids were designed and investigated. The most appropriate grids (having 2 and 2.5 cm cavity diameters for 6 and 18 MeV, respectively) provided the optimum dose delivery for superficial subcutaneous tumors locating around 1.5 and 5 cm depth for 6 and 18 MeV energies. Our comprehensive study provides reliable results that could be considered and developed more for a wider range of MeV electron grid therapies in routine clinical practices.

CTNI-44. DOSIMETRIC IMPACT OF TUMOR TREATING FIELDS ON CONCURRENT RADIATION THERAPY FOR PEDIATRIC BRAIN TUMORS

INTRODUCTION Early results suggest that tumor treating fields (TTF) concurrent with radiotherapy (RT) for glioblastoma yields acceptable dosimetry in adults; the impact on RT dose distribution in children is unknown. This study was undertaken to evaluate the dosimetric impact of TTF on concurrent photon RT for children with brain tumors. METHODS CT scans of an anthropomorphic pediatric head phantom (approximately 15-year-old) and an infant-head-sized spherical phantom were acquired with and without TTF attached. For each phantom, simulated supratentorial tumor targets were initially contoured on CT datasets acquired without TTF attached. Treatment plans using volumetric modulated arc therapy were created to deliver 60Gy and 50Gy to the gross tumor volume (GTV) and clinical tumor volume (CTV), respectively, in 30 fractions. The dose distributions of the same treatment plans were then re-computed with TTF attached. Target coverage metrics were compared between dose distributions with and without TTF. To measure skin dose, treatment plans were delivered with thermoluminescent dosimeters placed on the phantoms at various locations, with and without TTF attached. RESULTS The presence of TTF slightly reduced target coverage. For the two phantoms studied, D95 of the CTV was reduced by 0.65% and 1.03%, and D95 of the GTV was reduced by 0.7% and 1.05%, respectively. Electrodes under the direct beam path increased skin dose by an average of 43.3% (0.3Gy – 20.7Gy), but all skin dose measurements stayed within tolerances. TTF electrodes out of the RT field did not cause an increase in measured dose. CONCLUSIONS The dosimetric impact of TTF on pediatric head phantoms receiving concurrent RT resembles that reported in adult studies. Although the tumor dose is not significantly affected, the skin dose notably increases due to the bolus effect from the TTF electrodes, which may be mitigated by skin-sparing planning and shifting of the device during RT.

Patient doses in endovascular and hybrid revascularization of aortoiliac segment

Objectives: Constantly increasing number of procedures performed – endovascular or hybrid in patients with aortoiliac occlusive disease during the last decades finds its explanation in the lower morbidity and mortality rates, compared to bypass surgery. The purpose of the current survey was to estimate patients’ radiation exposure in aortoiliac segment after endovascular or hybrid revascularization and to study the main factors which have direct contribution. Methods: A retrospective study of 285 procedures conducted with the help of a mobile C-arm system in 223 patients was performed. Procedures were grouped according to criteria such as: type of intervention, vascular access, level of complexity and operating team. Different analyses were performed within the groups and dose values. Results: The median values of kerma–air product (KAP), the number of series and the peak skin dose (PSD) significantly increase with the increasing number of vascular accesses: for one access (16.68 Gy.cm2, 6 and 336 mGy), for two (56.93 Gy.cm2, 11 and 545 mGy), and for three (102.28 Gy.cm2, 15 and 781 mGy). Significant dependence was observed in the case of single access site between the type of access and the dose values: hybrid and retrograde common femoral artery/superficial femoral artery (CFA/SFA) endovascular accesses, 10.06 Gy.cm2/301 mGy and 13.23 Gy.cm2/318 mGy respectively, in contrast with the contralateral CFA and left brachial access, 33 Gy.cm2/421 mGy and 38.33 Gy.cm2/448 mGy respectively. Conclusion: The results demonstrate that the most important factors increasing the dose values are number and type of vascular accesses, followed by the combination and number of implanted stents with the complexity of the procedure. The PSD values for a single procedure were between 2 and 12 times lower than those IAEA proposed as trigger levels for radiation-induced erythema. This study shows that trigger levels were not reached even for patients with repeated procedures in the same segment in 1-year period. Advances in knowledge: The study gives important understanding and clarity on the growing awareness for dose-modifying factors during endovascular and hybrid revascularization of aortoiliac segment.