Hybrid-Vlasov modelling of three-dimensional dayside magnetopause reconnection

<p>Dayside magnetic reconnection at the magnetopause, which is a major driver of space weather, is studied for the first time in a three-dimensional (3D) realistic setup using the Vlasiator hybrid-Vlasov kinetic model. A noon–midnight meridional plane simulation is extended in the dawn–dusk direction to cover 7 Earth radii. The southward interplanetary magnetic field causes magnetic reconnection to occur at the subsolar magnetopause. Perturbations arising from kinetic instabilities in the magnetosheath appear to modulate the reconnection. Its characteristics are consistent with multiple, bursty, and patchy magnetopause reconnection. It is shown that the kinetic behavior of the plasma, as simulated by the model, has consequences on the applicability of methods such as the four-field junction to identify and analyse magnetic reconnection in 3D kinetic simulations.</p>

Mono versus dual isocentric technique for breast cancer radiotherapy: evaluation of planning, dosimetry and treatment delivery

Aim: To compare the dosimetry and reproducibility of set-up with monoisocentric technique (MIT) and dual isocentric technique (DIT) in adjuvant breast radiotherapy (RT). Material and methods: Breast cancer patients treated with MIT or DIT were retrospectively studied. The organ-at-risk dose was compared between two groups. All patients underwent set-up verification with an electronic portal imaging device, and set-up time was recorded for each fraction. Treatment reproducibility was assessed in terms of systematic and random error. Results: Twenty patients were included (11 right and 9 left-sided tumours) and ten received whole breast RT, while the rest received chest wall RT. Overall, the mean heart dose was less with MIT (0.40 versus 0.79, p = <0.001) as well as in left-sided tumours (0.37 versus 0.98, p = 0.003). The maximum dose at the field junction was significantly higher with DIT (43 Gy, 107%, p = 0.003). The maximum total error was 1 cm in lateral for supraclavicular field and 8 mm in superior–inferior in tangents for both techniques. There was no difference in set-up errors between the two techniques. Findings: MIT resulted in better dose homogeneity at the field junctions and reduced mean heart dose as compared to DIT. MIT is safe for implementation in clinical practice for breast cancer treatment. Conclusion: This study is one of the few studies comparing MIT with DIT in terms of the dosimetry and the first one to compare set-up errors between the two techniques. The ease of set-up and better dosimetry with MIT was achieved.

Field Junction Technique for Helical Tomotherapy-Based Total Body Irradiation

Purpose: Combination of total body irradiation (TBI) with chemotherapy is widely used technique for conditioning before hematopoietic stem cell transplantation for patient with hematological malignancies worldwide. Total body irradiation for patients with high height has to be divided into two parts: irradiation of upper part of the patient’s body (including head, body and part of legs) and irradiation of lower part of the patient’s body (including leg). There is an area in which the fields overlap each other – the junction area. The aim of this work is the development and verification of simple junction technique that would provide the dose distribution in the junction area from 90 to 125 % of prescribed dose. Material and methods: Total body irradiation was performed on the Tomotherapy machine using helical geometry of the beam delivery. Distribution of the dose in junction area was investigated. Simple solution was proposed: during the optimization of the radiotherapy plan certain margin should be maintained between upper and lower targets while dose distribution in junction area satisfies the uniformity requirements for the given irradiation geometry. The dimension of the margin was determined experimentally using a CheesePhantom and radiochromic EBT-2 films. The uniformity of dose distribution in the junction area was monitored by in vivo measurements using radiochromic EBT-2 films located on the skin surface of patients. Results: The dimension of the margin at which the dose in the junction area is within the range of 90 to 125 % of the prescribed dose was determined experimentally and amounted to 5.25 cm. The values of the measured dose were in the range from 97 to 105 %. In total 18 in vivo measurements of the junction area were performed. According to the results of in vivo dosimetry, the values of the doses measured in the junction area were in the range from 93 ± 3 % to 108 ± 4 %. Conclusion:The developed planning method with the selected plan geometry ensures satisfactory heterogeneity of the dose distribution in the area of field junction between the upper and lower irradiation regions, despite of the existing uncertainty of patient positioning. Results were confirmed by in vivo measurements. The obtained data can be used for total body irradiation of the patients using Helical Tomotherapy.