Topology of six degrees of freedom magnetic bearing

A novel magnetic topology has been designed for a six degrees of freedom, magnetically levitated and driven mirror, to be used in a three dimensional (3D) measurement system based on laser interferometry. The translations of the mirror are to be kept small, whereas the rotations are to be controlled over a large range with a high bandwidth and high accuracy. Finite element modelling (FEM) is used to analyze the proposed topology. For computational load reduction, a 2D FEM model has been derived from the actual 3D topology, which incorporates most of the magnetic subsystems. Simulations show that cross-influence between the actuators is small, that the forces and torques are proportional to the applied currents and that the angle of the rotor is of little influence. This allows the multiple in multiple out system to be regarded as multiple linear single in single out systems. ©2000 American Institute of Physics

Intensity-Modulated Radiotherapy in Patients with Cervical Cancer. An intra-individual Comparison of Prone and Supine Positioning

<p>Abstract</p> <p>Background</p> <p>Chemoradiation for cervical cancer patients is associated with considerable gastrointestinal toxicity. Intensity-modulated radiotherapy (IMRT) has demonstrated superiority in terms of target coverage and normal tissue sparing in comparison to conventional 3D planning in gynaecological malignancies. Whether IMRT in prone (PP) or supine position (SP) might be beneficial for cervical cancer patients remains partially unanswered.</p> <p>Methods</p> <p>10 patients on FIGO stage IB-III cervical cancer, 6 patients for definitive and 4 patients for adjuvant external beam pelvic RT, were planned in PP and SP using a 7-field IMRT technique. IMRT plans for PP and SP (mean dose, D_mean50.4 Gy) were optimized in terms of PTV coverage (1^stpriority) and small bowel sparing (2^ndpriority). A comparison of DVH parameters for PTV, small bowel, bladder, and rectum was performed.</p> <p>Results</p> <p>The comparison showed a similar PTV coverage of 95% of the prescribed dose and for target conformity in IMRT plans (PP, SP). PTV, rectum and bladder volumes were comparable for PP and SP. Significantly larger volumes of small bowel were found in PP (436 cc, + 35%, p = 0.01). PP decreased the volume of small bowel at 20-50.4 Gy (p < 0.05) and increased the rectum volumes covered by doses from 10-40 Gy (p < 0.01), the V50.4 was < 5% in both treatment positions. Bladder sparing was significant better at 50.4 Gy (p = 0.03) for PP.</p> <p>Conclusion</p> <p>In this dosimetric study, we demonstrated that pelvic IMRT in prone position for patients with cervical cancer seems to be beneficial in reducing small bowel volume at doses ≥20 Gy while providing similar target coverage and target conformity. The use of frequent image guidance with KV (kilovolt) or MV (megavolt) computertomography can reduce set-up deviations, and treatment in prone position can be done with a higher set-up accuracy. Clinical outcome studies are needed to affirm lower toxicity.</p

Influence of Daily Set-up Errors on Dose Distribution during Pelvis Radiotherapy

Radioterapija megavoltnim snopovima fotona linearnog akceleratora oblik je lokalnog onkološkog liječenja. Njezin je cilj predati propisanu apsorbiranu dozu ciljnom volumenu, uz što nižu dozu okolnomu zdravom tkivu. Proces se sastoji od niza koraka u kojima sudjeluje tim stručnjaka iz različitih područja. Veliki broj postupaka i ljudi uključenih u radioterapijski proces, nepouzdanosti uređaja te pomaci bolesnika i unutrašnjih organa tijekom terapije mogu uzrokovati odstupanja između planirane i stvarne raspodjele doze. U ovom radu procijenili smo utjecaj pogreške položaja bolesnika tijekom terapije na raspodjelu doze, te na veličine koje služe za procjenu kvalitete plana. Pogreške u položaju bolesnika ispitane su u skupini od 35 bolesnika. Te smo pogreške simulirali na pet bolesnika, koristeći se 3D programom za planiranje XiO (CMS Inc., St. Louis, MO, Elekta). Ispitali smo razlike u raspodjeli doze između planiranog i stvarnog položaja. Dodatno, utjecaj pogrešaka na odabir plana zračenja provjerili smo analizirajući promjene u dozno-volumnom histogramu (DVH), pokrivenost planiranog volumena (engl. PTV conformity index, CIPTV) i jednoličnost raspodjele doze u ciljnom volumenu (engl. homogeneity index, HI). Simulacije pokazuju da promjene u položaju bolesnika mogu uzrokovati značajne razlike između planirane i stvarne raspodjele doze. Kod nekih bolesnika ako se ne korigiraju pogreške, CIPTV postaje manji od 97 % što znači da plan više nije prihvatljiv za zračenje. Iznenađuje da HI nije ovisan o pomacima bolesnika kao CIPTV. Rezultati pokazuju potrebu za smanjenjem pogrešaka, što se može postići provođenjem kontrole kvalitete u radioterapiji.An external beam radiotherapy (EBRT) using megavoltage beam of linear accelerator is usually the treatment of choice in cancer patients. The goal of EBRT is to deliver the prescribed dose to the target volume, with as low as possible dose to the surrounding healthy tissue. A large number of procedures and different professions involved in radiotherapy, uncertainty of equipment and daily patient set-up errors can cause a difference between the planned and delivered dose. We investigated a part of this difference caused by measuring daily patient set-up errors for 35 patients. These set-up errors were simulated on five patients, using 3D treatment planning software XiO. The simulation investigated differences in dose distributions between the planned and shifted geometry. Additionally, we investigated the influence of the error on treatment plan selection by analysing changes in dose volume histograms, planning target volume conformity index (CIPTV), and homogeneity index (HI). Simulations showed that patient daily set-up errors can cause significant differences between the planned and actual dose distributions. Moreover, for some patients, those errors could affect the choice of treatment plan since CIPTV fell under 97 %. Surprisingly, HI was not as sensitive to set-up errors as CIPTV. Our results have confirmed the need to minimise daily set-up errors through quality assurance programmes