Flattening filter free beams in SBRT and IMRT: dosimetric assessment of peripheral doses

Purpose

Recently, there has been a growing interest in operating medical linear accelerators without a flattening filter. Due to reduced scatter, leaf transmission and radiation head leakage a reduction of out-of-field dose is expected for flattening filter free beams. The aim of the present study was to determine the impact of unflattened beams on peripheral dose for advanced treatment techniques with a large number of MUs.

Material and methods

An Elekta Precise linac was modified to provide 6 and 10 MV photon beams without a flattening filter. Basic beam data were collected and implemented into the TPS Oncentra Masterplan (Nucletron). Leakage radiation, which predominantly contributes to peripheral dose at larger distances from the field edge, was measured using a Farmer type ionisation chamber. SBRT (lung) and IMRT (prostate, head&neck) treatment plans were generated for 6 and 10 MV for both flattened and unflattened beams. All treatment plans were delivered to the relevant anatomic region of an anthropomorphic phantom which was extended by a solid water slab phantom. Dosimetric measurements were performed with TLD-700 rods, radiochromic films and a Farmer type ionisation chamber. The detectors were placed within the slab phantom and positioned along the isocentric longitudinal axis.

Results

Using unflattened beams results in a reduction of treatment head leakage by 52% for 6 and 65% for 10 MV. Thus, peripheral doses were in general smaller for treatment plans calculated with unflattened beams. At about 20 cm distance from the field edge the dose was on average reduced by 23 and 31% for the 6 and 10 MV SBRT plans. For the IMRT plans (10 MV) the average reduction was 16% for the prostate and 18% for the head&neck case, respectively. For all examined cases, the relative deviation between peripheral doses of flattened and unflattened beams was found to increase with increasing distance from the field.

Conclusions

Removing the flattening filter lead to reduced peripheral doses for advanced treatment techniques. The relative difference between peripheral doses of flattened and unflattened beams was more pronounced when the nominal beam energy was increased. Patients may benefit by decreased exposure of normal tissue to scattered dose outside the field.     

Intensity modulated radiotherapy (IMRT) with compensators

The irradiation with intensity-modulated fields is possible with static as well as dynamic methods. In our university hospital, the intensity-modulated radiotherapy (IMRT) with compensators was prepared and used for the first time for patient irradiation in July 2001. The compensators consist of a mixture of tin granulate and wax, which is filled in a milled negative mould. The treatment planning is performed with Helax-TMS (MDS Nordion). An additional software is used for editing the modulation matrix ("Modifix"). Before irradiation of the first patient, extensive measurements have been carried out in terms of quality assurance of treatment planning and production of compensators. The results of the verification measurements have shown that IMRT with compensators possesses high spatial and dosimetric exactness. The calculated dose distributions are applied correctly. The accuracy of the calculated monitor units is normally better than 3%; in small volumes, further dosimetric inaccuracies between calculated and measured dose distributions are mostly less than 3%. Therefore, the compensators contribute to the achievement of high-level IMRT even when apparatuses without MLC are used. This paper describes the use of the IMRT with compensators, presents the limits of this technology, and discusses the first practical experiences.     

煤炭科技期刊文献引文分析的思考

对44种煤炭科技期刊进行了引文分析,总结了煤炭科技工作者利用文献的现状与特点,对煤炭科技文献服务工作的改进与提高提出了建议。     

HBsAg定量检测在慢性乙型肝炎抗病毒治疗监测中的意义

迄今为止全世界大约有3.5亿-4.0亿慢性HBV感染者,约占全球人口总数1/3的人血清学证据表明既往或正在感染HBV.每年有超过100万的患者死于终末期肝病或肝细胞癌等慢性乙型肝炎相关疾病~[1].近年来美国肝病研究学会(AASLD 2008)、亚太肝脏研究学会(APASL 2008)和欧洲肝脏研究学会(EASL 2009)的指南对于抗病毒治疗的观点大多一致,即现有的抗病毒治疗方法还无法彻底清除或"治愈"HBV感染,当前抗病毒治疗的目标是预防或逆转肝病进展引起的肝硬化、肝衰竭和肝细胞癌(HCC)等并发症,降低病死率,减少肝移植.在开始抗病毒治疗前要综合考虑患者年龄、治疗疗程,应用肝活检或非侵入性检查以明确病情,治疗疗程最理想的是达到.HBsAg血清学转换,其次是达到HBeAg血清学转换和维持低水平的病毒载量.     

ITMAIS在听障婴幼儿早期干预中的临床应用

随着我国卫生部第64号令——《新生儿疾病筛查管理办法》的颁布实施.普遍新生儿听力筛查(universal newborn hearing screening,UNHS)在我国开展得越来越广泛,这必将促进婴幼儿听力问题的早发现、早诊断、早干预(“三早”).众所周知,听障婴幼儿的成功康复需要定期随访听觉言语发展情况、制订科学的个别化康复计划、有效评估听觉康复效果并根据评估结果进一步指导康复[1].     

Zur Wirksamkeit verschiedener Oxime auf die Rückbildung der Miosis nach lokaler Applikation von Cholinesterase-Inhibitoren

Zusammenfassung An 119 freiwilligen augengesunden männlichen und weiblichen Versuchspersonen wurde die Wirkung der Cholinesterasereaktivatoren Pralidoxim (PAM), Trimedoxim (TMB-4) und Dimedoxodoxim (Lü H 6; S 100) auf eine paraoxonbedingte Miosis untersucht.Während sich in einer prophylaktischer Anwendung entsprechenden Versuchsanordnung Trimedoxim am wirksamsten erwies, zeigte sich Dimedoxodoxim bei therapeutischem Einsatz den anderen Oximen überlegen.Da bei dem Einsatz von Cholinesterasereaktivatoren vorwiegend therapeutische Aspekte im Vordergrund des Interesses stehen, ist Dimedoxodoxim als Mittel der Wahl zur Durchbrechung einer Miosis nach Cholinesterasehemmstoffen anzusehen.

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Summary In 119 male and female voluntaries the effect of cholinesterase reactivators Pralidoxime, Trimedoxime and Dimedoxodoxime was investigated on the paraoxon induced miosis. In prophylactic conditions Trimedoxime was most effective. Dimedoxodoxime was found best in the mode of therapeutic application. In combination with premedication of Tropicamide Trimedoxime showed in both test series (therapeutically and prophylactically) most intensive decrease of miosis.With regard to the therapeutic administration for conteracting the local effects of cholinesterase inhibitors Dimedoxodoxime is considered as a suitable drug.

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Wir danken Herrn Dr. R. Schmiedel für die Herstellung von Dimedoxodoxim, Herrn Dr. Carstens (VEB Arzneimittelwerk Dresden) für die freundliche Überlassung von Trimedoxim und Frau G. Teichgräber für ihre gewissenhafte Mitarbeit bei der Durchführung der Versuche. Ebenso sind wir all denen zu Dank verpflichtet, die sich für die Untersuchungen freiwillig zur Verfügung stellten.     

Improvement of the accuracy of the Laplace transform method for the determination of radiotherapy spectra of clinical linear accelerators

The present study focused on the reconstruction of the bremsstrahlung spectrum of a clinical linear accelerator from the measured transmission curve, with the aim of improving the accuracy of this method. The essence of the method was the analytic inverse Laplace transform of a parameter function fitted to the measured transmission curve. We tested known fitting functions, however they resulted in considerable fitting inaccuracy, leading to inaccuracies of the bremsstrahlung spectrum. In order to minimise the fitting errors, we employed a linear combination of n equations with 2n-1 parameters. The fitting errors are now considerably smaller. The measurement of the transmission function requires that the energy-dependent detector response is taken into account. We analysed the underlying physical context and developed a function that corrects for the energy-dependent detector response. The factors of this function were experimentally determined or calculated from tabulated values.     

一种用于筛选降血糖及降血脂药物的动物模型

A new animal medel of hypergiycema and hyperlidemia was establislied bytreating normal Kunming mice with alloxan(iv);After different time intervals ,the levels of bloodglucose(BG)and serum triglyceride(TG)were determined.Forty-eight h after alloxanadministration ,the BG increased significantly,and the level of serum triglyceride reached maxium。The levels of blood glucose,serum triglyceride,cholesterol(CHOL )and free fatty acid(FFA )forvehicle treated mice were about 3.1,4.0,1.1,and 1. 7 times those of normal Kunming mice, respec-tively,To evaluate the new animal model, four drugs were use Two of them were antihyperglyce-mic drugs,metformin and a Recipe of Chinese Herbs(RC The other two were antihyperlipldemicagents ,clofibrate and fenofibrate。All drugs showed positlve effects on this kind of alloxan-diabeticKunming mice.It can be concluded that this kind of alloxan-treated Kunming mice is useful for testinghypoglycemic and hypolipidemic drugs。