Dosimetric Characteristics of the Siemens IGRT Carbon Fiber Tabletop

In this work, the dosimetric characteristics of a new commercial carbon fiber treatment table are investigated. The photon beam attenuation properties of the Siemens image-guided radiation therapy (IGRT) tabletop were studied in detail. Two sets of dosimetric measurements were performed. In the first experiment a polystyrene slab phantom was used: the central axis attenuation and the skin-sparing detriment were investigated. In the second experiment, the off-axis treatment table transmission was investigated using a polystyrene cylindrical phantom. Measurements were taken at the isocenter for a 360° rotation of the radiation beam. Our results show that the photon beam attenuation of the Siemens IGRT carbon fiber tabletop varies from a minimum of 2.1% (central axis) to a maximum of 4.6% (120° and 240° beam incidence). The beam entrance dose increases from 82% to 97% of the dose at the depth of maximum for a clinical 6-MV radiation field. The depth of maximum also decreases by 0.4 cm. Despite the wedge cross section of the table the beam attenuation properties of the IGRT tabletop remain constant along the longitudinal direction. American Association of Medical Dosimetrists.     

A surveillance study of patterns of reirradiation practice using external beam radiotherapy in Japan

The aim of this study was to survey the present status and patterns of reirradiation (Re-RT) practice using external beam radiotherapy in Japan. We distributed an e-mail questionnaire to the Japanese Society for Radiation Oncology partner institutions, which consisted of part 1 (number of Re-RT cases in 2008–2012 and 2013–2018) and part 2 (indications and treatment planning for Re-RT and eight case scenarios). Of the 85 institutions that replied to part 1, 75 (88%) performed Re-RTs. However, 59 of these 75 institutions (79%) reported difficulty in obtaining Re-RT case information from their databases. The responses from 37 institutions included the number of Re-RT cases, which totaled 508 in the period from 2009 to 2013 (institution median 3; 0–235), and an increase to 762 cases in the period from 2014 to 2018 (12.5; 0–295). A total of 47 physicians responded to part 2 of the survey. Important indications for Re-RT that were considered were age, performance status, life expectancy, absence of distant metastases and time interval since previous radiotherapy. In addition to clinical decision-making factors, previous total radiation dose, volume of irradiated tissue and the biologically equivalent dose were considered during Re-RT planning. From the eight site-specific scenarios presented to the respondents, >60% of radiation oncologists agreed to perform Re-RT. Re-RT cases have increased in number, and interest in Re-RT among radiation oncologists has increased recently due to advances in technology. However, several problems exist that emphasize the need for consensus building and the establishment of guidelines for practice and prospective evaluation.     

Modeling positioning uncertainties of prostate cancer external beam radiation therapy using pre-treatment data

Purpose

To investigate the influence of treatment plan data and image guidance (IG) on positioning uncertainty during prostate cancer (PCa) radiotherapy (RT).

Methods

Body mass index (BMI), planning target volume (PTV), bladder volume (BV), and rectal cross section area (RCS) were collected for 267 consecutive PCa patients undergoing daily IGRT. Radiographic isocenter corrections to intra-prostatic fiducials for 12,490 treatment fractions were used to derive random (RE) and systematic (SE) inter-fraction uncertainties for the cardinal axes. These data were used to simulate RE and SE for weekly IG and Action Level (AL)-IG treatment protocols.

Results

SE and RE were 2–5 and 3–4 mm in the cardinal axes, respectively, during simulation of no IG. Without IG, positive correlations (p < 0.01) were noted for (1) anterior-posterior RE vs. RCS and BV and (2) cranio–caudal RE vs. RCS, BV and BMI. The RE increase was 3 mm for the highest quartile of RCS, BV and BMI. Daily IGRT eliminated this relationship. 3D IG corrections of 1 cm or more occured in 27% of treatment fractions and in 97% of patients.

Conclusion

PCa patients with elevated pre-treatment BV, RCS and BMI have increased inter-fractionation positioning uncertainty and appear the primary candidates for daily IGRT.     

Image guided radiotherapy: Current status of soft tissue imaging

Image guided radiotherapy has evolved from two dimensional (2D) megavoltage imaging, which allowed verification with respect to bony anatomy, to three dimensional (3D) kilovoltage imaging which enables soft tissue structures to be used for verification. Alongside the technological developments, treatment delivery techniques have become more sophisticated and the potential to adapt treatment delivery to changes in tumour and/or organs at risk is increasing. This review explores the current status of soft tissue imaging techniques in conjunction with the potential clinical impact. The common tumour sites where the new treatment techniques are being investigated are identified and it is seen that to support the implementation of these techniques, investment in capital equipment and staff training is essential.     

Intrafraction monitoring of prostate motion during radiotherapy using the Clarity® Autoscan Transperineal Ultrasound (TPUS) system

IntroductionImplementation of the Clarity^® Autoscan (Elekta) Transperineal Ultrasound (TPUS) system in Bristol is the first of its kind in the UK and we have already shown its utility in interfractional Image Guided Radiotherapy (IGRT).¹⁴ This study establishes the extent of intrafraction prostate motion as measured by Clarity and explores the potential benefits of TPUS for intrafraction monitoring.MethodsMonitoring data was analysed for 526 fractions from 20 localised prostate cancer patients. Intrafraction prostate displacements exceeding thresholds of 3 mm, 7 mm and 10 mm along patient axes were assessed for frequency and duration of motion.ResultsProstate motion exceeds the above displacement thresholds during 52%, 8%, and 2% of fractions analysed. Displacement at the 3 mm threshold occurred for 100% of patients, 60% at 7 mm and 35% at 10 mm. The mean frequency and duration of displacements is low for the overall population. In contrast specific patients exhibit much higher displacement values. Posterior motion is most common, averaging at 24% of the treatment time at 3 mm, 3% at 7 mm and 1% at 10 mm, ranging up to 92%, 35% and 10% for individual patients.ConclusionsIntrafraction monitoring with Clarity has the potential to improve accuracy through application of in-treatment motion correction. This is most beneficial for specific patients who exhibit a higher frequency and/or duration of prostate motion. Consideration must be given to the added time implications and radiographer workload in clinical practice to correct for prostate motion. Clarity could help facilitate future protocols using tighter treatment margins, although further research is required.     

Évaluation et choix des protocoles d’imagerie sur le système de tomographie conique de basse énergie XVI® d’Elekta

PurposeThis work proposes an evaluation of the Elekta XVI^® kilovoltage cone-beam computed tomography imaging system. The average dose delivered for each acquisition protocol proposed by default by the manufacturer was measured with several detectors and compared to theoretical dose values given by Elekta. At the same time, an evaluation of image quality for pelvic protocols correlated to dose measurements in homogeneous and heterogeneous mediums allowed to optimize the use of the XVI^® system.Materials and methodsThe dose was measured for each acquisition protocol (varying filters, FOV and collimations) with four detectors (CT pencil ion chamber, 0.3 and 0.125 cm³ cylindrical ion chambers, radiothermoluminescent dosimeters) in a CTDI phantom. The dose evaluation in a heterogeneous medium was performed in an experimental anthropomorphic phantom simulating a male pelvis. Image quality was assessed with a Catphan^® 600 phantom.ResultsThe average dose measured in a homogeneous medium was about 17 mGy and 25 mGy per acquisition for Pelvis and Prostate protocols and about 17 mGy and 1 mGy for Lung and Head protocols. The study performed with different detectors showed that doses obtained were of the same order of magnitude (± 10%) and agreed with those supplied by the manufacturer. The evaluation of image quality correlated to the average dose measured allowed to optimize the use of XVI^® acquisition protocols. Measurement results in a heterogeneous medium showed a dose decrease by a factor 1.5 for bone and by a factor 2 for titanium.ConclusionThe study showed that theoretical values proposed by the manufacturer could be used to estimate the average dose delivered to the patient by the kV-CBCT imaging system. The analysis of all the results led to the implementation of a procedure allowing to optimize and account for the dose delivered to the patient by the CBCT imaging system and to report it in the patient folder.     

锥形束CT的常规检测方法和质量保证

目的探讨图像引导放疗中锥形束CT的常规检测方法和质量保证(QA)。方法从加速器中锥形束CT的相关项目验收程序入手,对锥形束CT系统进行日检、周检、月检,完成锥形束CT的系统安全、机械精度、图像质量等项目的质量保证。结果锥形束CT的系统安全性能正常,机械精度和图像质量的误差均在允许范围。锥形束CT的等中心与加速器机架等中心的误差为0.3 mm,SAD检测结果为85.2 cm,均在允许的误差范围内。锥形束CT在OBI模式下2D图像的低对比度分辨率和空间分辨率均合格,CBCT模式下CT图像空间分辨率在Ful-Fan和Half-Fan扫描模式下分别为7 lp/cm和6 lp/cm,低对比度分辨率合格,HU值的线性和一致性均合格,几何失真度小于1 mm,CT图像质量检查均为正常。结论锥形束CT的常规检测和质量保证可确保锥形束CT系统安全、精确。     

螺旋断层放射治疗胰腺癌摆位误差分析

目的分析在兆伏级电子计算机断层扫描(MVCT)图像引导下采用螺旋断层调强放疗技术治疗胰腺癌患者的摆位误差,并计算计划靶区(PTV)的边界(margin)外放。方法行MVCT图像扫描,将扫描获取的MVCT图像与计划CT图像进行配准,记录左右(X)、头脚(Y)、腹背(Z)轴方向和横断面旋转(roll)方向的误差值,并对误差值进行分析计算。结果共行592次MVCT扫描。X、Y、Z和roll方向摆位误差值分别为(-0.5±2.8)mm、(-1.1±6.4)mm、(6.0±4.4)mm和(-0.2±0.7)°。X、Y和Z 3个方向上的平均误差小于5 mm,所占比例分别为97.13%(575/592)、88.01%(521/592)和37.84%(224/592),roll方向旋转误差小于1°,所占比例为93.92%(556/592)。根据公式得出在X、Y和Z 3个方向PTV的边界外放间距值分别为5.2、9.9和7.5 mm。结论对MVCT图像引导下行螺旋断层放射治疗胰腺癌患者的摆位误差数据分析,为提高放射治疗精度及疗效,推荐胰腺癌放疗在X、Y和Z三维方向上CTV到PTV的margin外放值分别为5、10和8 mm。