Improved human observer performance in digital reconstructed radiograph verification in head and neck cancer radiotherapy.

Purpose: Digitally reconstructed radiographs (DRRs) are routinely used as an a priori reference for setup correction in radiotherapy. The spatial resolution of DRRs may be improved to reduce setup error in fractionated radiotherapy treatment protocols. The influence of finer CT slice thickness reconstruction (STR) and resultant increased resolution DRRs on physician setup accuracy was prospectively evaluated. Methods: Four head and neck patient CT-simulation images were acquired and used to create DRR cohorts by varying STRs at 0.5, 1, 2, 2.5, and 3 mm. DRRs were displaced relative to a fixed isocenter using 0–5 mm random shifts in the three cardinal axes. Physician observers reviewed DRRs of varying STRs and displacements and then aligned reference and test DRRs replicating daily KV imaging workflow. A total of 1,064 images were reviewed by four blinded physicians. Observer errors were analyzed using nonparametric statistics (Friedman’s test) to determine whether STR cohorts had detectably different displacement profiles. Post hoc bootstrap resampling was applied to evaluate potential generalizability. Results: The observer-based trial revealed a statistically significant difference between cohort means for observer displacement vector error (p = 0.02) and for Z-axis (p < 0.01). Bootstrap analysis suggests a 15% gain in isocenter translational setup error with reduction of STR from 3 mm to ≤2 mm, though interobserver variance was a larger feature than STR-associated measurement variance. Conclusions: Higher resolution DRRs generated using finer CT scan STR resulted in improved observer performance at shift detection and could decrease operator-dependent geometric error. Ideally, CT STRs ≤2 mm should be utilized for DRR generation in the head and break neck

Magneettikuvaukseen perustuvan sädehoidon suunnittelun käyttöönotto lantion alueella

Modern radiation therapy delivery techniques enable ever conformal delivery of the radiation increasing the likelihood for successful treatment and reducing complications in nearby healthy tissue. In order to improve the treatment outcomes, in addition to advanced radiation delivery techniques, more accurate knowledge about the location and spread of both disease and organs at risk (OAR) is needed. Thus, the use of magnetic resonance imaging (MRI) has increased substantially during recent years. In MRI, the contrast resolution for soft tissue is superior compared to other imaging modalities enabling precise target definition and contouring of the OARs. Currently, the use of MRI in radiation therapy is based on co-registration of the images facilitating the use of the information provided by MRI while computed tomography (CT) is used for dose computation and as a reference image for patient positioning. Unfortunately, the dual modality workflow is laborious and cost inefficient. In addition, the co-registration uncertainty propagates to treatment uncertainty causing systematic error. During recent years several research groups have published methods enabling the generation of so-called synthetic CT (sCT). It can be used like traditional CT for density information in dose computation and as positioning reference images. The use of sCT enables external beam radiation therapy workflow using only MR imaging. In this work we studied the commissioning and accuracy of MRI-only workflow for external beam radiation therapy (EBRT) of pelvic malignancies. The commissioning test shall cover all steps in the radiation therapy workflow where geometric or dosimetric accuracy is affected by the substitution of the CT by the sCT. In publications I and III, we assessed the dosimetric accuracy of sCT images in pelvis by comparing to dose distributions computed using CT images. In publications II and III, we studied the patient positioning accuracy when sCT images are used as reference images. In addition, in publications I and III we evaluated the impact of geometric distortions to the total accuracy of MRI-only workflow. According to our results, the use of studied sCT method is sufficiently accurate for clinical use for pelvic indications. In addition, image-guided radiation therapy based on MR images is accurate enough so that the total geometric accuracy improves compared to current CT based work-flow.Modernit sädehoitotekniikat mahdollistavat yhä tarkemman kohteenmukaisen sädehoidon antamisen, mikä lisää hoidon onnistumisen todennäköisyyttä ja vähentää komplikaatioita ympäröivissä terveissä kudoksissa. Parempiin hoitotuloksiin pääsemiseksi sädehoidossa tarvitaan kuitenkin, kehittyneiden hoitotekniikoiden lisäksi, yhä tarkempaa tietoa hoitokohteen ja riskielinten sijainnista. Tämän takia ionisoimattoman säteilyn käyttöön perustuvan magneettikuvauksen (MK) käyttö on lisääntynyt voimakkaasti sädehoidossa viime vuosina. MK:ssa pehmytkudosten välinen kontrasti on muita kuvausmodaliteetteja parempi, mikä mahdollistaa tarkemman kohteen määrittelyn ja riskielinten rajauksen. Nykyinen käytäntö MK-kuvien osalta sädehoidossa perustuu tietokonetomografia- (TT) ja MK-kuvien rekisteröintiin, jolloin MK-kuvien antama lisäinformaatio voidaan hyödyntää, vaikka itse hoitokenttien annoslaskenta ja potilaan kohdistus on TT-kuviin perustuvaa. Kahden kuvausmoda-liteetin käytöstä aiheutuu ylimääräistä työtä ja kustannuksia. Lisäksi kuvien rekisteröintiin liittyvä virhe lisää epävarmuutta hoidon tarkkuudessa. Viime aikoina useat tutkimusryhmät ovat julkaisseet menetelmiä, joiden avulla on mahdollista muodostaa sädehoidon annoslaskennassa tarvittava tiheyskartta (laskennallinen TT-kuva) suoraan magneettikuvausta käyttäen. Näin sädehoito on mahdollista toteuttaa pelkän magneettikuvan perusteella, jolloin yllä mainitut kahden kuvausmodaliteetin käytöstä aiheutuvat ongelmat voidaan välttää. Tässä työssä tutkittiin MK-kuviin perustuvan laskennallisen TT-kuvan käyttöönottoa ja tarkkuutta lantion alueen ulkoisessa sädehoidossa. Käyttöönottotestien tulee kattaa kaikki sellaiset vaiheet, jossa MK-pohjainen suunnittelu vaikuttaa joko geometriseen tai dosimetriseen tarkkuuteen. Ensimmäisessä ja kolmannessa osatyössä tutkittiin mahdollisuutta käyttää MK:ta sädehoitopotilaiden lantion alueen annoslaskennassa säteilyn vaimennuskorjaukseen. Toisessa ja kolmannessa osatyössä määritettiin potilasasemoinnin epätarkkuus käytettäessä MK-pohjaista menetelmää vertaamalla perinteiseen TT-kuvaan pohjautuvaan menetelmään. Lisäksi ensimmäisessä ja kolmannessa osatyössä arvioitiin MK:n geometrisen vääristymän vaikutuksia kokonaistarkkuuteen. Tutkimuksen perusteella menetelmän käyttö lantion alueella on riittävän tarkka kliiniseen käyttöön. Lisäksi kuvantaohjattu sädehoito magneettikuvien pohjalta on riittävän tarkkaa, jotta potilaan asemointitarkkuus ei huonone suhteessa nykyiseen TT-pohjaiseen suunnitteluun

Comparison of On Board Imager [OBI] and Cone Beam Computerized Tomography [CBCT] for position verification in pelvic malignancies on Image Guided Intensity Modulated Radiotherapy

OBJECTIVES: Comparison of On Board Imager [OBI] versus Cone Beam Computed Tomography [CBCT] in the verification of positional accuracy in pelvic malignancies in patients undergoing high precision Image guided intensity modulated radiotherapy [IG-IMRT] by quantifying and comparing the Systematic and Random Errors in the mediolateral, craniocaudal and antereoposterior directions to define the CTV to PTV margins and to analyze their time trend. METHODS: This is a descriptive study in which all the patients were diagnosed to have a pelvic malignancy, on IG-IMRT from January 2012 to August 2013 were included following an informed consent. The study included a total of 15 patients. Each patient had a CBCT and an OBI done on the first 3 days of treatment followed by the same weekly once. The shifts in the mediolateral, craniocaudal and antereoposterior directions were noted and the systematic, random and radial errors were calculated for both CBCT and OBI in each of the directions according to the standard formulae. The Wilcoxon signed rank test was used to determine the p value and a Bland- Altmann plot was done to compare the levels of agreement between the CBCT and the OBI with respect to all the errors. The CTV to PTV margin was calculated using the three formulae, namely Van Herk, Stroom’s and ICRU. The time trend for all the 15 patients were also analysed in the mediolateral, craniocaudal and the antereoposterior directions. RESULTS: The Systematic Errors were more as compared to the Random Errors. CBCT has detected more errors compared to the OBI. The systematic and random errors from our study were comparable to other studies and were within the tolerance limits in majority of patients. The CTV to PTV margin was also within the tolerance limits. There was a time trend seen showing that the errors decreased with time. CONCLUSIONS: The CBCT and the OBI are two different modalities which cannot be compared with each other and have to be used symbiotically

Uniform framework for the objective assessment and optimisation of radiotherapy image quality

Image guidance has rapidly become central to current radiotherapy practice. A uniform framework is developed for evaluating image quality across all imaging modalities by modelling the ‘universal phantom’: breaking any phantom down into its constituent fundamental test objects and applying appropriate analysis techniques to these through the construction of an automated analysis tree. This is implemented practically through the new software package ‘IQWorks’ and is applicable to both radiotherapy and diagnostic imaging. For electronic portal imaging (EPI), excellent agreement was observed with two commercial solutions: the QC-3V phantom and PIPS Pro software (Standard Imaging) and EPID QC phantom and epidSoft software (PTW). However, PIPS Pro’s noise correction strategy appears unnecessary for all but the highest frequency modulation transfer function (MTF) point and its contrast to noise ratio (CNR) calculation is not as described. Serious flaws identified in epid- Soft included erroneous file handling leading to incorrect MTF and signal to noise ratio (SNR) results, and a sensitivity to phantom alignment resulting in overestimation of MTF points by up to 150% for alignment errors of only ±1 pixel. The ‘QEPI1’ is introduced as a new EPI performance phantom. Being a simple lead square with a central square hole it is inexpensive and straightforward to manufacture yet enables calculation of a wide range of performance metrics at multiple locations across the field of view. Measured MTF curves agree with those of traditional bar pattern phantoms to within the limits of experimental uncertainty. An intercomparison of the Varian aS1000 and aS500-II detectors demonstrated an improvement in MTF for the aS1000 of 50–100% over the clinically relevant range 0.4–1 cycles/mm, yet with a corresponding reduction in CNR by a factor of p 2. Both detectors therefore offer advantages for different clinical applications. Characterisation of cone-beam CT (CBCT) facilities on two Varian On-Board Imaging (OBI) units revealed that only two out of six clinical modes had been calibrated by default, leading to errors of the order of 400 HU for some modes and materials – well outside the ±40 HU tolerance. Following calibration, all curves agreed sufficiently for dose calculation accuracy within 2%. CNR and MTF experiments demonstrated that a boost in MTF f50 of 20–30% is achievable by using a 5122 rather than a 3842 matrix, but with a reduction in CNR of the order of 30%. The MTF f50 of the single-pulse half-resolution radiographic mode of the Varian PaxScan 4030CB detector was measured in the plane of the detector as 1.0±0.1 cycles/mm using both a traditional tungsten edge and the new QEPI1 phantom. For digitally reconstructed radiographs (DRRs), a reduction in CT slice thickness resulted in an expected improvement in MTF in the patient scanning direction but a deterioration in the orthogonal direction, with the optimum slice thickness being 1–2 mm. Two general purposes display devices were calibrated against the DICOM Greyscale Standard Display Function (GSDF) to within the ±20% limit for Class 2 review devices. By providing an approach to image quality evaluation that is uniform across all radiotherapy imaging modalities this work enables consistent end-to-end optimisation of this fundamental part of the radiotherapy process, thereby supporting enhanced use of image-guidance at all relevant stages of radiotherapy and better supporting the clinical decisions based on it

CT Scanning

Since its introduction in 1972, X-ray computed tomography (CT) has evolved into an essential diagnostic imaging tool for a continually increasing variety of clinical applications. The goal of this book was not simply to summarize currently available CT imaging techniques but also to provide clinical perspectives, advances in hybrid technologies, new applications other than medicine and an outlook on future developments. Major experts in this growing field contributed to this book, which is geared to radiologists, orthopedic surgeons, engineers, and clinical and basic researchers. We believe that CT scanning is an effective and essential tools in treatment planning, basic understanding of physiology, and and tackling the ever-increasing challenge of diagnosis in our society