F. Van den Heuvel

Routine clinical on-line portal imaging followed by immediate field adjustment using a tele-controlled patient couch

We have evaluated the fluoroscopic on-line portal imaging (OPI) system developed by Siemens (Beamview-1, Concord, CA, U.S.A.) in routine clinical radiotherapy, involving the treatment of 883 fields (559 patient set-ups for treatment) on 21 patients. The image was typically generated by delivering 10 monitor units when used in single exposure or 1-2 monitor units on a large open field followed by 8-10 monitor units on the actual field when double exposure was used. Comparison between the portal image and the simulator film was done by eye. A region of tolerance was drawn on the simulator film and the field edges on the portal image had to project within this region. If this criterion was not met, adjustments followed by verification portal images were done before the remaining field dose was delivered. If possible, these adjustments were performed by moving the patient couch by remote control. The image quality was insufficient for evaluation in 75/883 (8.5%) fields. The abovementioned criterion was not met in 95/808 (11.8%) of the evaluable fields (26/559 patient set-ups were not evaluable). Of the 533 evaluable patient set-ups, 92 had to be adjusted (17.2%) including three (pelvic irradiations) set-ups that were adjusted on both field irradiated during the same radiotherapy session. In one case an incorrect tray (with wrong blocks) was detected and replaced. In one case (a 5.5 x 6.0 cm rectangular larynx field) the x and y axis of the field were interswitched. In one case incorrect focusing of a block was shown by the portal image. To make adjustments, the couch longitudinal position was changed 20 times (range -10 to +15 mm). The lateral position was changed 73 times (range -15 to +16 mm). The height position was changes 6 times (range -7 to +6 mm). Diaphragma rotation changes were performed 5 times (1 degree). The fraction of treatment time that was related to the use of OPI was 30.7% median (mean 32.4%, S.D. 14.1%). The range was 4.1 to 78.6%. On the basis of calculations assuming no OPI would have been used, field treatment time was increased by a median of 44.2% (mean 55.8%; S.D. 41.2%) by using OPI. The fraction of monitor units (fraction of the dose) to generate a satisfactory image was 10% median.(ABSTRACT TRUNCATED AT 400 WORDS)

Electronic Portal Imaging with On-Line Correction of Setup Error in Thoracic Irradiation: Clinical Evaluation

PURPOSE To analyze setup errors and the feasibility of their on-line correction using electronic portal imaging in the irradiation of lung tumors. METHODS AND MATERIALS Sixteen patients with lung cancer were irradiated through opposed anteroposterior fields. Localization images of anteroposterior fields were recorded with an electronic portal imaging device (EPID). Using an in-house developed algorithm for on-line comparison of portal images setup errors were measured and a correction of table position was performed with a remote couch control prior to treatment. In addition, residual errors were measured on the EPID verification image. Global and individual mean and standard deviation of setup errors were calculated and compared. The feasibility of the procedure was assessed measuring intra- and interobserver variability, influence of organ movement, reproducibility of error measurement, the extra time fraction needed for measuring and adjusting and the fraction of dose needed for imaging. RESULTS In two setups the procedure could not be finished normally due to problems inherent to the procedure. The reproducibility, intraobserver variability, and influence of organ movements were each described by a distribution with a mean value less than or equal to 1 mm and a standard deviation (SD) of less than 1.5 mm. The interobserver variability showed to be a little bit larger (mean: 0.3 mm, SD: 1.7 mm). The mean time to perform the irradiation of the anteroposterior field was 4 +/- 1 min. The mean time for the measurement and correction procedure approximated 2.5 min. The mean extra time fraction was 65 +/- 24% (1 SD) with more than half of this coming from the error measurement. The dose needed for generation of EPID images was 5.9 +/- 1.4% of total treatment dose. The mean and SD of setup errors were, respectively, 0.1 and 4.5 mm for longitudinal and -2.0 and 5.7 mm for transversal errors. Of 196 measured translational errors 120 (61%) exceeded the adjustment criteria. For individual patients systematic and random setup errors can be as high as, respectively, 15.8 and 7.5 mm. Mean residual error and SD were for longitudinal direction 0.08 and 1.2 mm and for transversal direction -0.9 and 1.0 mm (pooled data). For individuals, the mean residual errors were smaller than 1 mm, with a typical SD per patient of less than 2 mm. CONCLUSION Setup errors in thoracic radiation therapy are clinically important. On-line correction can be performed accurately with an objective measurement tool, although this prolongs the irradiation procedure for one field with 65%.

Interactive use of on-line portal imaging in pelvic radiation.

We have evaluated a fluoroscopic on-line portal imaging system in routine clinical radiotherapy, involving the treatment of 566 pelvic fields on 13 patients. The image was typically generated by delivering a radiation dose of 6-8 cGy. Comparison between portal image and simulator film was done by eye and all visible errors were corrected before continuing irradiation. If possible, these corrections were performed from outside the treatment room by moving the patient couch by remote control or by changing collimator parameters. Adjustments were performed on 289/530 (54.5%) evaluable fields or 229/278 (82.4%) evaluable patient set-ups. The lateral couch position was most frequently adjusted (n = 254). The absolute values of the adjustments were 6.8 mm mean (SD 6.6 mm) with a maximum of 40 mm. All absolute values of adjustments exceeding 25 mm were recorded in one patient and those exceeding 15 mm were observed in two patients. Both patients were obese females. Adjustments exceeding 5 mm were observed in all 13 patients. Related to the use of on-line portal imaging, treatment time was increased by a median of 36.5% (mean 45.8%; SD 42.1%). The range was 7.7 to 442%. The fraction of the total treatment time to perform corrections was 22.7% median (mean: 26.0; SD: 11.8%). Statistically significant systematic in-plane errors were found in 7/13 patients. A systematic error was detected on the lateral position of the field in five patients. In one patient a systematic error of the longitudinal field position and in one patient a rotational error was detected. For adjustments in the lateral direction the present method does not allow to detect lateral shifts of less than 2 mm. For adjustments in the longitudinal direction the sensitivity could not be estimated but the available data suggest that 80% of errors < or = 5 mm were not adjusted. In obese patients, random errors may be surprisingly large.

On-line portal imaging: Image quality defining parameters for pelvic fields—a clinical evaluation

PURPOSE A test of several image enhancement techniques, performed on on-line portal images in real clinical circumstances, is presented. In addition a score system enabling us to evaluate image quality on pelvic fields is proposed and validated. METHODS AND MATERIALS Localization images (n = 546) generated by an on-line portal imaging system during the treatment of 13 patients on pelvic fields were obtained by delivering a radiation dose of 6-8 cGy by an 18 MV photon beam, and recorded with a silicon intensified target video camera with adjustable gain, kV- and black level. Set-up errors were corrected before continuing irradiation. A scoring system based on the number of visible bone-soft tissue edges and transformed to a scale 0 to 5 was developed to judge image quality. A validation of this classification of images was performed with the use of transsectional bone-densities (bone-density*radiological path length) specified at the score defining landmarks. A high pass filter was used on all images, additional on-line open field subtraction was performed on 242 fields. Off-line study was performed in which a panel consisting of two groups (one composed of three radiation oncologists, the other of three radiotherapy technologists), scored 470 pelvic fields without further enhancement, and the same images with Contrast Limited Adaptive Histogram Equalization (CLAHE) (Pizer et al.). Two different clipping levels (3.0 and 5.0) were studied. RESULTS Gender and transsectional bone-densities were the most defining patient-related factors influencing image quality. Camera settings, gantry angle, and image post-processing were important non-patient-related factors. All investigators judged CLAHE to ameliorate low contrast images and to deteriorate good quality images (p < 0.001).

Relations of image quality in on-line portal images and individual patient parameters for pelvic field radiotherapy

AbstractWe have previously demonstrated that on-line portal imaging (OPI) can detect and correct significant errors in field set-up. Such errors occurred very frequently when irradiating the pelvic region and were typically detected after 10% of the field dose was delivered. The image quality on pelvic fields was, however, disappointing. The aims of the present study involving 566 pelvic fields on 13 patients were: 1.To study the machine- and patient-related factors influencing image quality.2.To study the factors related to machine, patient and patient set-up, influencing the errors of field set-up.3.To develop a method for predicting the camera settings. The OPI device consisted of a fluorescent screen scanned by a video camera. An image quality score on a scale 0–5 was given for 546/566 fields. In a univariate analysis, open field subtraction adversely affected the score (P < 0.001). The image score of anterior fields was significantly better than that of posterior fields (P < 0.001). Multivariate stepwise logistic regression showed that, in addition to anterior or posterior field (P < 0.001) and subtraction (P = 0.003), gender (P = 0.02) was also a significant predictor of image score. Errors requiring field adjustments were detected on 289/530 (54.5%) evaluable fields or 229/278 (82.4%) evaluable patient set-ups. Multivariate logistic regression showed that the probability of performing an adjustment was significantly related to gender (P < 0.001), image quality (P = 0.001) and AP-PA diameter (P = 0.04). The magnitude of adjustments made in the lateral direction correlated significantly (P < 0.0001) with patient bulk. The camera kV level with gain held constant showed an exponential dependency on dose rate at the image detector plate and can thus be predicted by treatment planning.

SU‐E‐J‐138: Fast 2‐D Fiducial Marker Detection on Sequential MV Projections in Arc Therapy

PURPOSE To automatically detect intrafraction motion during arc radiotherapy for prostate cancer patients by tracking fiducial markers in two-dimensional MV images acquired using the treatment beam, in order to adjust radiation dose accordingly. METHODS Four fiducial gold markers are implanted in a patients prostate. Patients are irradiated using a Varian Linac 2100 C/D with RapidArc upgrade (Varian Medical Systems, Palo Alto, CA). MV images (1024 × 768 pixels, 0.392 × 0.392 mm2 pixel size) acquired during a 360 degree gantry rotation at a one second interval (5 degrees) are preprocessed by subtracting a smoothed version of the image to retain only high image frequencies. Edge detection is then applied, followed by a one pixel wide dilation and erosion to transform the edges into contiguous regions. Next, our method searches the centers of visible markers (i.e. not covered by the MLC), constrained by marker estimates from the planning CT. This is done by finding all contiguous regions and maximizing a marker-region distance criterion for every visible marker. A two-dimensional estimate correction over consecutive projections is also implemented to improve marker estimates during gantry rotation. RESULTS We applied our method on four treatment fractions of the same patient. As such, a total of 191 projections with manually indicated marker ends as ground truth were used as validation. Markers were indicated twice on all images, to include observer errors. Results show a mean detection error of less than 0.5 mm in the projection image (standard deviation 0.6 mm), with an execution time of less than one second per image in matlab. Undetected markers and false positives mostly occurred at moving leaf boundaries, where marker visibility was determined by the observer. CONCLUSIONS Preliminary findings demonstrate that this method can be used to detect intrafraction motion during arc radiotherapy by only using projected MV images. Research sponsored by Varian Medical Systems, Palo Alto, CA.

PD-0462: Towards dosimetric tracking with adaptive VMAT?

was acquired at t = 0, 2, 4, 6, 8, and 10 minutes. Treatment and MR scanning was performed on voided bladders. The bladder CTV was delineated in all scans and each CTV was described using spherical coordinates with origin at the centre of volume of the first scan of the first session of each patient. Population-based 2D margin maps were derived by adapting a published margin recipe for bladder (Meijer et al, IJROBP 2003), characterising in the spherical coordinate system the intra-fractional changes (between the scans at t = 0 min to t = 10 min) in terms of systematic and random errors. Secondly, the possibility of deriving patient-specific intra-fractional margins was explored by using only the bladder expansions occurring in the first two series (the pretreatment and the first week series). A linear model was used to fit the radial changes occurring as a function of time. Focusing on the patients and directions where an expansion larger than 5 mm was observed, the patient-specific margin was defined as the upper 96% confidence limit of the linear coefficient multiplied by the relevant intra-fractional time (here assumed ten minutes). Results: The population-based 2D margin map is shown in Fig 1; when excluding the one female patient, the margins at superior and anterior directions were 14 mm, posterior 9 mm and the other directions (inferior, left and right) 5 mm. Intrafractional margins specific for each patient could be derived from the linear model fit (ranging up to 12 mm; R2 in the range: 0.33-0.68).

PO-0706: Individualised margin calculations for different target regions in anal cancer IMRT

Purpose/Objective: UK IMRT anal cancer treatment uses large fields to uninvolved nodal groups (40Gy) with simultaneous integrated boost to the primary tumour (50.4Gy T1/T2; 53.2Gy T3/T4) and involved nodes (50.4Gy). With a simple bony match online, iliac nodes are well covered and the margins required are well documented, however the margins for prophylactic inguinal nodes (pIN) and primary tumour are not well established as data from daily imaging are limited; these form the focus of this study. Materials and Methods: Anal cancer patients treated at a single institution under current UK IMRT guidelines were screened; 11 consecutive inguinal node negative patients were studied. Supine treatment comprised 28 fractions with daily imaging: CBCT fractions 1-5, 10, 15, 20 and 25; orthogonal kV imaging all other fractions. 99 CBCT’s were re-matched automatically to the planning CT. A bony match was performed using a clipbox encompassing the bony pelvis. Re-matches were performed within the same clipbox using the clinician defined tumour (GTVA) as a region of interest (ROI), then repeated with the pIN ROI. Accuracy of auto-matches were assessed visually to ensure clinical relevance. Bony match values were subtracted from the GTVA and pIN measurements to evaluate differences in the optimal treatment position for the tumour or the nodes relative to a simple bony match. Margins were calculated using van Herk’s recipe. Results: Differences (mm) between GTVA/ bony matches were larger than inguinal/ bony matches in all axes ( lat -3.1 to 4.2; -2 to 1.5, vert6.9 to 12.7; -3.6 to 2.9, long -13.3 to 17.2; -8.5 to 7.3 in GTV and pIN respectively). This was statistically significant in the long axis (p<0.05) shown in Fig.1. GTVA had consistently larger systematic and random errors than pIN, reflected in the margin calculations (mm): GTVA lat 2.8, long 9.8, vert 5.8; pIN lat 1.5, long 3.1, vert 3.1. Conclusions: With a simple bony match, the margin around pIN can be reduced to 1.5mm laterally and 3.1mm in all other directions potentially reducing toxicity to the groin, genitalia and bladder. The GTVA to PTV margin incorporates microscopic disease, the motion of the soft tissues of the anus which can be affected by tumour size, location, bowel filling and BMI; and the set up error. The margin reported in this study covers set up error and soft tissue motion of the anus. An individualised margin incorporating these factors can be calculated and applied during the treatment course with the aim of reducing toxicity in adjacent organs such as vagina, bladder and penile bulb. Further investigation is warranted to demonstrate reduced toxicities with these strategies.

PO-1001: Respiratory gating reduces heart doses for proton radiotherapy of the breast and internal mammary chain

PO-1001 Respiratory gating reduces heart doses for proton radiotherapy of the breast and internal mammary chain S. Hackett, F. Fiorini, S. Petillion, C. Taylor, C. Weltens, K. Vallis, S. Darby, F. Van den Heuvel University of Oxford, Department of Oncology, Oxford, United Kingdom Universitair Ziekenhuis Leuven, Radiotherapie-Oncologie, Leuven, Belgium University of Oxford, Clinical Trial Service Unit, Oxford, United Kingdom

PO-0847: A strategy for non-MU preserving adaptive radiotherapy

PO-0845 Evaluation of interplay effects of target and MLC during VMAT by using 3D gel measurements S. Ceberg, C. Ceberg, M. Falk, P. Munck af Rosenschöld, S. Bäck Skåne University Hospital, Medical Radiation Physics, Lund, Sweden Lund University, Medical Radiation Physics Department of Clinical Sciences Lund, Lund, Sweden Radiation Medicine Research Center, Radiation Oncology Rigshospitalet, Copenhagen, Denmark