1270 poster DOSIMETRIC IMPLICATIONS OF UNCORRECTED ROTATION OF THE PROSTATE DURING EXTERNAL BEAM RADIOTHERAPY

IGRT: VOLUME DEFINITION , TREATMENT MARGINS AND GEOMETRIC UNCERTAINTIES

1268 poster CHANGES AND DEFORMATIONS OF THE CORPUS UTERI DURING A COURSE OF RADIOTHERAPY FOR CERVICAL CANCER K. L. Jakobsen1 , C. F. Behrens1 , H. Johannesen2 1 C OPENHAGEN U NIVERSITY H OSPITAL , H ERLEV, Department of Oncology, Herlev, Denmark 2 C OPENHAGEN U NIVERSITY H OSPITAL , H ERLEV, Department of Radiology, Herlev, Denmark Purpose: The aim of this project is to evaluate the quality of CBCT-scans and to quantify the changes in the position of uterus during a course of radiotherapy.The hypothesis is that these changes are so dominant that some patients should be rescanned and replanned during the treatment course. Materials: Thirty patients with cervical cancer and weekly CBCT-scans were included. A radiologist (HHJ) retrospectively outlined the cervix and corpus uteri on the CBCT-scans and the sizes of the delineated volumes were analyzed.The outline was done in Eclipse 8.8 (Varian Medical Systems, Pal Alto, Californien). The CBCT-scans were acquired without contrast with 125 kV and 1040 mAs on Varian iX machines. Results: As the tumor shrinks so does the cervix and this can result in changes in the position of the corpus uteri. The daily differences in the bladder and bowel filling further changes the position of corpus uteri. For some patients dramatically changes of the irradiated volumes were observed.In general it was hard to distinguish between the different organs on the CBCTscans. Specifically it was difficult to distinguish between cervix and corpus uteri and between the lower part of cervix and the vagina.For a few patients the corpus uteri underwent large changes concerning size, position and shape. This is partly due to retention: The tumor in cervix has initially formed a plug and as the tumor shrinks the uterine cavity empties itself (see figure 1).

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IMAGES J. Li1 , J. Xing1 , Y. Zhang1 , F. Li1 , M. Xu1 S HANDONG C ANCER H OSPITAL, Department of Radiation Oncology, Jinan, China

1

Purpose: To compare the position and magnitude of internal target volume ( ITV) of the primary hepatocarcinomas delineated by the three methods based on four-dimensional computed tomography (4D-CT) and to investigate the relative factors affecting the position and magnitude . Materials: Twenty patients with primary hepatocarcinoma underwent big bore 4D-CT simulation scan of the thoraxand abdomenassisted by real-time position management (RPM) system for simultaneous record of the respiratory signals. The CT images with respiratory signal data were reconstructed and sorted into 10 phase group in a respiratory cycle, with 0% phase corresponding to end-inhale and 50% corresponding to end-exhale. Maximum intensity projection (MIP) image was generated. ITVs of the primary tumor of NSCLC were delineated using three methods as following: the gross tumor volume (GTV) on each of the ten respiratory phases of the 4D-CT image set were delineated and fused ten GTV to produce ITV10;the GTV delineated separately based on 0% and 50% phase were fused to produce ITVIN+EX;the visible tumor on the MIP images were delineated to produce ITVMIP. Twenty patients were divided into A group and B group based on the location of the target center and were divided into C group and D group based on the tumor D, the patients were divided into E group and F group based on the threedimensional (3D) motion vector of the target center. The position of the target center, the volume of target, the degree of inclusion (DI) and the matching index (MI) were compared reciprocally between ITV10, ITVIN+EX and ITVMIP, and the influence of the tumor position and 3D motion vector on the relative parameters were compared based on the grouping. Results: Average differences between the position of the center of ITVs on direction of X,Y and Z axes were less than 1.5 mm, with no statistically significant difference. The volume of ITV10 was larger than that of ITVIN+EX , but the difference was not statistically significant (t = 0.354P = 0.725), the volume of ITV10 was larger than that of ITVMIP, but the difference was not statistically significant (t = -0.392, P = 0.697). The ratio of ITVIN+EX with ITV10, ITVMIP with ITV10 were 0.75 ± 0.15 and 0.78 ± 0.14, respectively. DI of ITVIN+EX in ITV10, ITVMIP in ITV10 were (74.85 ± 15.09)% and (68.87 ± 13.69)%. MI between ITV10 and ITVIN+EX, ITV10 and ITVMIP were 0.75 ± 0.15, 0.67 ± 0.13, respectively. The median of ratio of ITVIN+EX / ITV10 was 0.57 in group A versus 0.87 in group B, the difference between group A and group B was statistically significantz = -3.300P = 0.001. the median of ratio of ITVMIP / ITV10 were 0.51 in group A versus 0.72 in group B, the difference between group A and group B was statistically significantz = -3.413P = 0.001. The median of ratio of ITVIN+EX / ITV10 was 0.79 in group C versus 0.74 in group D, with no statistically significant differencez = -0.920P = 0.358; the median of ratio of ITVMIP / ITV10 was 0.85 in group C versus 0.80 in group D, the difference was not statistically significantz = -0.568P = 0.570.The median of ratio of ITVIN+EX / ITV10 was 0.87 in group E versus 0.68 in group F, the difference between group E and group F was statistically significantz = -2.897P = 0.004. The median of ratio of ITVMIP / ITV10 were 0.85 in group E versus 0.81 in group F, the difference was not statistically significantz = -0.568P = 0.570. Conclusions: The center displacement of the ITVs delineated separately by the three different techniques based on 4D-CT images are not obvious; ITVIN+EX and ITVMIP can not replace ITV10, however, ITVIN+EX is more close to ITV10 comparing to ITVMIP. The ratio of ITV10 and ITVMIP is correlated to the 3D motion vector of the tumor. When the tumor in the upper part of the liver and with a 3D motion vector less than 9 mmITV10 should be the ideal ITV. 1270 poster

Conclusions: It is difficult to distinguish between cervix and corpus uteri and between cervix uteri and vagina on CBCT-scans. Especially the position of corpus uteri can change dramatically from day to day and during a treatment course. The daily position changes are accounted for with the CTV to PTV margin.The large deformations found in this study illustrate that a new CTscan must be acquired at some point during the course of treatment. The CBCT-scans can point out the relevant time for this rescan.Weekly MR-scans during radiotherapy would be advantageous. This will enable observation of the decrease in tumor burden and other morphological changes. The possibility of reducing the target volume by using MRI as a diagnostic scan should be investigated. The MRI allows better differtiation between tumor in the cervix and corpus uteri and between the benign tissues (cervix, corpus uteri and the vagina). 1269 poster COMPARATIVE STUDY BETWEEN THE THREE METHODS TO DELINEATE INTERNAL TARGET VOLUME OF THE PRIMARY HEPATOCARCINOMA BASED ON FOUR-DIMENSIONAL CT SIMULATION

DOSIMETRIC IMPLICATIONS OF UNCORRECTED ROTATION OF THE PROSTATE DURING EXTERNAL BEAM RADIOTHERAPY K. Burke1 , C. Lamb1 , N. Rosenfelder1 , M. Bidmead1 , N. van As1 , V. Khoo1 1 T HE R OYAL M ARSDEN NHS F OUNDATION T RUST, London, United Kingdom Purpose: 3D imaging of the prostate during external beam radiotherapy is now routine, either via orthogonal 2D kV imaging of fiducial markers or directly using cone beam CT. Correction of translational set-up errors is routine and can be achieved with a standard linac treatment couch. Dealing with rotational error (RE) is not so straightforward:Robotic tilting couch tops can only correct small RE, typically up to 3◦ . In our clinic RE > 10◦ have been observed in some patients.Rotating the couch top may induce secondary patient motion.As the prostate CTV is approximately spherical, it could be assumed that not correcting rotations may have a relatively small impact on set-up. The impact of this residual set-up error on dose coverage is not obvious however. We therefore undertook a dosimetric assessment of prostate rotation to assess its clinical impact and to see if a tolerance value could be established for RE. Materials: Ten patient datasets were selected for retrospective analysis. Each of these patients had been implanted with a minimum of three fiducial markers and had been imaged during treatment with an Exactrac (BrainLab)

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system.For each patient the CTV (consisting of prostate plus base of seminal vesicles) was rotated by 3, 5 and 10 degrees using in-house software. Rotations were applied in all three axes and were about the treatment isocentre. In addition to these standard rotations, for each patient the individual’s measured mean rotational error plus one standard deviation was applied to simulate a "typical" rotational set-up error (values ranged from -7.0 to +7.1 degrees). The rotated CTVs were then imported back into the TPS (Eclipse v8.6, Varian). It was assumed that the seminal vesicles moved in tandem with the prostate.A five field IMRT plan was then produced for each patient. Our technique utilises a simultaneous integrated boost: PTV2 (CTV plus 3mm, 5mm anteriorly, excludes rectum) is prescribed to 74 Gy, with PTV1 (CTV plus 10 mm, 7mm posteriorly) receiving 68 Gy. DVH data for each of the rotated CTVs, plus the original clinical CTV, were extracted for analysis. Results:

Conclusions: Significant (>1 mm) inter-marker distance changes have been observed during a course of post-operative radiotherapy. They are believed to be due to inter-and intra-fraction deformation of the prostate-bed. 1272 poster

Conclusions: CTV coverage at the D98% level is adequate (> 95%) in the face of rotations of 5 degrees or less and this may be an appropriate tolerance level. Minimum dose to CTV is adequate in the face of a 3 degree rotation but drops sharply at 10 degrees. Mean, D98% and maximum doses show little or no variation.These results suggest that our current margins are adequate to compensate for uncorrected REs up to 5 degrees. Any future reduction of margins would have to be evaluated in terms of the dosimetric impact of REs. 1271 poster EVALUATION OF PROSTATE-BED DEFORMATION WITH IMPLANTED FIDUCIAL MARKERS DURING POST-OPERATIVE RADIOTHERAPY OF PROSTATE CANCER G. Pang1 , P. Au1 , P. Cheung1 , G. Morton1 , H. Chung1 , A. Deabreu1 , H. Quon1 , A. Loblaw1 1 O DETTE C ANCER C ENTRE , S UNNYBROOK H EALTH S CIENCES C ENTRE, North York, Toronto, Canada Purpose: The objective of this study was to assess both inter-and intrafraction prostate-bed deformation during a course of post-operative radiotherapy. Materials: Patients with histologically confirmed recurrent or high risk adenocarcinoma of the prostate post-radical prostatectomy were eligible for this study (This is a part of a larger study aimed to assess the toxicities of delivering a hypofractionated accelerated course of adjuvant post-operative radiotherapy). Prior to CT simulation, 3 gold fiducial markers were implanted into the prostate bed: one at the bladder neck (marker #1), one at the urogenital diaphragm (marker #2), and one near the bladder/urethra anastomosis (marker #3). At least one week after insertion of the fiducial markers, a planning CT scan was performed with the patient supine with a custom vacuum lock bag for immobilization. Patients had a comfortably full bladder and were asked to empty their rectum prior to simulation and each treatment. The radiotherapy involved delivering 51 Gy in 17 fractions to the prostate bed using an optimized 7 field inverse planned IMRT technique. Before each fraction of radiotherapy, AP and RT lateral electronic portal images (pre-treatment EPI) were taken. The location of the centre of mass of three fiducial markers was used for daily online correction for setup errors and interfraction organ motion, while the change of inter-marker distance seen on the pre-treatment EPIs as compared to that on DRRs was used to assess the interfraction prostate bed deformation (and possible marker migration). Post-treatment electronic portal images (post-treatment EPIs) were also taken on days 1-6 and 11 and compared to the pre-treatment EPIs to assess the intrafraction prostate-bed deformation. Results: So far five patients have been enrolled into the study with four of them whose EPI data (a total of 192 images) were available for analysis. It was found that for all four patients significant (>1 mm) inter-marker distance changes were observed in both inter-and intra-fraction cases. The maximum inter-marker distance change observed was 5.7 mm in the interfraction case and 3.6 mm in the intrafraction case. The area of the triangle defined by the three markers was changed up to 27% between fractions and 17% during a fraction. Table 1 shows the mean and the standard deviation (in bracket) of the inter-marker distance change as well as the change of the triangle area defined by the three markers for these patients.

HIGH QUALITY MRI-CT CO-REGISTRATION WITH T2-WEIGHTED BLADE SEQUENCES FOR HEAD AND NECK RADIOTHERAPY. J. Maclean1 , S. punwani1 , S. Morley2 , D. Marsh1 , K. Sullivan1 , S. A. Moinuddin1 , C. Stacey1 , A. Bainbridge1 , D. Carnell1 , R. Mendes1 1 U NIVERSITY C OLLEGE L ONDON H OSPITAL NHS T RUST, London, United Kingdom 2 U NIVERSITY C OLLEGE L ONDON H OSPITAL NHS T RUST, Department of Clinical Oncology, London, United Kingdom Purpose: MRI is superior to CT at differentiating tumour from normal tissue in the head and neck region. Target volumes are traditionally defined on CT as electron density data directly derived from Hounsfield units are required for plan dosimetry. Co-registration of CT and MRI would improve accuracy of target volume definition. We aim to develop an optimised head and neck MRI-CT fusion protocol for use in our department and determine its accuracy. Materials: A customised flat acrylic baseboard compatible for both MRI and radiotherapy planning CT scan was designed and manufactured in-house. Body surface and flexible carotid coils were used in conjunction with the patients individualised thermoplastic (TP) radiotherapy head and neck shell. A TP bridge was fitted over the shell to support the body surface coil. Fig 1. 3 healthy volunteers underwent MRI scan in the TP shell to establish the tolerability of the procedure and to clarify the quality of anatomic clarity . Tolerability and acquisition time of the MRI was evaluated in 8 patients with head and neck cancer due to undergo radical radiotherapy. In these patients, planning CT and MRI scans were performed on the same day. Images were fused using a rigid co-registration programme on Eclipse (Varian Medical Systems) and the discrepancy between them was evaluated by measuring the rotations required to co-register the images in 3D.

Results: The T2-weighted BLADE MRI sequence with 3mm slices provided the highest resolution images with excellent soft tissue definition for co-registration. BLADE is Siemens version of the PROPELLER (Periodically Rotated Overlapping Parallel Lines with Enhanced Reconstruction) sequence which collects data to mitigate in-plane motion, through-plane motion and phase inconsistencies1 . The impact of involuntary patient motion, such as swallowing, was minimised without requiring additional distortion correction. All patients found the MRI scanning process acceptable and the median time to acquire the MRI images was 5 minutes. Mean rotation required for coregistration was <1◦ in all planes: mean x = 0.529◦ ; mean y = 0.463◦ ; mean z = 0.44◦ . Conclusions: Our method of CT-MRI image acquisition and co-registration is tolerable and accurate. T2-weighted BLADE sequences provide excellent image quality without the need for distortion correction. Evaluation of the clinical impact of this co-registration protocol on target volume definition is