THE USE OF LAXATIVE AND THE IMPACT ON RECTUM FILLING, INTRA-FRACTION MOTION AND DOSE TO PTV FOR PROSTATE PATIENTS

IGRT: T REATMENT DELIVERY AND PATIENT POSITIONING

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470 poster (Physics Track)

471 poster (Physics Track)

REAL-TIME MRI GUIDED IMRT BOOSTING FOR PELVIC LYMPH NODE METASTASES IN CERVICAL CANCER PATIENTS L. Dijkstra1 , E. Kerkhof1 , B. Raaymakers1 , I. M. Jürgenliemk-Schulz1 , J.

TECHNICAL PERFORMANCE OF A 3D-LASER SURFACE IMAGING SYSTEM T. Moser1 , S. Fleischhacker1 , K. Schubert2 , G. Sroka-Perez2 , C. P. Karger1 1 G ERMAN C ANCER R ESEARCH C ENTER (DKFZ), Department of Medical Physics in Radiation Oncology, Heidelberg, Germany 2 U NIV. K LINIKUM H EIDELBERG, Heidelberg, Germany

Lagendijk1 U.M.C. U TRECHT, Department of Radiotherapy, Utrecht, Netherlands

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Purpose: At our institution a 1.5 T MRI scanner integrated with a 6 MV radiotherapy accelerator is built together with Elekta and Philips Medical Systems. This machine would make real-time MRI guidance of radiotherapy possible, resulting in the use of smaller treatment margins. This treatment planning study investigates the dosimetric benefit of real-time MRI guided IMRT boosting of pelvic lymph node metastases in cervical cancer patients. The results are compared with our traditional sequential boost regimen, for which an offline portal imaging position verification technique is used. Materials: Ten patients with pelvic lymph node metastases are included in this study. Five patients had one, three had two, and two had five nodal GTVs. For this study an integrated IMRT boost of 15 Gy to a total dose of 60 Gy to the nodal GTVs was planned in 25 fractions of 2.4 Gy (EQD2 = 62.0 Gyα/β 10), with treatment margins of 4 mm around the nodal GTVs and CTVs. These plans were compared with the current clinically used regimen in which a sequential boost of 5 to 8 fractions of 2.0 Gy is applied after 25 fractions of 1.8 Gy to a total EQD2 of 54.3 to 60.3 Gyα/β 10. In this regimen AP-PA fields are used and a treatment margin of 10 mm is applied. Results: A typical example of the dose distributions of both boost regimens is shown in the figure. The dose to 99% of the nodal GTVs (D99) averaged over all patients after sequential boosting was 96.0% of the prescribed dose ranging from 54.3 to 60.3 Gyα/β 10. After integrated boosting the D99 was 98.5% of the prescribed dose of 62.0 Gyα/β 10. The volume of the bowel receiving more than 45 Gy was on average reduced from 17.6% to 5.2% by the integrated boost regimen, while the D99 is higher for the integrated boost in 9 of the 10 patients . Conclusions: An integrated IMRT boost regimen for pelvic lymph node metastases of cervical cancer with a treatment margin of 4 mm results in lower involvement of the surrounding bowel. Repeated planning based on real-time MRI guidance will reduce bowel involvement even further.

Purpose: Phantom measurements are performed to investigate the accuracy of a 3D-laser surface imaging system in determining inter-fractional setup corrections in radiotherapy. Materials: A 3D-laser-imaging system (Galaxy, LAP Laser, Lneburg, Germany) is used to reconstruct 3D patient surfaces with high resolution. The system is mounted at the ceiling of a tomotherapy unit (tomo, HiArt System, TomoTherapy, Madison, WI, USA). The reflections of the projected laser lines are recorded by a camera and a 3D-surface model of the patient is reconstructed. The actual patient position is then compared with that of a reference setup or with a contour of the skin imported from the treatment planning computed tomography (CT) via DICOM interface. As a result, a setup correction with 4 or 6 degrees of freedom is calculated. This correction can be used to improve the setup of the patient. To evaluate the technical performance of the surface imaging system for patient alignment, measurements were performed in a rigid anthropomorphic phantom. Multiple measurements with randomly selected 1D- and 3D-shifts were executed. By comparing the corrections detected by the surface imaging system with these predefined shifts, the accuracy of the surface imaging system was analyzed. Results: The accuracy in detecting the 3D-shifts were 0.30 ± 0.31 mm in lateral, 0.03 ± 0.10 mm in longitudinal, and 0.01 ± 0.12 mm in vertical direction. Regarding the reproducibility of the phantom position between reference and actual scan revealed an increased accuracy, if the phantom was rotated by 45◦ around the vertical axis (0.8 vs. 0.0 mm). At 0◦ setup of the phantom, a correlation between the corrections for the lateral direction and for the rotation around the longitudinal axis was found (r= 0.99). This is most probably due to the view of the camera system on the phantom as more information on the phantom shape can be obtained under a partially lateral view. Conclusions: The system is able to detect translations of a rigid phantom with an accuracy of better than 1-2 mm. With the phantom setup at 0◦ , we found a somewhat decreased reproducibility in lateral direction, which is, however, clinically acceptable. Current data in a phantom support the concept of improving patient alignment in fractionated radiotherapy with a surface imaging system. These results have to be confirmed by further investigations.The system is able to detect translations of a rigid phantom with an accuracy of better than 1-2 mm. With the phantom setup at 0◦ , we found a somewhat decreased reproducibility in lateral direction, which is, however, clinically acceptable. Current data in a phantom support the concept of improving patient alignment in fractionated radiotherapy with a surface imaging system. These results have to be confirmed by further investigations. Acknowledgement: This work was partly funded by LAP (Lneburg, Germany). 472 poster (Physics Track) THE USE OF LAXATIVE AND THE IMPACT ON RECTUM FILLING, INTRA-FRACTION MOTION AND DOSE TO PTV FOR PROSTATE PATIENTS C. Jensen1 , M. Johansen1 , M. Resell1 , A. Kylling1 , N. Skottner1 , O. Fremsæter1 Å LESUND HOSPITAL, Cancer Departement, Ålesund, Norway

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Purpose: At our clinic prostate patients routinely have gold seeds inserted to aid in the localization of the prostate during treatment. The patients are daily aligned to the gold seeds using the Elekta XVI system. Treatment is usually delivered by a standard four-field box technique. To minimise variations in rectum filling patients are prescribed a light laxative. The purposes of the present study were to evaluate the effect off the use of laxative on rectum filling; to investigate if laxatives have impact on intra-fraction motion and finally if the dose to the PTV was any different for patients using laxatives versus those not using it. Materials: Retrospectively the rectum volumes of 40 patients taking laxatives and 40 patients not taking laxatives were compared. The rectum was divided into three volumes: caudal, central and, cranial volumes. The minimum dose, Dmin, and D95 and D98 to the PTV were estimated for the two groups. A smaller group of patients was selected to calculate the margins needed to account for intra-fraction motion of the prostate. A total of 14 patients were included, of which 7 used laxative. The intra-fraction lateral, longitudinal and height translations were estimated for each treatment session using EPID images recorded after initial alignment to the gold seeds with the XVI system. In addition, intra-fraction rotation around the left-right axis was recorded. Results: Laxative reduces the median rectum volume by about 15%. The reduction is largest in the central rectum volume which is reduced by 25%. There are no significant differences in the margins needed to account for intra-fraction motion for the two groups. The median of the D95 and D98 to the PTV are similar for the two groups. However, the median Dmin is lower for the patient group receiving laxatives.

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Conclusions: The use of laxative for patients with gold seeds, for whom daily alignment is performed using the gold seeds, does not reduce intra-fraction motion. Rather the minimum dose to the PTV for these patients is about 11% lower than those not receiving laxatives. 473 poster (Physics Track) THE USE OF ORTHOGONAL KV IMAGE PAIRS OR CBCT FOR SETUP CORRECTION FOR STAGE III LUNG TUMORS B. Houben-Haring1 , J. Cuijpers1 , W. Verbakel1 , S. Senan1 1

VU U NIVERSITY M EDICAL C ENTER, Radiation Oncology, Amsterdam, Netherlands

Purpose: Stage III lung cancer patients may exhibit large set-up errors, which can be minimized with an online set-up protocol using either kV-Cone Beam CT (CBCT) or orthogonal kV-image pairs (OIP). CBCT allows volumetric imaging of the patient, and can thus be considered as a standard for achieving optimal set-up. However, use of CBCT is more time consuming, and administers more dose than OIP. We retrospectively compared setup errors using either OIP or CBCT to explore if these are equivalent if setup is based solely on bony structures. As roll of the patient cannot be determined using OIP, and OIP are unable to detect changes in soft tissue, these aspects were analyzed in the CBCT images. Materials: Patients were positioned using daily online setup correction using both OIP and CBCT (On-Board Imager, Varian Medical Systems) twice weekly, and OIP only for the remaining fractions. CBCT image registration was performed on the spinal column using automatic matching. Visual check was used to confirm that the tumor visible on the CBCT was encompassed by the PTV-contour. OIP were manual registered with the corresponding DRRs. A retrospective analysis was performed on the acquired images, including 664 OIP and 165 CBCT’s of 29 patients. Results: Table 1 shows the results of the CBCT match analysis and correlation and regression coefficients (β ) between OIP and CBCT matches. The correlation was high, except in the vertical direction which may be due to differences in length of the spinal column visible on the kV-images. Only for the vertical direction β differs statistically significant from 1, but the deviation is relatively small.

Analysis of 149 CBCT’s from 20 patients showed that 93% of all rotations were <2◦ . Two patients exhibited roll >2◦ for more than 30% of the fractions. In 5 out of 29 patients, the PTV did not encompass the visible tumor. Two patients showed large deformations in the internal anatomy caused by resolving atelectasis or tumor regression and two others showing a longitudinal shift of all thoracic structures. In one patient a large roll (up to 6◦ ), which was not detected on the OIP, caused the tumor to rotate out of the PTV. Conclusions: For bony setup correction, both orthogonal kV-images and CBCT are comparable. This suggests that orthogonal kV-imaging can be used as it is quicker and delivers significantly less dose. However, it is advisable to regularly acquire a CBCT to identify deviations in internal anatomy during treatment. The VU University Medical center has a research collaboration with Varian Medical Systems

tection system. Each patient was set-up daily using 3 external tattoos, corresponding to the isocentre ascertained at CT/Sim. All patients were instructed to follow departmental protocols regarding bladder and rectal filling. A total of 1,334 paired KV images were taken. A database of the daily displacements was created for each patient, allowing an analysis of the effectiveness of our off-line correction protocols. The 3 protocols examined were, no action level (NAL), shrinking action level (SAL) and daily pelvic bony matching, with any deviation >3mm considered an error. Results: For the NAL protocol (all corrections undone), that is aligning to tattoos only, prostate position varied > 3mm from the tattoos in 46.6% of the total treatments in the anterior/posterior (A/P) direction, 37.2% in the superior/inferior (S/I) direction and 13.8% in the left/right (L/R) direction. For the SAL protocol these figures were reduced to 36%, 23.5% and 6.8% in the A/P, S/I and L/R directions respectively. Furthermore, had daily bony matching been implemented an even greater improvement in prostate alignment was noted, being 23.9%, 21.7% and 0.5% in the A/P, S/I and L/R directions respectively. Conclusions: In the absence of fiducial markers for the accuracy of prostate position, the absolute minimum imaging requirement is daily on-line bony anatomy matching. However it is obvious from the above results that this method does not account for inter-fraction prostate motion, and hence treatment margins need to incorporate this. 475 poster (Radiotherapy Technologists (RTT) Track) BREAST CANCER IGRT USING EXACTRAC, CAN IT BE DONE? F. Wit1 , L. ter Beek2 R EINIER DE G RAAF G ROEP, Department for Radiotherapy and Oncology, Delft, Netherlands 2 R EINIER DE G RAAF G ROEP, Department for Radiotherapy and RadioOncology, Delft, Netherlands 1

Purpose: In 2008 an ExacTrac system including a robotic table was installed in our department. The treatment machine is not a Novalis, but a Varian Clinac 2100 CD. It has two X-ray energies (6 and 15 MV), and the maximum fieldsize is 400 x 400 mm. We do about 45 patients per day on this accelerator. Of these about 10 are ExacTrac patients. At the moment we use our ExacTrac system for all pelvic field treatments, including prostate, and for stereotactic treatments. Most other institutes use a Novalis, and so are limited to only treating to a maximum fieldsize of 100 x 100 mm.The aim of this project is to look at the possibilities of using ExacTrac to position breast patients. Breast fields are far larger than 100 x 100 mm, and so cannot be done on a Novalis. We aimed to evaluate both matching on clips and matching on bony structures for both the total breast field and the booster field. Materials: Measurements were performed using a CIRS model 002LFC IMRT Thorax phantom. (CIRS Norfolk, Virginia, USA). The phantom was constructed of tissue equivalent epoxy materials. To simulate breast material, real breast prostheses were used. (Sizes 9 and 5). These were manufactured by Amoena. (Kennesaw, Georgia, USA).Up to now we have performed two rounds of testing, using 4 phantom set ups. Each time real surgical clips R) (manufactured by Ethicon Endo-surgery,llc type Ligaclip  were placed inside the breast prostheses. We used 2 different size clips to determine which would be better visible on CT: ’small’ (3.8mm) and ’medium’ clips (6mm).De CT’s were performed using a Philips Gemini PET/CT imaging system. We used our standard radiotherapy acquisition protocol The slice thickness was 2 mm. Each phantom set-up went through the same route as our breast patients; simulation, CT followed by a simulator check. We let ExacTrac calculate and perform the necessary shift. For the breast fields the shifts as calculated and performed by ExacTrac were checked by portal imaging. The booster field’s shifts as calculated by ExacTrac were checked using the simulator. Results: For the booster fields the results are excellent, and we hope to use the ExacTrac for breast patients in a clinical setting soon. For the total breast fields this is work in progress.

474 poster (Radiotherapy Technologists (RTT) Track) A RETROSPECTIVE ANALYSIS OF THE EFFECTIVENESS OF STANDARD SET-UP PROTOCOLS VERSUS DAILY FIDUCIAL MARKER MATCHING S. George1 , Y. Tuohy1 , M. Maher1 , A. Gribbin1 , K. McKenzie2 , D. Berehoudougou3 M ATER P RIVATE H OSPITAL, Radiation Therapy, Dublin, Ireland Republic of 2 M ATER P RIVATE H OSPITAL, Radiation Research, Dublin, Ireland Republic of 3 M ATER P RIVATE H OSPITAL, Radiation Therapy Physics, Dublin, Ireland Republic of 1

Purpose: To retrospectively compare the deviations measured using an online automated fiducial marker matching system, to three commonly used off-line correction protocols, in order to determine the best protocol to follow in the absence of prostate fiducial markers. Materials: Forty prostate cancer patients, each with 3 implanted fiducial markers, received IMRT / IGRT treatments using an on-line automated de-

Conclusions: Our testing of ExacTrac for breast patients is still in progress, but preliminary results show very promising results for finding the isocenter