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Image Guided Radiation Therapy for Prostate IMRT: Rectum Volume Changes and Dosimetric Considerations
Proceedings of the 47th Annual ASTRO Meeting
(mean: 49cc; range: 22–106cc). The prostate, rectum, urethra, and bladder were all contoured. PV shells were calculated at uniformly spaced distances from the urethra and rectum. Seed-to-urethra (SU) and seed-to-rectum (SR) measurements were also determined. The base-to-apex length of the prostate and curvilinear prostatic urethral length was calculated. Results: Prostate mean length is 4.5 cm with 5th and 95th percentile at 3.0cm and 5.7cm. The mean curvilinear length of the prostatic urethra is at 4.4cm. The mean prostatic urethral bend is 30.3 ⫹ 12.2 degrees. Mean prostate-urethra and prostate-rectal surface distances are 2.7cm and 4.5cm (p⬍0.001,one-sided paired t-test) respectively. Mean SU and SR distances are 1.6cm and 2.3cm (p⬍0.001). The smallest and largest prostates were analyzed to determine the extent of anatomical variation. For the smallest prostate, the maximum prostate-urethra and prostate-rectum surface distance is 2.4cm and 3.7cm, respectively, whereas for the largest prostate, these distances are 3.9cm and 6.0cm. Figure 1(a) shows a plot of the normalized PV as a function of these distances. Figure 1(b) shows the cumulative percentage of seeds as a function of SU and SR distances, where the maxima for SU and SR are 3.3cm and 5.2cm, respectively. Conclusions: Based on the data analyzed in this study, an optimal TUUS imaging device would have a 4 – 6cm imaging field of view (FOV). In order to capture the entire prostate during one sweep, a custom TUUS device should have a 2.5–3cm radial imaging depth. Applications envisioned for a custom TUUS device include prostate brachytherapy, cryotherapy, thermal therapy, biopsy guidance and others.
Figure 1. Cumulative distance plots for 190 patient CT datasets. On the left, the plot shows the normalized PV as a function of radial distance for the smallest and largest prostate case. On the right, the total number of seeds within a given radial distance to the urethra and rectum is shown. Both plots confirm that a trans-urethral ultrasound imaging device requires a 3cm radial FOV versus a trans-rectal device which requires at least a 6cm FOV.
2530
Image Guided Radiation Therapy for Prostate IMRT: Rectum Volume Changes and Dosimetric Considerations
L. Chen, K. Paskalev, J. Zhu, X. Xu, L. Wang, R.A. Price, JR, E. Horwitz, S. Feigenberg, C.C. Ma, A. Pollack Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA Purpose/Objective: To investigate the change in rectal dose due to the inter-fractional rectum volume variation for patients with prostate cancer treated with intensity modulated radiation therapy (IMRT). Materials/Methods: Twenty prostate cancer patients treated with IMRT were included in this study. An enema was administered within 2 hours of simulation to empty the bowel for each patient. Each patient underwent sequential CT- and MRI simulations with a minimal rectal volume for treatment planning. MR and CT data sets were fused for target delineation. IMRT treatment planning was performed on the CT image. Inter-fractional prostate motion was corrected using a CT-on-Rails system prior to treatment. CT images were also taken after the treatment to compare target variation and isocenter shift during a treatment. In this study, rectal contours were generated on both simulation CT images and subsequent treatment CT images. IMRT plans were generated based on our clinical acceptance criterion. The subsequent treatment CT images for each patient from the CT-on-Rails system were used to recompute the patient dose distributions with the same leaf sequences used for treatment. The isocenter was shifted relative to the simulation CT, as required by the protocol, to ensure appropriate target coverage. The rectal doses based on the subsequent treatment CT were compared with the original doses planned on the simulation CT scans using our clinical acceptance criteria. Results: The results show that patient rectal volume varies significantly between fractions, and as a response to the radiation dose, decreases during the course of treatment for some patients. Figure 1 shows the dose-volume histograms for a patient, whose rectal volume changed between 50.2 and 161.7 cc during the treatment course. For an IMRT plan based on an empty rectum, all subsequent rectal DVH values satisfy our clinical acceptance criteria (V40 ⬍ 35%; V65 ⬍17%). If the IMRT plan was based on one of the subsequent CT scans without controlling the rectal volume some of the rectal DVH values might not meet the requirements of our clinical protocol. Conclusions: Due to the large inter-fractional variation of the rectal volume it is more favorable to plan prostate IMRT based on an empty rectum. Smaller rectal volumes are generally more difficult to plan for the same target volume and acceptance criteria because they represent the worst-case scenario. Accurate prostate localization is also needed to ensure that the actual doses received by the rectum will not fail the clinical treatment criteria during a treatment course. An enema
S549
S550
I. J. Radiation Oncology
● Biology ● Physics
Volume 63, Number 2, Supplement, 2005
administered within 2 hours of simulation to empty the bowel followed by treatment plans based on minimal rectal volumes often provide best-achievable rectal sparing and can lead to better clinical outcome for prostate IMRT. A study of the correlation between observed variations and treatment complications is under way.
2531
Prostate External Beam Radiotherapy Post Permanent Seed Implantation: Dosimetric Implications
D.A. Dimitroyannis Radiation Oncology, BIDMC-Harvard Medical School, Boston, MA Purpose/Objective: A dose enhancement is expected in the vicinity of high atomic number materials embedded in tissue when irradiated by bremsstrahlung photons from radiotherapy linacs. We investigate this dose enhancement in the case of prostate external beam radiotherapy post permanent radioactive seed implantation. Materials/Methods: Monte Carlo simulations were used to examine the dose enhancement effects in the tissue in the vicinity of seed implants. The GEANT4 code was used in the present work. A tissue embedded seed implant was irradiated in simulation under realistic bremsstrahlung beam energies (6MV, 10MV and 18 MV nominal) and beam apertures (15⫻15, 25⫻25 and 100⫻100 mm squared). Polymer gel dosimetry was used to verify a subset of the numerically obtained data. Results: For a popular iodine-125 radioactive seed implant encased in titanium, we found dose enhancements of 8%, 18% and 29% in the vicinity of the implant for beam energies of 6 MV, 10 MV and 18 MV respectively and for the smaller of the beam apertures simulated. Depending on the pattern of seed implantation, beam energy and irradiation field patterns, an overall dose enhancement to the prostate of up to 12% of the a-priori prescribed dose was observed. For a fixed implantation pattern, the dose enhancement increased with beam energy and decreased with beam aperture. Conclusions: Salvage external beam radiotherapy to the prostate, post seed implantation, delivered by small aperture beams, IMRT-like, warrants careful dosimetric considerations due to expected dose enhancements.
2532
Correction of Patient Positioning Errors based on In-Line Cone Beam CTs: Clinical Implementation and First Experiences
C. Thilmann,1,3 S. Nill,2 T. Tucking,2 B. Hesse,2 L. Dietrich,2 B. Rhein,2 P. Haering,2 U. Oelfke,2 J. Debus,3 P. Huber1 Dept. of Radiooncology, German Cancer Research Center, Heidelberg, Germany, 2Dept. of Medical Physics, German Cancer Research Center, Heidelberg, Germany, 3Clinical Radiology, University of Heidelberg, Heidelberg, Germany
1
Purpose/Objective: Clinical implementation of a kV cone beam CT (CBCT) for setup correction in radiotherapy (RT). Materials/Methods: For evaluation of the setup correction workflow, 6 tumor patients (prostate and lung cancer, sacral chordoma, head-and-neck and paraspinal tumor) were selected. All patients were treated with fractionated stereotactic RT, 5 of them with IMRT. For patient fixation, a scotch cast body frame or a vacuum pillow was used. The in-line imaging equipment consisted of an X-ray tube and a flat panel imager (FPI) attached to a Siemens LINAC: the imaging beam is opposite to the treatment beam sharing the same isocenter (fig.1). For dose delivery, the treatment beam has to traverse the FPI which is mounted below the multileaf collimator. For each patient, at least 220 projections over the range of 220° were acquired. The fast reconstruction of the 3D-CBCT dataset was done with an implementation of the FDK algorithm. For the registration process of the treatment planning CT with the acquired CBCT, an in-house developed mutual-information matcher was used. Results: Bony landmarks were easily detected and a table shift for correction of the setup-deviation could be automatically calculated in all cases. The image quality was sufficient for verifying the required table shift by comparison of the desired target point with the isocenter visible on the CBCT. The detected maximum setup-deviation was 3mm for patients fixed with the body frame, and 8mm for patients positioned on a vacuum pillow. Due to an action level of 2mm, a target point correction was carried out in 4 cases. In the lung and prostate cases, in particular, soft tissue was visualized with adequate image quality. The additional workload of the described workflow (fig.2) compared to a normal treatment fraction leads to an extra time of about 5–10 minutes, which will most likely be reduced further by streamlining the different steps.
(mean: 49cc; range: 22–106cc). The prostate, rectum, urethra, and bladder were all contoured. PV shells were calculated at uniformly spaced distances from the urethra and rectum. Seed-to-urethra (SU) and seed-to-rectum (SR) measurements were also determined. The base-to-apex length of the prostate and curvilinear prostatic urethral length was calculated. Results: Prostate mean length is 4.5 cm with 5th and 95th percentile at 3.0cm and 5.7cm. The mean curvilinear length of the prostatic urethra is at 4.4cm. The mean prostatic urethral bend is 30.3 ⫹ 12.2 degrees. Mean prostate-urethra and prostate-rectal surface distances are 2.7cm and 4.5cm (p⬍0.001,one-sided paired t-test) respectively. Mean SU and SR distances are 1.6cm and 2.3cm (p⬍0.001). The smallest and largest prostates were analyzed to determine the extent of anatomical variation. For the smallest prostate, the maximum prostate-urethra and prostate-rectum surface distance is 2.4cm and 3.7cm, respectively, whereas for the largest prostate, these distances are 3.9cm and 6.0cm. Figure 1(a) shows a plot of the normalized PV as a function of these distances. Figure 1(b) shows the cumulative percentage of seeds as a function of SU and SR distances, where the maxima for SU and SR are 3.3cm and 5.2cm, respectively. Conclusions: Based on the data analyzed in this study, an optimal TUUS imaging device would have a 4 – 6cm imaging field of view (FOV). In order to capture the entire prostate during one sweep, a custom TUUS device should have a 2.5–3cm radial imaging depth. Applications envisioned for a custom TUUS device include prostate brachytherapy, cryotherapy, thermal therapy, biopsy guidance and others.
Figure 1. Cumulative distance plots for 190 patient CT datasets. On the left, the plot shows the normalized PV as a function of radial distance for the smallest and largest prostate case. On the right, the total number of seeds within a given radial distance to the urethra and rectum is shown. Both plots confirm that a trans-urethral ultrasound imaging device requires a 3cm radial FOV versus a trans-rectal device which requires at least a 6cm FOV.
2530
Image Guided Radiation Therapy for Prostate IMRT: Rectum Volume Changes and Dosimetric Considerations
L. Chen, K. Paskalev, J. Zhu, X. Xu, L. Wang, R.A. Price, JR, E. Horwitz, S. Feigenberg, C.C. Ma, A. Pollack Radiation Oncology, Fox Chase Cancer Center, Philadelphia, PA Purpose/Objective: To investigate the change in rectal dose due to the inter-fractional rectum volume variation for patients with prostate cancer treated with intensity modulated radiation therapy (IMRT). Materials/Methods: Twenty prostate cancer patients treated with IMRT were included in this study. An enema was administered within 2 hours of simulation to empty the bowel for each patient. Each patient underwent sequential CT- and MRI simulations with a minimal rectal volume for treatment planning. MR and CT data sets were fused for target delineation. IMRT treatment planning was performed on the CT image. Inter-fractional prostate motion was corrected using a CT-on-Rails system prior to treatment. CT images were also taken after the treatment to compare target variation and isocenter shift during a treatment. In this study, rectal contours were generated on both simulation CT images and subsequent treatment CT images. IMRT plans were generated based on our clinical acceptance criterion. The subsequent treatment CT images for each patient from the CT-on-Rails system were used to recompute the patient dose distributions with the same leaf sequences used for treatment. The isocenter was shifted relative to the simulation CT, as required by the protocol, to ensure appropriate target coverage. The rectal doses based on the subsequent treatment CT were compared with the original doses planned on the simulation CT scans using our clinical acceptance criteria. Results: The results show that patient rectal volume varies significantly between fractions, and as a response to the radiation dose, decreases during the course of treatment for some patients. Figure 1 shows the dose-volume histograms for a patient, whose rectal volume changed between 50.2 and 161.7 cc during the treatment course. For an IMRT plan based on an empty rectum, all subsequent rectal DVH values satisfy our clinical acceptance criteria (V40 ⬍ 35%; V65 ⬍17%). If the IMRT plan was based on one of the subsequent CT scans without controlling the rectal volume some of the rectal DVH values might not meet the requirements of our clinical protocol. Conclusions: Due to the large inter-fractional variation of the rectal volume it is more favorable to plan prostate IMRT based on an empty rectum. Smaller rectal volumes are generally more difficult to plan for the same target volume and acceptance criteria because they represent the worst-case scenario. Accurate prostate localization is also needed to ensure that the actual doses received by the rectum will not fail the clinical treatment criteria during a treatment course. An enema
S549
S550
I. J. Radiation Oncology
● Biology ● Physics
Volume 63, Number 2, Supplement, 2005
administered within 2 hours of simulation to empty the bowel followed by treatment plans based on minimal rectal volumes often provide best-achievable rectal sparing and can lead to better clinical outcome for prostate IMRT. A study of the correlation between observed variations and treatment complications is under way.
2531
Prostate External Beam Radiotherapy Post Permanent Seed Implantation: Dosimetric Implications
D.A. Dimitroyannis Radiation Oncology, BIDMC-Harvard Medical School, Boston, MA Purpose/Objective: A dose enhancement is expected in the vicinity of high atomic number materials embedded in tissue when irradiated by bremsstrahlung photons from radiotherapy linacs. We investigate this dose enhancement in the case of prostate external beam radiotherapy post permanent radioactive seed implantation. Materials/Methods: Monte Carlo simulations were used to examine the dose enhancement effects in the tissue in the vicinity of seed implants. The GEANT4 code was used in the present work. A tissue embedded seed implant was irradiated in simulation under realistic bremsstrahlung beam energies (6MV, 10MV and 18 MV nominal) and beam apertures (15⫻15, 25⫻25 and 100⫻100 mm squared). Polymer gel dosimetry was used to verify a subset of the numerically obtained data. Results: For a popular iodine-125 radioactive seed implant encased in titanium, we found dose enhancements of 8%, 18% and 29% in the vicinity of the implant for beam energies of 6 MV, 10 MV and 18 MV respectively and for the smaller of the beam apertures simulated. Depending on the pattern of seed implantation, beam energy and irradiation field patterns, an overall dose enhancement to the prostate of up to 12% of the a-priori prescribed dose was observed. For a fixed implantation pattern, the dose enhancement increased with beam energy and decreased with beam aperture. Conclusions: Salvage external beam radiotherapy to the prostate, post seed implantation, delivered by small aperture beams, IMRT-like, warrants careful dosimetric considerations due to expected dose enhancements.
2532
Correction of Patient Positioning Errors based on In-Line Cone Beam CTs: Clinical Implementation and First Experiences
C. Thilmann,1,3 S. Nill,2 T. Tucking,2 B. Hesse,2 L. Dietrich,2 B. Rhein,2 P. Haering,2 U. Oelfke,2 J. Debus,3 P. Huber1 Dept. of Radiooncology, German Cancer Research Center, Heidelberg, Germany, 2Dept. of Medical Physics, German Cancer Research Center, Heidelberg, Germany, 3Clinical Radiology, University of Heidelberg, Heidelberg, Germany
1
Purpose/Objective: Clinical implementation of a kV cone beam CT (CBCT) for setup correction in radiotherapy (RT). Materials/Methods: For evaluation of the setup correction workflow, 6 tumor patients (prostate and lung cancer, sacral chordoma, head-and-neck and paraspinal tumor) were selected. All patients were treated with fractionated stereotactic RT, 5 of them with IMRT. For patient fixation, a scotch cast body frame or a vacuum pillow was used. The in-line imaging equipment consisted of an X-ray tube and a flat panel imager (FPI) attached to a Siemens LINAC: the imaging beam is opposite to the treatment beam sharing the same isocenter (fig.1). For dose delivery, the treatment beam has to traverse the FPI which is mounted below the multileaf collimator. For each patient, at least 220 projections over the range of 220° were acquired. The fast reconstruction of the 3D-CBCT dataset was done with an implementation of the FDK algorithm. For the registration process of the treatment planning CT with the acquired CBCT, an in-house developed mutual-information matcher was used. Results: Bony landmarks were easily detected and a table shift for correction of the setup-deviation could be automatically calculated in all cases. The image quality was sufficient for verifying the required table shift by comparison of the desired target point with the isocenter visible on the CBCT. The detected maximum setup-deviation was 3mm for patients fixed with the body frame, and 8mm for patients positioned on a vacuum pillow. Due to an action level of 2mm, a target point correction was carried out in 4 cases. In the lung and prostate cases, in particular, soft tissue was visualized with adequate image quality. The additional workload of the described workflow (fig.2) compared to a normal treatment fraction leads to an extra time of about 5–10 minutes, which will most likely be reduced further by streamlining the different steps.