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Feasibility of Adapting Real-time Ultrasound Guidance for the Treatment of Prostate Cancer
Proceedings of the 53rd Annual ASTRO Meeting contours should reflect any change is size, shape and location of the OARs and GTV found in the mid-treatment CT image. A quantitative analysis of the agreement between the warped pre-treatment contours and free-drawn oncologist contours was performed using the DICE index. As the DICE index approaches 1 this indicates that the agreement between the computers generated contours and the contours drawn by the radiation oncologist is improving. The dosimetric impact of using the MIM Maestro generated contours was evaluated in two stages. Firstly, to determine the feasibility of using MIM Maestro generated contours as a quantitative indicator of the need to perform an adaptive re-plan. Secondly, to determine the impact of directly using the unmodified MIM Maestro generated contours as the basis of the adaptive re-plan. Both stages would be compared with the doses reported by radiation oncologist drawn contours on the mid-treatment CT scan. Results: The average DICE score across the 10 patients for the GTVand OARs was 0.82 ± 0.09 and 0.81 ± 0.11, respectively. It is expected that the dosimetric differences between plans derived using MIM Maestro generated contours and plans derived from radiation oncologist drawn contours will be clinically insignificant. Conclusions: The MIM Maestro deformable registration algorithm can be used to automatically generate mid-treatment contours for evaluation and re-planning purposes; thereby, improving the efficiency and making routine ART feasible. Author Disclosure: A. Ravi: None. G. Smith: None. J. Lee: None. R. Tirona: None.
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Feasibility of Adapting Real-time Ultrasound Guidance for the Treatment of Prostate Cancer
P. Qi1, K. Stephans1, J. Wong2, P. Xia1 1
Cleveland Clinic, Cleveland, OH, 2John Hopkins University, Baltimore, MD
Purpose/Objective(s): An integrated ultrasound-CBCT system has been proposed and is under development to improve realtime localization of soft tissues for IMRT or SBRT treatment of prostate cancer. With this system, the ultrasound probe can be positioned in trans-perineal and trans-abdominal locations. In this study, we investigated the latter setup since that can be also applied for other abdominal treatment sites. One major concern is that the presence of the ultrasound probe during treatment may attenuate beam intensities. To address this issue and to maintain the same treatment goals, we redesigned beam configurations for IMRT planning to ensure that beams do not pass through the probe before reaching the tumor. Materials/Methods: Ten patients with prostate cancer, who underwent intensity modulated radiation therapy (IMRT), were randomly selected for this study. The clinical plans were created with a standard configuration of 5 coplanar beams, with beam angles of 260 , 310 , 0 , 50 , and 100 . To avoid possible interferences with an ultrasound probe, the anterior beam was replaced by two posterior oblique beams. A new plan was then created for each patient, following the same planning guidelines for the clinical plans. Two types of IMRT plans were compared based on the dosimetric endpoints such as doses to 95% (D95) of the PTV, doses received by 15% of the volume (D15) of bladder and rectum, and mean doses to the femoral heads. In addition, the homogeneity index (HI) and the conformality index (CI) were calculated. Because the prescription doses varied among 10 patients, the dosimetric data was calculated relative to the corresponding prescription dose. The student t-test was used for statistical analysis. Results: Relative to the prescription dose, the ratios of D95 of the PTV were 100.149 ± 0.005 % and 100.297 ± 0.001 % for the new and original plans, respectively. The corresponding ratios of D15 of the rectum were 75.079 ± 1.742 % and 74.388 ± 2.173 %, respectively. Similarly, the corresponding ratios of D15 of the bladder were 60.625 ± 14.117 % and 60.969 ± 14.523%, respectively. Regarding the mean doses to the left and right femoral head, the ratios were 30.292 ± 0.165 % and 32.151 ± 0.110 % for new plans, and 30.381 ± 0.209 % and 31.860 ± 0.161 % for the original plans. P-value of t-test for each dosimetric endpoint was consistently . 0.3. The average HI was 1.055 and 1.051 for new and original IMRT plans while the average CI was 1.169 and 1.167 for new and original plans. Conclusions: For patients with prostate cancer, a real-time ultrasound-CBCT system can be deployed during IMRT treatment without any sacrifice of treatment plan quality. Author Disclosure: P. Qi: None. K. Stephans: None. J. Wong: None. P. Xia: None.
3318
Individual Target Margin for Stereotactic Body Radiation Therapy of Lung Cancer
B. Zheng, Y. Di William Beaumont Hospital, Royal Oak, MI Purpose/Objective(s): Treatment planning target volume (PTV) depends not only on the treatment position variation, but also on the target size/shape and individual dose distribution. To explore the effect of the individual factors on PTV design, direct construction of the individualized target margin was performed and evaluated using lung cancer patients treated in hypofractionated stereotactic radiotherapy (SBRT). Materials/Methods: Twenty-two lung cancer SBRT patients with prescription dose of 48Gy at 12Gy per fraction were included in the individualized target margin design and evaluation. Individualized target margin construction required inputs of treatment position variation including the systematic and the random variations, the number of treatment fractions, and the individual dose distribution around the clinical target volume (CTV), and is operated recursively. A pilot planning with CTV alone without extra margin was performed to obtain an initial dose distribution; while a position displacement sampling process was formed using the systematic and random variations, as well as the treatment fraction number. Applying the random samples of position displacement on each surface point of CTV and convolved them with the dose distribution, the target margin was calculated along each normal direction on the CTV surface. An individualized PTV was then formed to ensure CTV receive full prescription dose with a confidence level 90%, and verified by the sampling process again once a new plan was created using the PTV. Two treatment plans created respectively using the individualized PTVand a PTV with 5mm margin calculated based on the conventional margin recipe were compared. The target volume, as well as dose volume parameters of organs of interest, was evaluated. Results: The conventional PTV size was 25.7 - 99.8 cc (average 55.4 cc); while the individualized PTV was 18.4 - 85.2 cc (average 44.4 cc). Using the individualized PTV, the likelihood of CTV receiving full prescribed dose was 93.6%, compared to 68.3% when the conventional PTV was used.
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3317
Feasibility of Adapting Real-time Ultrasound Guidance for the Treatment of Prostate Cancer
P. Qi1, K. Stephans1, J. Wong2, P. Xia1 1
Cleveland Clinic, Cleveland, OH, 2John Hopkins University, Baltimore, MD
Purpose/Objective(s): An integrated ultrasound-CBCT system has been proposed and is under development to improve realtime localization of soft tissues for IMRT or SBRT treatment of prostate cancer. With this system, the ultrasound probe can be positioned in trans-perineal and trans-abdominal locations. In this study, we investigated the latter setup since that can be also applied for other abdominal treatment sites. One major concern is that the presence of the ultrasound probe during treatment may attenuate beam intensities. To address this issue and to maintain the same treatment goals, we redesigned beam configurations for IMRT planning to ensure that beams do not pass through the probe before reaching the tumor. Materials/Methods: Ten patients with prostate cancer, who underwent intensity modulated radiation therapy (IMRT), were randomly selected for this study. The clinical plans were created with a standard configuration of 5 coplanar beams, with beam angles of 260 , 310 , 0 , 50 , and 100 . To avoid possible interferences with an ultrasound probe, the anterior beam was replaced by two posterior oblique beams. A new plan was then created for each patient, following the same planning guidelines for the clinical plans. Two types of IMRT plans were compared based on the dosimetric endpoints such as doses to 95% (D95) of the PTV, doses received by 15% of the volume (D15) of bladder and rectum, and mean doses to the femoral heads. In addition, the homogeneity index (HI) and the conformality index (CI) were calculated. Because the prescription doses varied among 10 patients, the dosimetric data was calculated relative to the corresponding prescription dose. The student t-test was used for statistical analysis. Results: Relative to the prescription dose, the ratios of D95 of the PTV were 100.149 ± 0.005 % and 100.297 ± 0.001 % for the new and original plans, respectively. The corresponding ratios of D15 of the rectum were 75.079 ± 1.742 % and 74.388 ± 2.173 %, respectively. Similarly, the corresponding ratios of D15 of the bladder were 60.625 ± 14.117 % and 60.969 ± 14.523%, respectively. Regarding the mean doses to the left and right femoral head, the ratios were 30.292 ± 0.165 % and 32.151 ± 0.110 % for new plans, and 30.381 ± 0.209 % and 31.860 ± 0.161 % for the original plans. P-value of t-test for each dosimetric endpoint was consistently . 0.3. The average HI was 1.055 and 1.051 for new and original IMRT plans while the average CI was 1.169 and 1.167 for new and original plans. Conclusions: For patients with prostate cancer, a real-time ultrasound-CBCT system can be deployed during IMRT treatment without any sacrifice of treatment plan quality. Author Disclosure: P. Qi: None. K. Stephans: None. J. Wong: None. P. Xia: None.
3318
Individual Target Margin for Stereotactic Body Radiation Therapy of Lung Cancer
B. Zheng, Y. Di William Beaumont Hospital, Royal Oak, MI Purpose/Objective(s): Treatment planning target volume (PTV) depends not only on the treatment position variation, but also on the target size/shape and individual dose distribution. To explore the effect of the individual factors on PTV design, direct construction of the individualized target margin was performed and evaluated using lung cancer patients treated in hypofractionated stereotactic radiotherapy (SBRT). Materials/Methods: Twenty-two lung cancer SBRT patients with prescription dose of 48Gy at 12Gy per fraction were included in the individualized target margin design and evaluation. Individualized target margin construction required inputs of treatment position variation including the systematic and the random variations, the number of treatment fractions, and the individual dose distribution around the clinical target volume (CTV), and is operated recursively. A pilot planning with CTV alone without extra margin was performed to obtain an initial dose distribution; while a position displacement sampling process was formed using the systematic and random variations, as well as the treatment fraction number. Applying the random samples of position displacement on each surface point of CTV and convolved them with the dose distribution, the target margin was calculated along each normal direction on the CTV surface. An individualized PTV was then formed to ensure CTV receive full prescription dose with a confidence level 90%, and verified by the sampling process again once a new plan was created using the PTV. Two treatment plans created respectively using the individualized PTVand a PTV with 5mm margin calculated based on the conventional margin recipe were compared. The target volume, as well as dose volume parameters of organs of interest, was evaluated. Results: The conventional PTV size was 25.7 - 99.8 cc (average 55.4 cc); while the individualized PTV was 18.4 - 85.2 cc (average 44.4 cc). Using the individualized PTV, the likelihood of CTV receiving full prescribed dose was 93.6%, compared to 68.3% when the conventional PTV was used.
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