Prostate IGRT using Implanted Fiducials: Influence of Patient Immobilization on Target Localization
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2851 Probabilistic Treatment Planning Applied to RTOG0126 Prostate Plans J. J. Gordon, N. Sayah, J. Moore, E. Weiss, J. Siebers Virginia Commonwealth University, Richmond, VA Purpose/Objective(s): To verify that probabilistic treatment planning (PTP) can generate prostate IMRT plans that are as or more robust to geometric uncertainties than conventional planning target volume (PTV) based plans. Materials/Methods: In this virtual study, plans were generated for two prostate patients using PTP versus the high dose arm of RTOG0126. RTOG PTVs expanded the clinical target volume (CTV) 5 mm posteriorly and 1 cm in other directions. PTP does not utilize PTVs. Instead it uses the inverse planning algorithm to generate dosimetric margins around target structures, guided by prob- abilistic coverage criteria. In this implementation, the desired coverage criterion was to deliver the prescription dose (PD) of 79.2 Gy to the prostate for 95% of uncertainties. That is, each patient was intended to have a 95% chance of receiving a minimum CTV dose of at least 79.2 Gy. Geometric (e.g., setup) uncertainties were assumed to be normally distributed with 3 mm standard devi- ations. Random uncertainties were incorporated into the treatment planning system (TPS) via fluence convolution. Prostate cov- erage for systematic uncertainties was then calculated using an in-house developed research objective within the Pinnacle treatment planning system (TPS). By sampling dose versus distance in multiple 3D directions, this objective found the isodose surface that provided the desired 95% coverage, and penalized voxels with dose falling between that isodose and the PD. In this way, the stan- dard Pinnacle optimizer generated a dose distribution that minimized the objective by maximizing prostate coverage at the PD. Results: For both patients, the critical tradeoff was between prostate and rectal dose. RTOG plans were able to meet plan dose volume histogram (DVH) criteria but, constrained by the pre-defined PTVs, remained somewhat below the rectum dose limits. When uncertainties were accounted for, the RTOG plans delivered the PD to the prostate for 62% and 58% of uncertainties, re- spectively. Being unconstrained by a PTV, the PTP plans were able to increase target coverage by pushing up against the rectum DVH limits. They delivered the PD for 89% and 87% of uncertainties, most likely ensuring a higher tumor control probability. Conclusions: PTP creates non-uniform dosimetric margins around CTVs. This enables it to achieve higher coverage probabilities by maximizing dosimetric margins subject to normal tissue constraints, and / or by being permissive of geometric uncertainties in directions that do not negatively impact normal tissues. The results illustrate the potential of PTP to adapt to individual anatomies, and so produce plans that are more robust to geometric uncertainties. (Supported by NIH P01CA116602.) Author Disclosure: J.J. Gordon, None; N. Sayah, None; J. Moore, None; E. Weiss, None; J. Siebers, None. 2852 Prostate IGRT using Implanted Fiducials: Influence of Patient Immobilization on Target Localization C. J. Hampton D. B. Fried Wake Forest University School of Medicine, Winston-Salem, NC Purpose/Objective(s): In an effort to enhance patient comfort and improve interfraction setup variability, a simplified custom immobilization strategy was implemented in our clinic for prostate RT patients. Using captured IGRT target localization data, an analysis was performed to determine the influence of two different methods of patient immobilization on interfraction target localization. Materials/Methods: A total of 46 prostate patients receiving radiation therapy between Feb 2006 and Feb 2008 were analyzed. Each patient was treated daily with IMRT. Patients were setup supinely and custom immobilized with either the thermoplastic Hip- Fix system, or beginning in May 2007, the Combifix knee and foot immobilization system (both by Civco Medical Solutions, Kalona, IA, USA). Daily localization was performed using commercial software (Isoloc, Civco) and 2 oblique orthogonal mega- voltage portal images. 3 gold markers implanted under TRUS guidance prior to RT were easily identified on the portal images. Stereoscopic localization techniques were used to determine lateral (RL), superior/inferior (SI), and anterior/posterior (AP) trans- lations needed to position the prostate (center-of-mass of implanted fiducials) at the treatment isocenter. Statistical analysis of the magnitude of systematic and random errors (represented by mean and standard deviation of sample data) in each direction (RL, SI, AP) were calculated per patient and for sample cohorts of patients (n = 10 per cohort) immobilized with the HipFix and Combifix systems. A two-sided Student’s t test was used to assess the significance of differences between the groups. Results: A total of 61% of all analyzed IGRT localizations (n = 1,253) had a displacement .5 mm. A majority of the localization displacements .5 mm were in the longitudinal direction for HipFix, and in the vertical direction for Combifix. No significant dif- ference was seen between the HipFix and Combifix systems with regard to the mean systematic error in any direction (RL: 3.13 mm vs. 3.65 mm; SI: 4.44 mm vs. 3.54 mm; and AP: 4.04 mm vs. 4.14 mm). The magnitude of the mean random error was significantly less in the SI (3.07 mm vs. 2.28 mm) and AP (3.30 mm vs. 2.43 mm) directions using the Combifix immobilization system (p \ 0.005). Conclusions: While the use of a daily IGRT correction strategy effectively removes both systematic and random uncertainties, the Combifix immobilization system provides custom patient immobilization while significantly reducing the mean random error in the SI and AP directions versus the HipFix system. Author Disclosure: C.J. Hampton, None; D.B. Fried, None. 2853 Quantitative Evaluation of Cone Beam Computed Tomography in Target Volume Definition for Offline Image Guided Radiation Therapy of Prostate Cancer W. Wang 1 , Q. Wu 2 , D. Yan 2 1 Cancer Hospital, Chinese Academy of Medical Sciences, Beijing, China, 2 William Beaumont Hospital, Royal Oak, MI Purpose/Objective(s): In fractionated radiotherapy treatment of prostate cancer, the position and shape of target volume can change significantly from day to day. The target motion is accounted in our offline adaptive radiotherapy treatment (ART) process by a patient-specific internal target volume (ITV) constructed from planning CT and daily helical CTs (HCT) from first week. S550 I. J. Radiation Oncology d Biology d Physics Volume 72, Number 1, Supplement, 2008
Transcript of Prostate IGRT using Implanted Fiducials: Influence of Patient Immobilization on Target Localization
S550 I. J. Radiation Oncology d Biology d Physics Volume 72, Number 1, Supplement, 2008
2851 Probabilistic Treatment Planning Applied to RTOG0126 Prostate Plans
J. J. Gordon, N. Sayah, J. Moore, E. Weiss, J. Siebers
Virginia Commonwealth University, Richmond, VA
Purpose/Objective(s): To verify that probabilistic treatment planning (PTP) can generate prostate IMRT plans that are as or morerobust to geometric uncertainties than conventional planning target volume (PTV) based plans.
Materials/Methods: In this virtual study, plans were generated for two prostate patients using PTP versus the high dose arm ofRTOG0126. RTOG PTVs expanded the clinical target volume (CTV) 5 mm posteriorly and 1 cm in other directions. PTP does notutilize PTVs. Instead it uses the inverse planning algorithm to generate dosimetric margins around target structures, guided by prob-abilistic coverage criteria. In this implementation, the desired coverage criterion was to deliver the prescription dose (PD) of 79.2Gy to the prostate for 95% of uncertainties. That is, each patient was intended to have a 95% chance of receiving a minimum CTVdose of at least 79.2 Gy. Geometric (e.g., setup) uncertainties were assumed to be normally distributed with 3 mm standard devi-ations. Random uncertainties were incorporated into the treatment planning system (TPS) via fluence convolution. Prostate cov-erage for systematic uncertainties was then calculated using an in-house developed research objective within the Pinnacle treatmentplanning system (TPS). By sampling dose versus distance in multiple 3D directions, this objective found the isodose surface thatprovided the desired 95% coverage, and penalized voxels with dose falling between that isodose and the PD. In this way, the stan-dard Pinnacle optimizer generated a dose distribution that minimized the objective by maximizing prostate coverage at the PD.
Results: For both patients, the critical tradeoff was between prostate and rectal dose. RTOG plans were able to meet plan dosevolume histogram (DVH) criteria but, constrained by the pre-defined PTVs, remained somewhat below the rectum dose limits.When uncertainties were accounted for, the RTOG plans delivered the PD to the prostate for 62% and 58% of uncertainties, re-spectively. Being unconstrained by a PTV, the PTP plans were able to increase target coverage by pushing up against the rectumDVH limits. They delivered the PD for 89% and 87% of uncertainties, most likely ensuring a higher tumor control probability.
Conclusions: PTP creates non-uniform dosimetric margins around CTVs. This enables it to achieve higher coverage probabilitiesby maximizing dosimetric margins subject to normal tissue constraints, and / or by being permissive of geometric uncertainties indirections that do not negatively impact normal tissues. The results illustrate the potential of PTP to adapt to individual anatomies,and so produce plans that are more robust to geometric uncertainties. (Supported by NIH P01CA116602.)
Author Disclosure: J.J. Gordon, None; N. Sayah, None; J. Moore, None; E. Weiss, None; J. Siebers, None.
2852 Prostate IGRT using Implanted Fiducials: Influence of Patient Immobilization on Target Localization
C. J. Hampton D. B. Fried
Wake Forest University School of Medicine, Winston-Salem, NC
Purpose/Objective(s): In an effort to enhance patient comfort and improve interfraction setup variability, a simplified customimmobilization strategy was implemented in our clinic for prostate RT patients. Using captured IGRT target localization data,an analysis was performed to determine the influence of two different methods of patient immobilization on interfraction targetlocalization.
Materials/Methods: A total of 46 prostate patients receiving radiation therapy between Feb 2006 and Feb 2008 were analyzed.Each patient was treated daily with IMRT. Patients were setup supinely and custom immobilized with either the thermoplastic Hip-Fix system, or beginning in May 2007, the Combifix knee and foot immobilization system (both by Civco Medical Solutions,Kalona, IA, USA). Daily localization was performed using commercial software (Isoloc, Civco) and 2 oblique orthogonal mega-voltage portal images. 3 gold markers implanted under TRUS guidance prior to RT were easily identified on the portal images.Stereoscopic localization techniques were used to determine lateral (RL), superior/inferior (SI), and anterior/posterior (AP) trans-lations needed to position the prostate (center-of-mass of implanted fiducials) at the treatment isocenter. Statistical analysis of themagnitude of systematic and random errors (represented by mean and standard deviation of sample data) in each direction (RL, SI,AP) were calculated per patient and for sample cohorts of patients (n = 10 per cohort) immobilized with the HipFix and Combifixsystems. A two-sided Student’s t test was used to assess the significance of differences between the groups.
Results: A total of 61% of all analyzed IGRT localizations (n = 1,253) had a displacement .5 mm. A majority of the localizationdisplacements .5 mm were in the longitudinal direction for HipFix, and in the vertical direction for Combifix. No significant dif-ference was seen between the HipFix and Combifix systems with regard to the mean systematic error in any direction (RL: 3.13 mmvs. 3.65 mm; SI: 4.44 mm vs. 3.54 mm; and AP: 4.04 mm vs. 4.14 mm). The magnitude of the mean random error was significantlyless in the SI (3.07 mm vs. 2.28 mm) and AP (3.30 mm vs. 2.43 mm) directions using the Combifix immobilization system(p \ 0.005).
Conclusions: While the use of a daily IGRT correction strategy effectively removes both systematic and random uncertainties, theCombifix immobilization system provides custom patient immobilization while significantly reducing the mean random error in theSI and AP directions versus the HipFix system.
Author Disclosure: C.J. Hampton, None; D.B. Fried, None.
2853 Quantitative Evaluation of Cone Beam Computed Tomography in Target Volume Definition for Offline
Image Guided Radiation Therapy of Prostate CancerW. Wang1, Q. Wu2, D. Yan2
1Cancer Hospital, Chinese Academy of Medical Sciences, Beijing, China, 2William Beaumont Hospital, Royal Oak, MI
Purpose/Objective(s): In fractionated radiotherapy treatment of prostate cancer, the position and shape of target volume canchange significantly from day to day. The target motion is accounted in our offline adaptive radiotherapy treatment (ART) processby a patient-specific internal target volume (ITV) constructed from planning CT and daily helical CTs (HCT) from first week.
