Pancreatic Stereotactic Body Radiation Therapy Research Articles

To Compress, or to Gate? Abdominal Compression versus Respiratory Gating in Pancreatic Stereotactic Body Radiation Therapy

To Compress, or to Gate? Abdominal Compression versus Respiratory Gating in Pancreatic Stereotactic Body Radiation Therapy

Stereotactic body radiation therapy for pancreatic cancer: Assessing motion with and without abdominal compression.

234 Background: Stereotactic body radiation therapy (SBRT) is an emerging treatment option for locally advanced pancreatic cancer. This ablative therapy requires highly accurate delivery due to nearby organs at risk. To minimize tumor motion, our institution applies abdominal compression during computed tomography (CT) simulation. The purpose of this study is to evaluate the effect of compression in the context of pancreatic SBRT. Methods: In the last 6 months, 32 patients who completed SBRT to the pancreas at our institution were selected for analysis. In each patient, two 4DCT images were acquired, one with and one without abdominal compression. Abdominal compression was achieved with an indexed compression belt with a customized degree of inflation. Each patient had fiducial markers implanted in or near the pancreatic tumor prior to simulation. These fiducials were contoured on both planning CT scans for each gated phase. Motion was assessed by fiducial position changes throughout each gated phase. Results: In the anterior to posterior, transverse, and superior to inferior dimension, compression decreased motion in 19 of 32 cases (59%), 21 of 32 cases (66%), and 28 of 32 cases (88%) respectively. In the anterior to posterior (AP) dimension compression decreased motion by a mean of 0.43mm ± 1.7mm with a range of -2.1-6.5mm (p = 0.16). The mean decrease in motion with compression in the transverse dimension was 0.93mm ± 1.9mm with a range of -1.6-8.6mm (p = 0.01). In the superior to inferior dimension, compression decreased motion by a mean of 2.72mm ± 2.8mm with a range of -1.2-11.5mm (p < 0.001). Displacement of tissue due to compression led to increased patient AP separation at the level of T12 by a mean of 9.1±5.8mm (p < 0.001). Conclusions: Abdominal compression significantly reduced tumor motion in the superior to inferior and transverse directions for patients undergoing SBRT to the pancreas. This decrease in motion allows for significant reductions in the size of the volume necessary to treat the tumor. Given our findings, we would recommend using abdominal compression over free-breathing for pancreatic SBRT.

SU‐E‐J‐170: Dosimetric Consequences of Uncorrected Rotational Setup Errors During Stereotactic Body Radiation Therapy (SBRT) Treatment of Pancreatic Cancers

Purpose:To explore the dosimetric consequences of uncorrected rotational setup errors during SBRT for pancreatic cancer patients.Methods:This was a retrospective study utilizing data from ten (n=10) previously treated SBRT pancreas patients. For each original planning CT, we applied rotational transformations to derive additional CT images representative of possible rotational setup errors. This resulted in 6 different sets of rotational combinations, creating a total of 60 CT planning images. The patients’ clinical dosimetric plans were then applied to their corresponding rotated CT images. The 6 rotation sets encompassed a 3, 2 and 1‐degree rotation in each rotational direction and a 3‐degree in just the pitch, a 3‐degree in just the yaw and a 3‐degree in just the roll. After the dosimetric plan was applied to the rotated CT images, the resulting plan was then evaluated and compared with the clinical plan for tumor coverage and normal tissue sparing.Results:PTV coverage, defined here by V33 throughout all of the patients’ clinical plans, ranged from 92–98%. After an n degree rotation in each rotational direction that range decreased to 68–87%, 85–92%, and 88– 94% for n=3, 2 and 1 respectively. Normal tissue sparing defined here by the proximal stomach V15 throughout all of the patients’ clinical plans ranged from 0–8.9 cc. After an n degree rotation in each rotational direction that range increased to 0–17 cc, 0–12 cc, and 0–10 cc for n=3, 2, and 1 respectively.Conclusion:For pancreatic SBRT, small rotational setup errors in the pitch, yaw and roll direction on average caused under dosage to PTV and over dosage to proximal normal tissue. The 1‐degree rotation was on average the least detrimental to the normal tissue and the coverage of the PTV. The 3‐degree yaw created on average the lowest increase in volume coverage to normal tissue.This research was sponsored by the AAPM Education Council through the AAPM Education and Research Fund for the AAPM Summer Undergraduate Fellowship Program.

Patient Setup Management for Pancreatic Stereotactic Body Radiation Therapy: Interfraction Adjustment Between Spine Alignment and the Pancreatic Fiducial During Breath-Hold kV Imaging

Active Breathing Coordinator (ABC) is a widely used motion management strategy for the radiation therapy of mobile tumors. During treatment, aligning the patient based on spine alone does not automatically lead to an acceptable alignment with respect to soft tissue targets and adjustment is necessary. Determining the magnitude of such an adjustment is an important quality assurance measure that will guide expectations for each treatment site. It will also allow creation of guidelines to determine acceptable shifts between the spine and a given target (fiducial) for a given site. Estimating the magnitude of this adjustment for pancreatic cancer stereotactic body radiation therapy (SBRT) was the purpose of this work. This was a retrospective study of nineteen (n = 19) pancreatic SBRT patients, average age 65y (range 47 - 88y), treated between 12/2012 - 6/2013. Radio-opaque fiducial markers were implanted by endoscopic guidance into or adjacent to the pancreatic tumor to provide a target for accurate localization. For each fraction, a free-breathing CBCT was registered to a reference breath-hold CT for alignment to spine. Then, two perpendicular breath-hold kV projection images were acquired and compared with corresponding reference digitally reconstructed radiographs (DRR) to further fine-tune the alignment with the fiducial marker. By comparing the breath-hold kV projection images from subsequent treatment fractions with those from fraction 1, 3D variability of the fiducial position was derived. A 2D-to-2D image registration code was written that registered each day’s 2D projection image to the projection image from day 1. We applied normalized cross-correlation for this registration where the template was chosen at the spine. After fusing the two images at the spine region, the inter-fraction variability of the anatomical breath-hold state was then derived by analyzing the relative position of the fiducials from day n (n>1) compared to day 1 images. We observed an average inter-fraction reproducibility of 1.73 ± 0.84 mm, 1.98 ± 1.42 mm, and 3.19 ± 2.49 mm in the LR, AP and SI directions, respectively. The average excursion from free breathing spine alignment to breath-hold fiducial alignment was 1.51 ± 1.38 mm, 2.05 ± 1.91 mm, and 3.01 ± 2.04 mm in the LR, AP and SI directions, respectively. The range of excursions which can be interpreted as the average expected adjustment required after spine alignment was observed as 0.18 - 5.07 mm, 0.07 - 5.89mm, and 0.61 - 7.82 mm in the LR, AP and SI directions respectively. Our study shows that inter-fraction shifts from free-breathing spine to ABC breath-hold target can be as high as 8 mm. Values that deviate significantly from this limit should be investigated to rule out potential problems such as bowel gas or fiducial shift prior to proceeding with treatment.

OC-0505: Patient-specific motion and treatment margins in pancreatic stereotactic body radiation therapy

OC-0505: Patient-specific motion and treatment margins in pancreatic stereotactic body radiation therapy

MO‐F‐108‐02: A Retrospective Study On the Potential Clinical Gain of On‐Line Adaptive Re‐Planning for Pancreatic Cancer SBRT

Purpose: Stereotactic body radiotherapy (SBRT) shows promise in unresectable pancreatic cancer with high rates of local control compared to conventionally fractionated radiotherapy. One critical limitation with pancreas SBRT arises from toxicity in the nearby duodenum, which is a dynamic and distensible organ that changes shape daily. The purpose of this pilot study was to evaluate the potential gains of daily adaptive re‐planning with SBRT in pancreatic cancer. Methods: Pancreas SBRT patients were re‐planned using our previously developed GPU‐based supercomputing online re‐planning environment (SCORE), which automatically and quickly generates plans from daily cone‐beam CT (CBCT) data with the original plan and DVH as a reference. For each patient, the tumor volume and normal structure contours were deformed from the original planning CT to each daily CBCT. The deformed contours were reviewed and edited by a physician, and then the plan was re‐optimized to account for daily changes in anatomy. The daily adaptive re‐planned dose was compared to the actual dose received for the target volume and nearby duodenum. Results: Fourteen daily CBCT scans were used to generate daily adaptive radiation plans. For all patients, the average target coverage for the PTV improved, and the cold spot decreased with daily adaptive re‐planning. The cold spot (minimum dose to 1% of PTV) decreased on average from 25.1% to 4.6% (p=0.002, paired t‐test). With daily adaptive re‐planning the hot spot in the duodenum (volume that received 100% of dose) decreased from 2.8% to 1.7% (p=0.09). Doses to other organs were clinically insignificant, and did not significantly change with daily adaptive radiotherapy. Conclusion: This pilot study shows that daily adaptive re‐planning with pancreas SBRT has the potential to improve target coverage, and reduce dose to normal organs. Further study into the technical implementation of daily adaptive re‐planning is ongoing. Varian Medical Systems through a Master Research Agreement

SU‐E‐T‐412: What Is the Benefit of Fiducial Marker Implantation for Pancreatic SBRT?

Purpose: Localization of pancreatic tumors during Stereotactic Body Radiation Therapy (SBRT) is difficult due to the poor soft‐tissue contrast of abdominal cone‐beam CT (CBCT). The purpose of this study was to measure the increased accuracy of pancreas localization when radiopaque fiducial markers (RFM) are implanted into the tumor and peri‐pancreatic tissue to facilitate a reduction of margins and escalation of dose. Methods: 15 CBCT scans were acquired of patients implanted with RFM enrolled in a prospective study involving SBRT for pancreatic cancer, receiving 30 Gy in five fractions with PTV margins of 3–5 mm. In order to quantify the effect of RFM, matching sets of markerless CBCT images were created. An automated tracking algorithm was developed to identify the location of markers in each CBCT projection, and 2D Gaussian fitting was used to filter these markers from the images. The 3D dataset was re‐reconstructed using these filtered projections, yielding a CBCT image of the same patient geometry with no RFM. These markerless images were aligned to the reference CT images using the anterior vertebral surface/aorta and the pancreatic stent as a guide. Using the RFM images as a gold standard, the differences in position represent the error in localization due to the lack of RFM information. Results: The automated algorithm successfully yielded CBCT images without RFM, star artifacts due to markers, or artifacts due to the filtration process. When the vertebral bodies and aorta were used for localization, the average error (compared to RFM images) was 15.0±7.4 mm. When a pancreatic stent was used for localization, the average error was 3.9±2.1 mm. Conclusion: Implantation of RFM increases localization accuracy by 4–15 mm compared to conventional techniques. Marker implantation is necessary when using tight margins and/or increasing tumor dose, and can be used to increase the therapeutic ratio.

Stereotactic radiation treatment planning with volumetric modulated arc therapy: Impact of duodenal sparing on pancreatic tumor coverage.

301 Background: Volumetric modulated arc therapy (VMAT) allows for intensity-modulated radiation delivery with faster treatment times and fewer delivered monitor units (MU). The dose-limiting structure for pancreatic stereotactic body radiation therapy (SBRT) is the duodenum. We evaluate VMAT dose distribution, delivery times, and the effect of duodenal sparing (DS) for pancreas SBRT. Methods: Plans of 15 patients with unresectable pancreatic cancer (14 head/1 tail) were selected. VMAT treatment planning with the “SmartArc” function of Pinnacle v. 8.9 was used to plan one fraction of 25 Gy to the PTV (gross tumor + 2 mm expansion) normalized to the 80% isodose line. Two VMAT SBRT plans were conducted for each case; the first did not attempt to spare the duodenum (non DS) while the second did (DS). Constraints were stomach/duodenum any point max <30 Gy (for DS plan), liver D50 < 5 Gy, ipsilateral kidney D25 < 5 Gy, cord Dmax < 5 Gy and stomach D4 < 22.5 Gy. Results: Gross tumor volume ranged from 58.4cm3 to 320.3 cm3. The average overlap volume between PTV and the duodenum was 8.4 cm3. In 10/15 non-DS plans, the duodenal Dmax exceeded 30 Gy. With DS optimization, only 1/15 plans exceeded the 30 Gy threshold. These differences were statistically significant (p<0.001). Typical MU and delivery times, as calculated by the planning software, were 5494 MU and 775 secs vs. 5296 MU and 703 secs for the DS and non-DS plans, respectively. The difference in delivery times was significant (p=0.01), but amounted to only 1.2 min on average. The average duodenal Dmax for non-DS plans was 30.4Gy, D4% was 23.4 Gy. With DS, the average Dmax was reduced to 28.1Gy and D4% to <19.7 Gy (p<0.001). As expected, VMAT plans with greater overlap between the duodenum and PTV had a higher duodenal Dmax. Conclusions: This study demonstrates the feasibility of VMAT for high-dose SBRT treatment of pancreatic cancer incorporating constraints to limit the dose to the duodenum. Future studies will evaluate whether VMAT with fractionated SBRT results in improved duodenal sparing more efficiently than traditional IMRT. No significant financial relationships to disclose.