Physics Contribution
Characterization and Management of Interfractional Anatomic Changes for Pancreatic Cancer Radiotherapy

https://doi.org/10.1016/j.ijrobp.2011.12.073Get rights and content

Purpose

To quantitatively characterize interfractional anatomic variations in pancreatic cancer radiotherapy (RT) and to study dosimetric advantages for using an online adaptive replanning scheme to account for these variations.

Methods and Materials

Targets and organs at risk (OAR) were delineated by autosegmentation based on daily computed tomography (CT) images acquired using a respiration-gated in-room CT during daily image-guided RT (IGRT) for 10 pancreatic cancer patients. Various parameters, including the maximum overlap ratio (MOR) between the volumes based on planning and daily CTs for a structure, while the overlapping volumes were maximized, were used to quantify the interfractional organ deformation with the intrafractional variations largely excluded. An online adaptive RT (ART) was applied to these daily CTs. To evaluate the dosimetric benefits of ART, the dose distributions from the online ART were compared to those from the repositioning in the current standard IGRT practice.

Results

The interfractional anatomic variations, particularly the organ deformation, are significant during pancreas irradiation. For the patients studied, the average MORs of all daily CTs were 80.2%, 61.7%, and 72.2% for pancreatic head, duodenum, and stomach, respectively. The online ART leads to improved dosimetric plan with better target coverage and/or OAR sparing than IGRT repositioning. For the patients studied, the mean V50.4 Gy (volume covered by 50.4 Gy) for the duodenum was reduced from 43.4% for IGRT to 15.6% for the online ART scheme.

Conclusions

The online adaptive RT scheme can effectively account for the significant interfractional anatomic variations observed in pancreas irradiation. The dosimetric advantages with the online ART may enable safe dose escalation in radiation therapy for pancreatic cancer.

Introduction

Pancreatic cancer is the fourth leading cause of cancer death in the United States, with a 5-year survival rate consistently below 5% (1). The only curative option is surgery, but most patients have locally advanced, unresectable, or metastatic disease when diagnosed. Concurrent chemotherapy and radiotherapy (RT) are combined to improve treatment of unresectable disease. During multifractional RT for pancreatic cancer, inter- and intrafractional variations have been reported to be significant 2, 3, 4, 5. Such a large variation demands a large margin between the planning target volume (PTV) and clinical target volume (CTV). Because of such a large margin, the delivery of curative radiation dose to the pancreatic tumor is prohibited by the surrounding radiation sensitive organs at risk (OARs), including the duodenum, stomach, liver, kidneys, bowels, and spinal cord. The duodenum, which is adjacent to the pancreas, becomes the most dose-limiting organ because gastrointestinal bleeding ulcers and perforation increase in frequency with doses greater than 54 Gy (6).

To reduce the large margins used in the RT for pancreatic cancer, a variety of techniques have been introduced to manage inter- and intrafractional variations 7, 8, 9, 10, 11, 12, 13, 14, 15. For example, the intrafractional variation, mostly respiratory motion in the abdomen, can be managed by gating 7, 8. Image-guided RT (IGRT) is being used to address interfractional setup variations and translational motion 8, 9, 10, 11, 12, 13, 14, 15. Currently, the most commonly used IGRT method is to reposition the patient based on the rigid-body registration of the planning computed tomography (CT) and the CT of the day, acquired immediately before the treatment delivery (12). This method is incapable of addressing organ deformation and rotation and independent motion between different organs, which are the major components of interfractional motion of pancreatic tumors. As a result of this incapability, the CTV-to-PTV margins used in IGRT for pancreatic cancer remain large.

With the availability of high-contrast imaging of soft tissue (e.g., diagnostic quality CT from in-room CT and on-board kV cone beam CT) for IGRT, innovative techniques are being developed to perform online adaptive RT (ART) by either modifying (14) or reoptimizing completely (15) the treatment plan and then delivering the adaptive plan without moving the patient. Online ART enables tailoring the dose distribution to conform to the anatomy of the day, with the capability of addressing organ rotation and deformation and independent motion between different organs that are not taken into account by the current standard of IGRT practice. Among a number of online ART strategies 7, 8, 9, 10, 11, 12, 13, 14, a real-time adaptive replanning scheme (11) has been reported recently that features two distinct steps: (a) morphing beam segment shapes to match the new locations and shapes of the target and normal structures, and (b) optimizing the weights for the new apertures. The scheme has been explored for several anatomic sites including prostate (16), head, and neck (17). It has been found that the online replanning can maintain the plan quality as the original plan based on the planning CT and can be completed within 8 min for prostate RT (16). This scheme has now been implemented in a commercial planning system (RealArt, Prowess Inc.).

Online ART is appealing for the treatment of pancreatic cancer, which has not been previously explored. In this study, we retrospectively analyzed the daily CT data sets recently acquired with a respiration-gated in-room CT during IGRT for pancreatic cancer. First, we quantitatively characterized the interfractional anatomic variations (particularly organ deformation), and second, we examined the effectiveness of using the online ART strategy to address these variations by comparing the dosimetric benefits of intensity-modulated RT (IMRT) plans of the online ART over the standard repositioning scheme. Unlike most of the studies reported previously, where inter- and intrafractional variations were coupled, the intrafractional (respiration) motion was small (<3 mm) in the data analyzed in this work, as respiration gating was used for daily CT acquisition as well as for radiation delivery when the respiration motion was greater than 3 mm.

Section snippets

Methods and Materials

A total of 107 daily kV CTs selected for 10 pancreatic cancer patients treated with daily CT-guided repositioning using a CT-on-Rails (CTVision, Siemens) were analyzed retrospectively. Each of these daily CT sets was randomly selected from every 3 consecutive fractions, so that the selected daily CT sets were evenly distributed among all fractions and were representative of each patient. Intrafractional respiratory motions were either small (<3 mm) or were reduced to less than 3 mm by

Results and Discussion

Substantial interfractional anatomic variations were observed in most daily CT sets analyzed. For example, Figure 1 presents planning CT contours in axial, sagittal, and coronal views overlaid with daily CT contours (others) for pancreatic head of one patient by aligning the COM of pancreatic head (CTV) of daily CT contours to that of the planning CT contours, mimicking the repositioning scheme based on soft-tissue alignment. These data were used to assess the CTV-to-PTV margin for the

Conclusions

With the intrafractional motion largely excluded, we have quantified the interfractional organ variations in pancreatic head irradiation by using various parameters including dice similarity coefficient and maximum overlap ratio and found that the interfractional organ deformations for abdominal organs are significant. We propose to use an online adaptive replanning scheme to account for the large interfractional variations in pancreatic head RT. The plan comparison shows that an adaptive plan

References (19)

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This work was supported in part by the Medical College of Wisconsin Cancer Center Meinerz Foundation, Siemens Oncology Care Systems, and Prowess Inc.

Conflict of interest: none.

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