Gastrointestinal Cancer

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SU_1_2008 - The Utility of Positron Emission Tomography (PET) To Guide Radiation Therapy Target Volume Determination As Compared With Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) for Rectal Cancer

Sunday, October 21
1:15 PM - 2:45 PM
Location: Innovation Hub, Exhibit Hall 3

The Utility of Positron Emission Tomography (PET) To Guide Radiation Therapy Target Volume Determination As Compared With Magnetic Resonance Imaging (MRI) and Computed Tomography (CT) for Rectal Cancer
R. Fain III1, J. Lorenz1, X. Chen1, A. Li1, B. A. Erickson1, and W. A. Hall2,3; 1Medical College of Wisconsin, Milwaukee, WI, 2Medical College of Wisconsin and Clement J Zablocki VA Medical Center, Milwaukee, WI, 3Clement J Zablocki VA Medical Center, Milwaukee, WI

Purpose/Objective(s): The role of positron emission tomography (PET) in the management of locally advanced and oligometastatic rectal cancer to guide radiation therapy (RT) treatment volumes is unclear. We investigated the pre-treatment use of PET scans obtained during initial workup to determine its effect on the RT treatment volumes in patients with locally advanced and oligometastatic rectal cancer.

Materials/Methods: A list of locally advanced and oligometastatic rectal cancer patients treated in the department of radiation oncology from 01/2004 through 11/2017 was compiled. Those that received palliative RT were excluded from analysis. A review of each patient’s medical record identified those that had undergone a PET scan prior to initiation of any oncologic interventions. Oncology provider notes were reviewed, and the reports and images of PET scans were compared to those of computed tomography (CT) and magnetic resonance imaging (MRI) staging studies. In addition, PET scans were individually reviewed to determine if RT treatment volumes, including fields and contour volumes, were altered by the PET results.

Results: One hundred ninety six patients were identified. Of those, 50 patients (26%) had undergone PET scans prior to treatment. The majority of PET scans were acquired as part of routine initial work up (n=40), some were specifically obtained to evaluate an equivocal finding seen on CT imaging (n=7) or secondary to a patient’s inability to have IV contrast (n=3). The median age of patients undergoing PET was 56 and the median distance of the primary tumor from the anal verge was 7.5 cm. The majority of patients (66%) had clinical stage III disease followed by 18% and 16% of patients being classified as Stage IV and Stage II, respectively. The results of the PET scan changed the clinical stage in 2 (4%) patients and the RT plan in 3 (6%) patients. Of the 3 patients with a change in RT plan, 1 patient had additional metastatic disease identified and as a result definitive RT was not offered. The additional 2 patients had alterations in radiation fields and contour volumes to cover additional nodal disease seen on PET which was not appreciated on the original CT and MRI imaging. Additionally, MRI identified a greater number of suspicious lymph nodes compared to PET. The mean number of lymph nodes identified by MRI was 3.1 (range: 0-23) compared to 1.7 by PET (range: 0-12).

Conclusion: Pre-treatment PET scans in patients with rectal cancer did not influence the RT plan or management in the vast majority of cases reviewed. These results demonstrate that PET appears to provide little information beyond CT and MRI to adjust RT volumes in rectal cancer. PET scans may hold utility when a patient’s co-morbidities prevent administration of IV contrast or when staging CT or MRI studies produce equivocal findings.

Author Disclosure: R. Fain: Employee; Medical College of Wisconsin. J. Lorenz: None. X. Chen: None. A. Li: None. B.A. Erickson: Employee; ProHealth Care, Waukesha, WI. Steering committee; American Brachytherapy Society.

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