Successful treatment of a free-moving abdominal mass with radiation therapy guided by cone-beam computed tomography: a case report
© Dabaja et al; licensee BioMed Central Ltd. 2010
Received: 3 December 2009
Accepted: 19 October 2010
Published: 19 October 2010
Because tumors in the abdomen can change position, targeting these tumors for radiation therapy should be done with caution; use of daily image-guided radiation therapy is advised.
We report the case of a 72-year-old Caucasian man with recurrent mantle cell lymphoma who was referred for palliative radiation therapy for an abdominopelvic tumor. Computed tomography was used to generate images for radiation treatment planning. Comparison of those planning images with a positron emission tomography/computed tomography scan ordered during the planning period revealed that the tumor had moved from one side of the abdomen to the other during the three-day interval between scans. To account for this unusual tumor movement, we obtained a second set of planning computed tomography scans and used a Varian cone-beam computed tomography scanner with on-board imaging capability to target the tumor before each daily treatment session, leading to successful treatment and complete resolution of the mass.
Abdominal masses associated with the mesentery should be considered highly mobile; thus, radiation therapy for such masses should be used with the utmost caution. Modern radiation therapy techniques offer the ability to verify the tumor location in real time and shift the treatment ports accordingly over the course of treatment.
One of the most important challenges for the safe delivery of radiation therapy is the accurate application of three-dimensional conformal radiation therapy (3DCRT). The application of computed tomography (CT) in the 1970s to generate beam's-eye view images spurred the development of CT-based treatment simulation and planning for 3DCRT [1, 2]. The benefit of conformal therapy lies in targeting the tumor area with smaller radiation fields while sparing the surrounding critical organs. But the benefit of better targeting came with the additional challenge of creating consistently reproducible means of positioning patients for multiple treatment sessions. Maintaining reproducibility among treatments involves multiple issues, including the devices used for patient immobilization and accounting for differences in set-up between sessions, changes in tumor size or volume between sessions, and the motion of internal organs during and between sessions. The International Commission on Radiation Units and Measurements (ICRU) addressed the issue of consistency in volume and dose specifications in radiation therapy in consecutive reports published between 1978 and 1999 [3–5]. These reports gave the radiation oncology community a consistent language and methodology for image-based, tumor volume-based treatment planning. Nevertheless, some patients present with tumors in locations that do not conform to known rules, and therefore treatments prescribed according to guidelines such as the ICRU reports can potentially miss the target and mistreat the patient. Here we describe the case of a patient who presented with an abdominopelvic lymphoma mass that could have been completely missed with conventionally planned treatment ports because of the extensive motion of the tumor within the abdomen.
Radiation treatment was simulated and planned based on CT scanning as follows. CT scans (5-mm slices) were obtained over the course of several days for planning purposes; the target volume was outlined on those scans, and a radiation therapy plan was generated by a Pinnacle treatment planning system (version 8.0, Philips Medical Systems, Madison, Wisconsin, USA). During the planning process, the medical oncologist ordered a positron emission tomography (PET)/CT scan. When the results of that scan became available three days later, the radiation oncologist noticed that the location of the tumor mass on the radiation planning CT scan was completely different than its location on the PET/CT scan. At that time, the patient was brought back to radiation oncology, another planning CT scan series was obtained, and the two sets of planning CT scans taken five days apart were compared. We found and confirmed that the tumor mass had moved in three dimensions, from the left side to the right side, from the lower pelvis to the above the pelvic rim, and from a mid anteroposterior location to a more anterior location, over those five days (Figures 1 and 2). We decided at that point to proceed with the treatment using a cone-beam CT device equipped with on-board imaging (Varian Medical Systems Inc., Palo Alto, California, USA). Cone-beam CT provides volumetric images in real time while the patient is immobilized in the treatment position immediately before each treatment session. We obtained cone-beam CT scans immediately before each daily treatment, which the treating radiation oncologist used to move the beam's-eye views (anteroposterior and posteroanterior) such that the tumor was contained within the radiation port. Because the extent of tumor motion ranged between 3 and 7 cm in all three dimensions and because we could not predict the direction or the extent of movement, we concluded that using cone-beam CT for daily verification of tumor position was the only way to effectively treat this mass.
The patient completed radiation therapy to a total dose of 36 Gy given in 18 treatment sessions. The volume of the mass decreased in from 213.9 cm3 before the radiation therapy to 70.2 cm3 at the completion of the radiation therapy.
Accounting for internal organ motion is a major problem in treating abdominal tumors with radiation therapy. Tumors attached to the mesentery can move significantly more than tumors located in the retroperitoneal region. Before the era of 3DCRT, the abdominal mass in this patient would have been missed in the course of daily treatments. Our use of cone-beam CT with on-board imaging capability was extremely useful in this case and allowed us to successfully treat this patient. Cone-beam CT was originally explored by Simpson et al.  as a way of generating single-slice tomograms with one gantry rotation of the linear particle accelerator (LINAC). Currently, several solutions involving CT image acquisition have been introduced into routine clinical use [7–9]. The concept of cone-based CT is based on integrating a kilovoltage (kV) x-ray source and a large-area flat panel detector on a standard LINAC to allow simultaneous fluoroscopy, radiography and volumetric kV cone-beam CT imaging. A volumetric CT image is reconstructed from data collected during a single gantry rotation.
The uncertainty of the dose distribution that would have been received by the tumor if conventional radiation planning techniques had been used is the core message of this report. We showed that the tumor would have been almost completely missed in several instances. Moreover, the cone-beam approach also allowed us to account for decreases in tumor size as well as position over the course of treatment, so the difference in planned dose distribution would have been even greater if daily changes in the tumor volume had not been accounted for. In light of the variable and unpredictable daily movement of the mesenteric mass, the tumor mass would definitely have received inadequate coverage and some days would have been completely missed by the radiation fields. We conclude from this experience that image-guided radiation therapy is both valid and useful for tracking the motion of highly mobile abdominal masses.
This report is intended as a cautionary note to the radiation oncology community to use care when treating mesenteric-based masses in the abdomen because such masses can move substantial distances and can easily be missed if treatment is planned according to the current 3DCRT guidelines.
Written informed consent was obtained from the patient for publication of this case report and accompanying images. A copy of the written consent is available for review by the Editor-in-Chief of this journal.
positron emission tomography
three-dimensional conformal radiotherapy.
This report was derived in the course of the authors' normal duties; no funding sources were involved.
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