Abstract
PurposeTo evaluate use of breath-hold CTs and implanted fiducials for definition of the internal target volume (ITV) margin for upper abdominal stereotactic body radiation therapy (SBRT). To study the statistics of inter- and intra-fractional motion information.Methods and materials11 patients treated with SBRT for locally advanced pancreatic cancer (LAPC) or liver cancer were included in the study. Patients underwent fiducial implantation, free-breathing CT and breath-hold CTs at end inhalation/exhalation. All patients were planned and treated with SBRT using volumetric modulated arc therapy (VMAT). Two margin strategies were studied: Strategy I uses PTV = ITV + 3 mm; Strategy II uses PTV = GTV + 1.5 cm. Both CBCT and kV orthogonal images were taken and analyzed for setup before patient treatments. Tumor motion statistics based on skeletal registration and on fiducial registration were analyzed by fitting to Gaussian functions.ResultsAll 11 patients met SBRT planning dose constraints using strategy I. Average ITV margins for the 11 patients were 2 mm RL, 6 mm AP, and 6 mm SI. Skeletal registration resulted in high probability (RL = 69%, AP = 4.6%, SI = 39%) that part of the tumor will be outside the ITV. With the 3 mm ITV expansion (Strategy 1), the probability reduced to RL 32%, AP 0.3%, SI 20% for skeletal registration; and RL 1.2%, AP 0%, SI 7% for fiducial registration. All 7 pancreatic patients and 2 liver patients failed to meet SBRT dose constraints using strategy II. The liver dose was increased by 36% for the other 2 liver patients that met the SBRT dose constraints with strategy II.ConclusionsImage guidance matching to skeletal anatomy is inadequate for SBRT positioning in the upper abdomen and usage of fiducials is highly recommended. Even with fiducial implantation and definition of an ITV, a minimal 3 mm planning margin around the ITV is needed to accommodate intra-fractional uncertainties.
Highlights
Image guided radiotherapy (IGRT) has improved the accuracy of radiation therapy (RT) by providing 3D imaging registration based on volumetric anatomic information
The liver dose was increased by 36% for the other 2 liver patients that met the stereotactic body radiation therapy (SBRT) dose constraints with strategy II
Image guidance matching to skeletal anatomy is inadequate for SBRT positioning in the upper abdomen and usage of fiducials is highly recommended
Summary
Image guided radiotherapy (IGRT) has improved the accuracy of radiation therapy (RT) by providing 3D imaging registration based on volumetric anatomic information. A linear accelerator (LINAC) treatment machine equipped with cone beam computed tomography (CBCT) can acquire high resolution images with excellent skeletal anatomy contrast and useful soft tissue contrast. This allows for significantly improved registration and tumor for the large geometrical uncertainties in the tumor position in this anatomical region. In addition to allowing more accurate daily tumor targeting, fiducial markers provide an excellent opportunity to examine inter- and intra-fractional motion of the tumor relative to the skeletal anatomy, with the ultimate aim of minimizing target volume margins in a patient-specific manner. The relative uncertainty associated with predicting inter- and intra-fractional motion during free-breathing SBRT of abdominal tumors can have a significant impact on planning dosimetry and potentially even clinical outcomes
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