International Journal of Radiation Oncology*Biology*Physics
Clinical investigationsTranscranial sonography: integration into target volume definition for glioblastoma multiforme
Introduction
The application of sonography in the delineation of malignant brain tumors has so far been focused on the intraoperative setting: The method’s ability to differentiate between malignant glioma and normal brain tissue at the resection margin has been confirmed by histopathologic data to be superior to microscopic inspection by the neurosurgeon (1). Recent reports from the neurology department of our institution have advanced the use of transcranial sonography (TCS) in outlining the extension of malignant brain tumors 2, 3, 4. Initially, computed tomography (CT) and TCS were compared in their ability to predict the presence of solid tumor in multiple stereotactic biopsies from 25 patients with malignant brain tumors, mainly malignant glioma (2). Solid tumor was found in 91% of 33 biopsies from CT contrast-enhancing areas, but also in 63% of biopsies from 40 low-density, nonenhancing areas, representing edema. Hyperechogenic echo-feature (the characteristic of tumor) on TCS was detected in 58 biopsy sites, 95% of which were confirmed as solid tumor, whereas only 13% of iso- or hypoechogenic locations were associated with solid tumor. Very recently, a role for TCS in tumor definition could also be demonstrated in the postoperative situation (4). Among 21 patients operated for high-grade glioma, early postoperative TCS, CT, and magnetic resonance imaging (MRI) were able to correctly predict residual tumor at the resection site in all 19, 16, and 11 cases, respectively, of the 19 patients with tumor in the biopsy from the margin.
The capability of TCS to distinguish solid tumor from edema (but not edema from normal tissue) and to determine solid tumor extension makes the method attractive for integration into radiation therapy boost planning. The fact that high-grade gliomas recur close to the initial tumor margin (5) and almost exclusively within the irradiated volume (6) has led to trials investigating the use of small-volume boost irradiation by brachytherapy (7), stereotactic radiotherapy 8, 9, or hyperfractionated proton/photon treatment (10). The use of a stereotactic radiotherapy boost is currently evaluated in randomized trials by the Radiation Therapy Oncology Group (RTOG) (11) and the European Organization for Research and Treatment of Cancer (EORTC) (12). These approaches require a precise definition of postoperative tumor remnants, to which such boost treatment is to be confined. The addition of TCS to the established imaging methods may help to avoid underestimation of boost target volumes which may be associated with target volume definition based only on contrast-enhancement on CT or MRI. The current study is a first report on the effects of integrating TCS into treatment planning for a potential stereotactic boost in 13 patients referred for sonography after subtotal resection of glioblastoma multiforme (GBM).
Section snippets
Patients and actual treatment
From January 1998 to March 1999, 13 patients with contrast enhancement on planning CT were referred for TCS as part of postoperative radiotherapy after subtotal resection of GBM. Maximum diameter on preoperative gadolinium-enhanced, T1-weighted MRI scan was 1.8 to 5.9 (median 4.0) cm. Patients were not selected for tumor size and location, performance status, and extent of subtotal resection. Whereas no sufficient temporal bone window for TCS was present in two patients (15%), TCS could be
GTV and stereotactic boost PTV defined by CT and TCS
A summary of the size of the composite gross tumor volumes (GTVs) as defined by CT and TCS and the contribution of overlap of both methods, CT only and TCS only is given in Table 1. Contributions of both methods to the composite GTV in individual patients are illustrated in Fig. 2. The median stereotactic boost planning target volume (PTV; 3-mm margin) resulting from use of CT information alone was 34 ml (range 2–134 ml). The median corresponding PTV based on inclusion of CT + TCS was 74 ml
Discussion
TCS, a noninvasive, low-cost procedure, has recently been demonstrated histologically to have a higher sensitivity than CT and MRI in predicting the presence of residual tumor at the surgical margin in patients undergoing resection of malignant glioma (4). However, a slightly lower specificity of TCS was detected. Because TCS distinguishes solid tumor from edema and normal brain, but not edema from normal brain, the method is of particular interest for high-dose radiotherapy (e.g., stereotactic
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