International Journal of Radiation Oncology*Biology*Physics
Clinical investigationBrainEvaluation of Peritumoral Edema in the Delineation of Radiotherapy Clinical Target Volumes for Glioblastoma
Introduction
In glioblastoma multiforme (GBM), a clear relationship between recurrence pattern and peritumoral edema has not been established. Defining the correlation between the two is necessary if routine intentional inclusion of edema within the radiation field is to be advocated and justified. In the absence of such data, the inclusion of peritumoral edema within the clinical target volume (CTV) has not been universally adopted. According to Leibel and Phillips’ Textbook on Radiation Oncology, “… little agreement exists as to the definition of the CTV or PTV” (1). The method of target delineation practiced since 1981 at the University of Texas M. D. Anderson Cancer Center (MDACC) has been to define the CTV as the gross tumor volume (GTV) + 2 cm and a planning target volume (PTV) as CTV + 0.5 cm. An alternate method articulated by the Radiation Therapy Oncology Group (RTOG) states that the initial field is defined as the peritumoral edema + 2 cm and prescribed to 46 Gy. The boost field is defined as GTV + 2.5 cm and prescribed to 60 Gy (2, 3). The rationale for including peritumoral edema is that such areas are believed to contain high concentrations of tumor cells (4, 5, 6). A potential disadvantage to such an approach is that this may lead to larger radiation treatment fields that irradiate a larger volume of brain than is necessary when there is significant edema. Proximity to the gross tumor rather than the presence of peritumoral edema may be a more important factor in predicting the initial site of recurrence (7, 8, 9, 10, 11). Although some investigators believe that peritumoral edema is directly related to infiltrating tumors cells, an alternate plausible hypothesis may be that peritumoral edema merely co-exists with infiltrating tumor cells in what is a spatial coincidence but actually reflects two independent processes. In other words, peritumoral edema may simply be the result of mass effect and vascular permeability factors secreted by the visible tumor while there is a concentration gradient of infiltrating tumor cells that is greatest when in close proximity to the GTV (12). The objective of this study was to analyze the recurrence patterns of GBM in relation to its peritumoral edema to gain a better understanding of whether peritumoral edema should be intentionally included within the CTV.
Section snippets
Patient characteristics
Forty-eight GBM patients had undergone consecutive treatment with three-dimensional (3D) conformal radiotherapy at The University of Texas M. D. Anderson Cancer Center (MDACC) between July 2000 and June 2002 and constituted the study group. All had subsequently recurred as determined by follow-up neuroimaging and its interpretation by the clinicians involved in their care. Approval for the conduct of this retrospective study was obtained from the MDACC Institutional Review Board on April 1,
Patient characteristics and tumor parameters
The study comprised 48 patients whose clinical characteristics are outlined in Table 1. The median GTV was 33.3 cm3 (range, 9.9–120.8 cm3). Median MDACC-defined CTV was 226.3 cm3 (range, 97–425.3 cm3). The median edema volume was 78.6 cm3 (range, 11.8–249.1 cm3). The median whole brain volume was 1,363.9 cm3 (range, 1,071.3–1,665.9 cm3). The median recurrence volume was 23.5 cm3 (range, 1.5–260.1 cm3). The median recurrence outside edema was 3.3 cm3 (range, 0–199 cm3). The relationship between
Discussion
After the introduction of CT as a planning method for the irradiation of intracranial tumors, many investigators have tried to define the optimal treatment volume for the malignant gliomas. The inclusion of peritumoral edema within the CTV has been an area of disagreement in planning postoperative radiotherapy for GBM. Historical RTOG protocols, including RTOG 83-02, RTOG 86-12, RTOG 97-10, and current RTOG protocols, typically include radiotherapy guidelines that include peritumoral edema in
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Conflict of interest: none.