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Targeting Glioblastoma Multiforme

see identifying the target, and target 2

Normally radiation is given to the gross tumor with a margin (to account for microscopic spread beyond the obvious visible, or gross, disease)  A common tactic would be to add 15mm around the GTV to get the CTV and another 3mm to get the PTV as noted in the RTOG 98-03 protocol the following fields are used: (in this study the PTV gets 46Gy and the boost field (PTV2 = GTV + 3mm) gets 66, 72, 78 or 84Gy.) see other RTOG trials for target volumes

Targeting Radiation

The gross tumor volume (GTV) is defined by the physician as all known gross disease as defined by the treatment planning CT and/or postoperative MRI scan. CT to MRI image registration should be used when available. The GTV will encompass the residual gross tumor as seen on a contrast enhanced MRI scan of the head after surgery. Every attempt should be made to obtain postoperative MRI < 72 hours after surgery.

The clinical target volumes (CTV) are the GTV plus areas considered to contain microscopic disease, delineated by the treating physician, and for this study will be defined as follows: CTV1 is the residual tumor plus at least a 15 mm margin. CTV2 is the GTV (post operative residual) with no margin. Superior and inferior margins for CTV1 should be at least 15 mm +/- 1 mm depending on the resolution of the CT scan slicing. The CTVs will be limited to the brain and adjacent nervous tissue. The CTVs need not extend beyond the cranial contents. The 15 mm margin refers to a margin of non-enhancing brain or nervous tissue adjacent to the GTV.

The planning target volume (PTV) will provide a margin around the CTV to compensate for variability in treatment setup and patient motion. A minimum of 3 mm around the CTV is required to define each respective PTV. Note: The PTV does not indicate the block edge. Additional margin around the PTV to define the beam aperture is required to account for beam penumbra (block margin in a BEV display).



Radiation Therapy

2.0 Gy x 30 fractions, 5 days a week x 6 weeks for a total dose of 60 Gy

The target volume for both the initial volume and the conedown volume shall be based on the preoperative CT/MRI. This initial target volume shall include the contrast-enhancing lesion and surrounding edema (if it exists) demonstrated on CT/MRI plus a 2.0 centimeter margin. If no surrounding edema is present, the initial target volume should include the contrast enhancing lesion plus a 2.5 centimeter margin. The initial target volume will be treated to 46 Gy in 23 fractions. After 46 Gy, the tumor volume for the conedown treatment should include the contrast-enhancing lesion (without edema) on the pre-surgery CT/MRI scan plus a 2.5 centimeter margin.

Dose Limitation to Critical Structures

The lens and cervical spine must be shielded from the direct beam at all times. When possible to do without shielding gross tumor, attempts should be made to limit the dose to the optic chiasm to 54 Gy, the retina of at least one eye (but preferably both) to 50 Gy, and the brain stem to 60 Gy. When the optic chiasm must be included in the full dose, then there may be a finite unknown risk of developing blindness.