Malignant CNS tumors demand target volume delineation and field setup that start with current imaging, anatomy-aware margins, and a disciplined clinical workup. Before contouring begins, the chapter calls for a detailed history, a neurologic-focused examination, appropriate laboratory studies including hormonal assessment when relevant, baseline blood counts for patients receiving chemotherapy, visual field and visual acuity testing, audiometry, and baseline neurocognitive evaluation.
When the lesion is surgically accessible, maximal safe resection with the goal of gross-total resection remains the backbone of management. Definitive radiotherapy is reserved for biopsy-only cases, while most patients move to postoperative treatment. For a broader view of the subject, see our Target Volume Delineation and Field Setup – Complete Clinical Guide.
General principles of target delineation in malignant CNS tumors
The short answer is that MRI timing and sequence selection shape the whole plan. The chapter recommends treatment-planning MRI obtained close to CT simulation, because post-contrast T1 and FLAIR define tumor more clearly, while T1 helps with hippocampal contouring and 3D T2 or CISS sequences help trace cranial nerves and normal anatomy.
In practice, that imaging combination cuts down uncertainty before margins are ever expanded. The chapter also reviews the available delivery options, including 3D-CRT, FSRT, IMRT, VMAT, SRS, and proton beam radiotherapy, but the point is clear: technique choice only works when the target and organs at risk are contoured correctly. Figure 30.1 makes that concrete in a right temporal anaplastic astrocytoma with a additional parietal lesion: the first volume goes to 50.4 Gy from the post-contrast FLAIR-defined disease, the second to 59.4 Gy from the cavity and residual tumor on post-contrast T1, and the anatomic expansions are deliberately kept from crossing midline, entering the prepontine cistern, extending into skull, or passing the tentorium into the posterior fossa.
Patient positioning, immobilization, and simulation
The goal here is reproducible treatment geometry from the first scan onward. Patients with malignant primary brain tumors are typically simulated supine, with the arms parallel to the body and the shoulders in a natural resting position.
An indexed 3-point thermoplastic mask is the standard for CT simulation and treatment. A 5-point mask is added when skull-base disease or proximity to the optic apparatus makes neck positioning more critical. For MRI simulation and treatment, the chapter recommends a clam shell mask. Head and chin stay neutral unless there are specific skull-base instructions.
Axial CT images should cover the full head through the shoulders, using 1 mm slices for SRS, FSRT, or proton therapy and 2 mm slices for 3D-CRT, IMRT, or VMAT. Co-registration of diagnostic MRI is strongly recommended unless there is a clear medical or clinical contraindication. Intravenous contrast can also help outline the primary tumor, the postoperative cavity, or the fusion with pretreatment MRI.
Normal structures
For primary intracranial tumors, organs at risk belong on the planning CT with direct support from pretreatment MRI. When disease approaches critical structures, the chapter advises creating planning risk volumes to guide dosimetry and plan review.
Table 30.1. Suggested organs at risk for primary brain tumors
This list is the chapter’s default checklist before a plan is signed off. It is intentionally broad because several of these contours directly limit the allowed anatomic expansion and change how the final dose distribution is judged.
| Structure |
|---|
| Brain |
| Uninvolved brain (brain minus GTV or CTV, depending on the clinical scenario) |
| Brainstem (brainstem core, brainstem surface) |
| Spinal cord |
| Right cochlea |
| Left cochlea |
| Right globe |
| Left globe |
| Right lens |
| Left lens |
| Right optic nerve |
| Left optic nerve |
| Optic chiasm |
| Right retina |
| Left retina |
| Right lacrimal gland |
| Left lacrimal gland |
| Right temporal lobe |
| Left temporal lobe |
| Right hippocampus |
| Left hippocampus |
| Hypothalamus |
| Pituitary |
Source: Target Volume Delineation and Field Setup: A Practical Guide for Conformal and Intensity-Modulated Radiation Therapy, 2nd Edition (Table 30.1)
The chapter notes that contouring examples for primary intracranial cases are shown in the benign CNS chapter. Even without those figures in front of you, the practical message is obvious: uninvolved brain, optic pathways, brainstem, cochleae, and hippocampi are central planning structures, not optional extras.
High-grade glioma
For high-grade glioma, the chapter stays disciplined: maximal safe resection first, chemotherapy when indicated, and margins that follow the disease pattern seen on MRI. Patients with high-grade astrocytoma and oligodendroglioma undergo resection for diagnosis, molecular characterization, and removal of as much gross disease as can be safely taken out.
Standard treatment is conventionally fractionated radiotherapy to 59.4-60 Gy with chemotherapy in the concurrent or adjuvant setting. The chapter also makes room for hypofractionated schedules in poor-risk, elderly, or frail patients, including 40.05 Gy in 15 fractions or 25 Gy in 5 fractions, with reduced 0.5-1 cm margins and with or without chemotherapy. Figure 30.4 is a useful reminder that a smaller CTV is still selective rather than casual: in that frontal glioblastoma case, the 0.5 cm CTV still had to include potential contralateral spread routes such as the genu of the corpus callosum.
The case examples sharpen the differences between workflows. In Figure 30.2, a non-enhancing anaplastic astrocytoma treated to 59.4 Gy in 33 fractions after gross-total resection used a post-contrast FLAIR-defined GTV that included the cavity, followed by a 1.5 cm anatomically constrained CTV and a 0.3 cm PTV. In Figure 30.3, a right parietal glioblastoma after subtotal resection received two sequential volumes, 46 Gy to the T2/FLAIR-defined disease and 60 Gy to the cavity and residual enhancement on post-contrast T1, both with 2 cm anatomically constrained CTVs and a 0.3 cm PTV. Figure 30.5 then shows a different situation: a posterior temporal glioblastoma with no meaningful FLAIR extension beyond the enhancing tumor was treated to 40 Gy in 15 fractions to a single volume with a 1.0 cm CTV and 0.3 cm PTV, while Figure 30.6 describes 30 Gy in 5 fractions for a cerebellar glioblastoma after gross-total resection, using a 0.5 cm CTV and a 0.3 cm PTV.
Table 30.2. Recommended target volumes for high-grade glioma
This table is where the chapter becomes operational. It brings together the dose schedules, the MRI reference for each GTV, and the CTV expansion that separates enhancing anaplastic disease, non-enhancing anaplastic disease, glioblastoma, and gliosarcoma.
| Tumor type | Recommended dose/fractionation | GTV definition | Suggested CTV expansions | PTV expansions |
|---|---|---|---|---|
| Anaplastic glioma (enhancing tumor) | Sequential cone down: PTV1 50.4 Gy at 1.8 Gy/fraction; PTV2 59.4 Gy at 1.8 Gy/fraction. Simultaneous integrated boost: PTV1 54.45 Gy at 1.65 Gy/fraction; PTV2 59.4 Gy at 1.8 Gy/fraction. |
GTV1 is defined by the T2 or FLAIR volume. GTV2 is defined by the post-operative cavity and residual tumor on post-contrast T1 MRI. |
CTV1 is defined by a 1.5 cm expansion, reduced around natural barriers to tumor spread. CTV2 is defined by a 1.0 cm expansion, reduced around natural barriers to tumor spread. |
0.3-0.5 cm, depending on IGRT frequency, radiotherapy technique, and daily patient-positioning technology. |
| Anaplastic glioma (non-enhancing tumor) / IDH-wildtype diffuse astrocytoma | PTV1 59.4 Gy at 1.8 Gy/fraction. | GTV is defined by the post-operative cavity volume and residual tumor on T2 or FLAIR. | CTV is defined by a 1.5 cm expansion, reduced around natural barriers to tumor spread. | 0.3-0.5 cm, depending on IGRT frequency, radiotherapy technique, and daily patient-positioning technology. |
| Glioblastoma | PTV1 46 Gy at 2 Gy/fraction; PTV2 60 Gy at 2 Gy/fraction (sequential cone down). PTV1 50-51 Gy at 1.67-1.7 Gy/fraction; PTV2 60 Gy at 2 Gy/fraction (simultaneous integrated boost). |
GTV1 is defined by the T2 or FLAIR volume. GTV2 is defined by the post-operative cavity and residual tumor on post-contrast T1 MRI. |
CTV1 is defined by a 2 cm expansion, reduced around natural barriers to tumor spread. CTV2 is defined by a 2 cm expansion, reduced around natural barriers to tumor spread. |
0.3-0.5 cm, depending on IGRT frequency, radiotherapy technique, and daily patient-positioning technology. |
| Gliosarcoma | PTV1 46 Gy at 2 Gy/fraction; PTV2 60 Gy at 2 Gy/fraction (sequential cone down). PTV1 50-51 Gy at 1.67-1.7 Gy/fraction; PTV2 60 Gy at 2 Gy/fraction (simultaneous integrated boost). |
GTV1 is defined by the T2 or FLAIR volume. GTV2 is defined by the post-operative cavity and residual tumor on post-contrast T1 MRI. |
CTV1 is defined by a 1.5-2 cm expansion, reduced around natural barriers to tumor spread. CTV2 is defined by a 1.5-2 cm expansion, reduced around natural barriers to tumor spread. |
0.3-0.5 cm, depending on IGRT frequency, radiotherapy technique, and daily patient-positioning technology. |
Source: Target Volume Delineation and Field Setup: A Practical Guide for Conformal and Intensity-Modulated Radiation Therapy, 2nd Edition (Table 30.2)
The chapter treats gliosarcoma as a glioblastoma-equivalent planning problem. Figure 30.7 follows that rule with two volumes, 46 Gy and 60 Gy, FLAIR-based and post-contrast T1-based GTVs, 1.5 cm anatomically constrained CTVs, and a 0.3 cm PTV. When the disease is metastatic rather than primary, the planning logic changes substantially, so it is worth comparing this chapter with our article on Brain Metastases: WBRT, SRS, and Delineation.
Meningioma and hemangiopericytoma
Here the planning problem shifts away from infiltrative edema and toward reconstruction of preoperative extent. In grade II and grade III meningioma, as well as hemangiopericytoma, dural attachments, involved bone, and any brain invasion determine what the postoperative target should really contain.
The chapter reminds the reader that meningiomas are the most common primary intracranial tumors in adults, even though fewer than 30% are atypical WHO grade II or malignant WHO grade III lesions. After gross-total resection of a grade II meningioma, adjuvant radiotherapy can be considered; after subtotal resection, it is recommended. For grade III meningioma, adjuvant radiotherapy is recommended regardless of resection extent.
One practical caution matters here: skull and normal brain are not automatically natural barriers when grade II or III meningiomas invade bone or brain. If operative or pathology findings show brain invasion, the margin should include normal brain. After resection of a hemangiopericytoma, adjuvant radiotherapy is also recommended.
Table 30.3. Recommended target volumes for grade II/III meningioma and hemangiopericytoma
This table condenses the situations where preoperative dural attachment and osseous involvement change both the GTV definition and the CTV width. In high-grade meningioma, that detail carries as much weight as the final prescription dose.
| Tumor type | Recommended dose/fractionation | GTV definition | Suggested CTV expansions | PTV expansions |
|---|---|---|---|---|
| Grade II meningioma (upfront) | PTV 54-59.4 Gy at 1.8 Gy/fraction. | GTV is defined by the post-operative cavity and residual tumor including suspicious dural and/or bone involvement on post-contrast T1 MRI. | CTV is defined by a 0.5 cm expansion, reduced around natural barriers to tumor spread. | 0.3-0.5 cm, depending on IGRT frequency, radiotherapy technique, and daily patient-positioning technology. |
| Grade II meningioma (recurrent) | PTV 54-59.4 Gy at 1.8 Gy/fraction. | GTV is defined by the post-operative cavity and residual tumor including suspicious dural and/or bone involvement on post-contrast T1 MRI. Evaluation of prior dural attachment at initial diagnosis is also recommended. | CTV is defined by a 0.5-1.0 cm expansion, reduced around natural barriers to tumor spread. | 0.3-0.5 cm, depending on IGRT frequency, radiotherapy technique, and daily patient-positioning technology. |
| Grade III meningioma (upfront or recurrent) | PTV 59.4-60 Gy at 1.8-2 Gy/fraction. | GTV is defined by the post-operative cavity and residual tumor including suspicious dural and/or bone involvement on post-contrast T1 MRI. Evaluation of prior dural attachment at initial diagnosis is also recommended. | CTV is defined by a 1-1.5 cm expansion, reduced around natural barriers to tumor spread. | 0.3-0.5 cm, depending on IGRT frequency, radiotherapy technique, and daily patient-positioning technology. |
| Hemangiopericytoma | PTV 59.4-60 Gy at 1.8-2 Gy/fraction. | GTV is defined by the post-operative cavity and residual tumor including suspicious dural and/or bone involvement on post-contrast T1 MRI. | CTV is defined by a 1.5 cm expansion, reduced around natural barriers to tumor spread, but should include the entirety of involved bone. | 0.3-0.5 cm, depending on IGRT frequency, radiotherapy technique, and daily patient-positioning technology. |
Source: Target Volume Delineation and Field Setup: A Practical Guide for Conformal and Intensity-Modulated Radiation Therapy, 2nd Edition (Table 30.3)
Figure 30.8 shows the logic in an atypical parafalcine meningioma: the GTV includes the surgical bed, the original dural attachments, and residual nodularity at the medial cavity margin involving the falx, followed by a 0.5 cm CTV and a 0.3 cm PTV. Figure 30.9 keeps the same emphasis on preoperative extent in a malignant parasagittal frontoparietal meningioma, but the anatomically constrained margin increases to 1.0 cm before the 0.3 cm PTV is added.
If one practical summary has to survive from this chapter, it is this: in malignant CNS tumors, the best plan comes from the right MRI, the right respect for anatomic barriers, and the right margin for the histology in front of you. Technique and fractionation only become meaningful after that foundation is in place.
