Craniospinal Irradiation in Medulloblastoma
For average-risk medulloblastoma (M0 disease, classic histology, GTR/NTR), use 23.4 Gy CSI with involved field boost to 54 Gy; for high-risk disease (M+ or STR, large cell/anaplastic histology), use 36 Gy CSI with boost to 54-55.8 Gy; for very high-risk disease (MYC amplification), use 36 Gy CSI with boost to 54-55.8 Gy plus carboplatin prior to each fraction for Group 3 tumors. 1
Risk Stratification Framework
Risk stratification is critical to determining the appropriate CSI dose 1. The NCCN 2025 guidelines provide clear criteria:
Average Risk
Must meet all of the following:
- M0 disease (no metastases)
- Classic histology
- Gross total resection (GTR) or near-total resection (NTR)
Treatment: 23.4 Gy CSI + 54 Gy involved field boost 1
High Risk
Includes Groups 3 and 4 (Non-WNT/non-SHH) with:
- M+ disease (metastatic) OR
- Subtotal resection (STR) OR
- Large cell/anaplastic histology
Treatment: 36 Gy CSI + 54-55.8 Gy involved field boost 1
Very High Risk
- MYC amplification
Treatment: 36 Gy CSI + 54-55.8 Gy boost + carboplatin prior to each RT fraction for Group 3 tumors only 1
Critical Technical Considerations
Boost Volume Definition
The involved field boost should target the tumor bed with a 2-cm clinical target volume margin around the surgical bed 2, 3. This approach has demonstrated excellent local control with a 5-year posterior fossa control rate of 90.5% 2.
Age Restrictions
Radiation is not recommended for patients < 3 years of age 1. For patients > 3 years, radiation-avoiding strategies may be considered at the treating physician's discretion, though these guidelines apply to radiation-inclusive treatment strategies 1.
Evidence Supporting Dose Selection
Average-Risk Disease
The reduced-dose CSI approach (23.4 Gy) is supported by prospective multi-institutional data showing 83% 5-year event-free survival when combined with conformal boost and dose-intensive chemotherapy 3. However, standard-dose CSI may be more efficient than low-dose CSI based on COG trial data 1. A historical trial comparing 23.4 Gy versus 36 Gy showed increased early relapse risk with the reduced dose 4, but modern protocols incorporating conformal boost techniques and intensive chemotherapy have improved outcomes.
High-Risk Disease
The 36 Gy CSI dose for high-risk patients is based on prospective data showing 70% 5-year event-free survival in high-risk patients 5. This represents a significant improvement over historical outcomes.
Carboplatin Radiosensitization
For very high-risk Group 3 tumors with MYC amplification, carboplatin should be given prior to each RT fraction 1. A randomized phase III trial demonstrated that carboplatin during radiation improved event-free survival by 19% specifically in high-risk Group 3 medulloblastoma 1.
Common Pitfalls to Avoid
Do not use reduced-dose CSI (23.4 Gy) for high-risk patients - This is associated with increased relapse risk 4
Ensure adequate molecular characterization - Groups 3 and 4 are now combined as "Non-WNT/non-SHH" medulloblastoma, but Group 3 tumors with MYC amplification require carboplatin radiosensitization 1
Monitor for incomplete response after RT - Failure to achieve complete response after RT is an independent adverse prognostic factor (5-year EFS 39.3% vs 75.7%, p=0.007) 6
Large cell/anaplastic histology alone as a risk factor remains unclear - The guidelines note uncertainty about whether this histology alone constitutes high-risk disease 1
Concurrent and Adjuvant Therapy
All risk categories require chemoradiation followed by maintenance chemotherapy 1. For average-risk disease, weekly vincristine during RT followed by COG or St. Jude maintenance protocols is recommended 1. For high/very-high-risk disease, use St. Jude or ACNS0332 protocols (6 cycles of cisplatin, cyclophosphamide, vincristine) 1.
Monitor for vincristine-associated neuropathy and cisplatin-associated ototoxicity throughout treatment 1.