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Craniospinal irradiation (CSI) is an integral component of multimodality management in medulloblastoma (MB). Hematological toxicity is one of the commonest toxicity observed during CSI leading to treatment breaks and increased overall treatment time. Arc based RT techniques i.e. VMAT expose large volume of normal tissue to low RT dose, which might increase the risk of bone marrow toxicity. The aim of the current study is to compare the incidence of hematological toxicity in 3DCRT and VMAT technique. 

The study was conducted in the department of radiotherapy and oncology, over a period of 18 months. MB patients who received CSI during the said period were analysed.  All patients were treated in the supine position on a 6 or 15 MV linac. 3DCRT was the most commonly employed technique. VMAT was used only if homogeneity or dosimetric constraints were not achieved. In 3DCRT technique, 2 lateral fields were used for cranial field, matched to the spinal field by collimation and couch rotation. VMAT was planned by 4-6 full arcs using 2-3 isocentres. Complete blood counts were checked twice weekly during CSI. Details of patients experiencing toxicity and treatment gap were documented in Microsoft Excel sheet. SPSS v 23 was used for statistical analysis.

A total of 20 patients were available for analysis (median age 15 years). Out of this, 13 and 7 patients were treated with 3DCRT and VMAT respectively. Seven patients experienced treatment interruption due to hematological toxicity. Details of hematological toxicity and treatment interruption is shown in Table 1. In VMAT group, 5 out of 7 (71.4%) patients had at-least grade 2 hematological toxicity as compared to only 2 out of 13 (15.4%) patients in 3DCRT group. Majority of the patients experiencing hematological toxicities were below 14 years of age. Most of the treatment interruptions were started in the 2^nd week of CSI. The incidence of hematological toxicity was significantly associated with bone marrow volume receiving low dose radiation, particularly 3 and 5 Gy (p-0.04).  Figure 1 is showing dose colour wash of bone marrow volume receiving (A) 3 Gy in 3DCRT group; (B) 3 Gy in VMAT group (C) 5 Gy in 3DCRT group and (D) 5 Gy in VMAT group. No significant association of hematological toxicity was found with the age or baseline blood counts.

There is no substantial evidence to suggest excess bone marrow suppression in medulloblastoma patients treated by VMAT as compared to 3DCRT. Higher incidence of hematological toxicity in the index study, in the VMAT group, can be explained by the high bone marrow volume exposing to low dose radiation. Though the results from our study cannot be generalized due to small sample size and selection bias, VMAT should be used carefully after considering bone marrow sparing in the treatment plan optimization to reduce the incidence and severity of hematological toxicity.