Abstract:

Objective To explore the dosimetric characteristics of intensity modulated proton therapy （IMPT），intensity modulated radiation therapy （IMRT） and tomotherapy （TOMO） techniques applied in the irradiation of pediatric whole central nervous system tumors. Methods Taking the target area of a 14-year-old pediatric patient clinically diagnosed with atypical teratoid/rhabdomyoid tumor，meningeal metastasis by Shandong Cancer Hospital and Institute，and undergoing craniospinal irradiation （CSI） as an example，IMPT，IMRT and TOMO plans were designed respectively based on the clinical prescription of the target area and the limit requirements of organs at risk （OARs）. The conformal index （CI），homogeneity index （HI） and gradient index （GI） of each planning target volume，as well as the dose volume index of normal tissues，were evaluated to compare the dosimetric characteristics of the three types of plans. Results The CI （0.71），HI （0.05） and GI （3.13） of the IMPT plan were comparable to those of IMRT plan （0.80，0.08，3.14）. The HI （0.03） and GI （2.54） of the TOMO plan were excellent，which were all within the clinically acceptable range. The irradiation dose to parallel organs in the IMPT plan was lower than that in the IMRT and TOMO plan. In the IMPT plan，V₅ of lungs was 2.9%，IMRT plan was 37.6%，and TOMO plan was 43.5%. The D_mean of liver in the IMPT plan was 0.01 Gy（RBE），IMRT plan was 6.12 Gy，and TOMO plan was 6.39 Gy. In the IMPT plan，none of the bladder，rectum，and femoral head received the dose，while there was low-dose radiation in both IMRT and TOMO plan. For serial organs adjacent to and within the target area，the D_max of spinal cord and brainstem in IMPT plan was 39.89 and 39.88 Gy（RBE），respectively； in IMRT plan，they were 39.43 and 38.59 Gy，respectively； and in TOMO plan，they were 38.41 and 37.69 Gy，respectively. The low-dose area in the IMPT plan was significantly better than the photon radiotherapy plans. Among them，the absolute volume IMPT plan occupied by 10% of the prescribed dose area in the patient's body was reduced by 70.10% compared with IMRT plan and 76.96% compared with TOMO plan； the 30% prescribed dose volume IMPT plan was reduced by 53.49% compared with IMRT plan and 62.51% compared with TOMO plan； the 50% prescribed dose volume IMPT plan was reduced by 39.06% compared with IMRT plan and 42.23% compared with TOMO plan. Conclusions The IMPT plan demonstrated significantly reduced low-dose exposure and lower doses to parallel OARs compared to both IMRT and TOMO plans in pediatric CSI. The CI，HI and GI of the three plans can all meet the clinical requirements. However，for serial organs adjacent to and within the target area，the D_max of the IMPT plan may be higher than that of IMRT and TOMO plans.

Key words: Central nervous system neoplasms, Radiotherapy, Proton therapy, Radiotherapy dosage