Abstract:

Objective To analyze the effects of different dose calculation grid size of Monaco system on the physical and biological dosimetry of target area and organ at risk （OAR） in T₄ nasopharyngeal carcinoma. Methods A total of 18 patients with stage T₄ nasopharyngeal carcinoma who received radiotherapy in the Department of Radiotherapy of Yunnan Cancer Hospital from October 2020 to April 2022 were selected to complete the delineation of target areas and OAR in the Monaco 5.11.03 system，and the volumetric intensity modulated arc therapy （VMAT） plan was developed on the 3 mm grid with the optimization mode of target area priority. The 3 mm grid group plan was replicated without changing any other parameters，and the physical plan was re-established on the 1，2，4 and 5 mm grids，and then the five plans were normalized to the prescription dose to cover 95% of the target volume. The planning time，D_2%，D_50%，D_98%，conformity index （CI），homogeneity index （HI），gradient index （GI），tumor control probability （TCP），D_2% and D_mean of important OAR around the target area were calculated and statistically analyzed. Results Planning primary tumor gross target volume（PGTVp）：The D_2% of 1，2，3，4 and 5 mm groups were （76.94±0.66），（75.98±0.76），（75.56±0.67），（75.67±0.73） and （75.94±0.85） Gy，respectively，with a statistically significant difference （F=9.86，P<0.001）. The CI of 1，2，3，4 and 5 mm groups were 0.75±0.05，0.78±0.04，0.78±0.05，0.79±0.04 and 0.78±0.04，respectively，with a statistically significant difference （F=2.61，P=0.041）. There were statistically significant differences in D_50%，D_98%，HI，equivalent uniform dose （EUD） and tumor control probability （TCP） among the groups （H=17.14，P=0.002； F=9.35，P<0.001； H=25.43，P<0.001 ； F=5.85，P<0.001； H=17.65，P=0.001）. There was no statistically significant difference in GI among the groups （P>0.05）. Pairwise comparison showed that D_2% in 2，3，4，5 mm groups compared with 1 mm group，D_50% in 5 mm group compared with 2，3 mm groups，D_98% in 4 mm group compared with 1，2 mm groups，D_98% in 5 mm group compared with 1，2，3 mm groups，CI in 5 mm group compared with 1 mm group，HI in 2，3，4，5 mm groups compared with 1 mm group，EUD in 3 mm group was compared with 1 mm group，EUD in 5 mm group compared with 2，3 mm groups，TCP in 3 mm group compared with 1 mm group，and TCP in 5 mm group compared with 3 mm group，there were statistically significant differences （all P<0.05）. Planning nodal gross target volume （PGTVn）：The D_2% of 1，2，3，4 and 5 mm groups were （76.36±0.59），（75.36±0.62），（75.04±0.68），（75.25±0.72） and （75.39±0.77） Gy，respectively，with a statistically significant difference （F=10.32，P<0.001）. The HI of 1，2，3，4 and 5 mm groups were 1.08（1.08，1.08），1.07（1.06，1.07），1.06（1.06，1.07），1.06（1.06，1.07），1.06（1.06，1.07），1.06（1.06，1.08），respectively，with a statistically significant difference （H=22.00，P<0.001）；There were statistically significant differences in D_50%，D_98% and EUD among the groups （H=11.79，P=0.019； H=20.49，P<0.001； F=12.14，P=0.016）. Pairwise comparison showed that there were statistically significant differences in D_2% between 2，3，4，5 mm groups and 1 mm group，D_98% between 4 mm group and 1 mm group，D_98% between 5 mm group and 1，2 mm groups，HI between 2，3，4 mm groups and 1 mm group，and EUD between 3 mm group and 1 mm group （all P<0.05）. Planning primary tumor clinical target volume 1 （PCTVp1）： The D_2% of 1，2，3，4 and 5 mm groups were （76.59±0.63），（75.64±0.65），（75.64±0.98），（75.41±0.70） and （75.71±0.84） Gy，respectively，with a statistically significant difference （F=9.53，P<0.001）. The D_50% of 1，2，3，4，5 mm groups were （72.09±0.34），（71.85±0.39），（71.82±0.45），（72.04±0.56），（72.43±0.66） Gy，respectively，with a statistically significant difference （F=4.20，P=0.019）. There was no statistically significant difference in the other indexes among the groups （all P>0.05）. Pairwise comparison showed that there were statistically significant differences in D_2% between 2，3，4，5 mm groups and 1 mm group，and in D_50% between 2，3 mm groups and 1 mm group （all P<0.05）. Planning nodal clinical target volume 1（PCTVn1）： There were no statistically significant differences in all indexes among the groups （all P>0.05）. Planning clinical target volume 2 （PCTV2）： The D_2% of 1，2，3，4 and 5 mm groups were （75.57±0.50），（74.87±0.67），（74.51±0.51），（74.61±0.63） and （75.00±0.74） Gy，respectively，with a statistically significant difference （F=8.27，P<0.001）. Pairwise comparison showed that the D_2% of the 2，3，4 mm groups were significantly different from that of the 1 mm group （all P<0.05）. The calculation time of physical plan in 1，2，4 and 5 mm groups was 987.00（848.00，1 091.00），120.50（99.75，134.00），26.00 （24.00，34.25） and 21.50（18.75，34.75）s，respectively，with a statistically significant difference （H=61.62，P<0.001）. Pairwise comparison showed that there were statistically significant differences in the calculation time between 4 mm group and 1，2 mm groups，5 mm group and 1，2 mm groups （all P<0.05）. There was no statistically significant difference in the dosimetric parameters of OAR around the target area among the groups （all P>0.05）. Conclusion The physical dose and biological dose of the important OAR around the target area and the target area change with the change of dose calculation grid size when formulating the physical plan of radiotherapy for T₄ nasopharyngeal carcinoma. Considering the quality of the physical plan and the calculation time，when the Monaco system formulates the VMAT plan for T₄ nasopharyngeal carcinoma patients，the plan can be optimized on the 3 mm computing grid and copied to the 1 mm computing grid for recalculation.

Key words: Nasopharyngeal carcinoma, Radiotherapy dosage, Organs at risk, Radiotherapy