Influence of Absolute Dose Variation on Dose Verification of Portal Dosimetry for
Large Fields in IMRT Plans of Nasopharyngeal Carcinoma
SHI Feiyue1,2, ZHUANG Jieying1, WANG Min1, QIN Wei1, ZHAO Huanyu1, WEI Xiaowei1
1. Radiation Therapy Center, Nanjing First Hospital, Nanjing Medical University (Nanjing First Hospital), Nanjing Jiangsu 210006,
China; 2. Research Center of Medical Physics, Nanjing Medical University, Nanjing Jiangsu 210029, China
Abstract：Objective To study the influence of absolute dose variation of linac on dose verification results of Portal Dosimetry (PD)
for large fields in intensity modulated radiotherapy (IMRT) plans of nasopharyngeal carcinoma. Methods The IMRT treatment plans
of 12 patients with nasopharyngeal carcinoma were selected. For each treatment plan, the field with gantry angle of 0o (AA0) was
selected, which was a large field and consisted of two split fields. For fields of AA0, corresponding PD verification plans were created
and executed on a Clinac iX linac. The PD dose distribution matrix of the large field AA0 was obtained by adding two matrices of
corresponding two PD split fields. The absolute dose variations of linac were simulated by multiplying the measured dose distribution
matrix of the AA0 field with different coefficients. Using the 3 mm/3% Gamma analysis parameters, values of Gamma passing rate
(G) of the AA0 field under different absolute dose variations were analyzed using the PD software module, and detailed analyses
were performed. The symbol dp was used to represent the relative percentage variation of absolute dose of the linac. Variations of
G(dp) were investigated when dp was in range of -5.0% to 5.0% at an interval of 0.5%. The data of G(dp) were fitted using a quartic
polynomial, and the maximum value of G (Gmax) and the range of dp variation (R95) corresponding to G≥95% were calculated using
the fitting formula. Results All the 12 values of G(0) of AA0 was ≥95% (96.91%-99.38%). When dp deviated in the positive or
negative direction, value of G showed a downward trend by and large, but value of G(0) was not the maximum of G(dp), and G
showed an arch shape as dp changes. The ranges of Gmax value and R95 value were 97.08%-99.41% and 6.3%-12.0%, respectively.
Values of G for all the 12 AA0 fields were≥95%, when -1.5%≤dp≤2.3%. The dp corresponding to the average Gmax was 0.6%.
Conclusion The absolute dose variation of linac has an influence to a certain extent on the PD dose verification results for large fields
in IMRT plans of nasopharyngeal carcinoma. In order to obtain good PD dose verification results, the absolute dose variation of linac
should be limited within a reasonable range. This study provides useful data reference for evaluating the PD dose verification results
and for the quality control work of measuring and calibrating the absolute dose of linac.
时飞跃1,2，庄洁颖1，王敏1，秦伟1，赵环宇1，魏晓为1. 绝对剂量的变化对鼻咽癌调强放疗计划
大野Portal Dosimetry剂量验证的影响[J]. 中国医疗设备, 2023, 38(7): 12-16.
SHI Feiyue1,2, ZHUANG Jieying1, WANG Min1, QIN Wei1, ZHAO Huanyu1, WEI Xiaowei1. Influence of Absolute Dose Variation on Dose Verification of Portal Dosimetry for
Large Fields in IMRT Plans of Nasopharyngeal Carcinoma. China Medical Devices, 2023, 38(7): 12-16.
