ABSTRACT
Objective To investigate the impact of adopting different dose grid resolution during volumetric modulated arc therapy (VMAT) planning on COMPASS pass rate.Methods A total of 10 patients with cervical cancer were enrolled.Four types of VMAT plans (plan1,plan2,plan3,plan4) were designed for each patient,with dose grid resolution of 0.2 cm × 0.2 cm × 0.2 cm,0.3 cm × 0.3 cm ×0.3 cm,0.4 cm ×0.4 cm ×0.4 cm and 0.5 cm ×0.5 cm ×0.5 cm,respectively.The plans were exported to Linac and conducted,then measured by COMPASS.The discrepancies were analyzed,which were obtained by comparing reconstructed dose from COMPASS and dose from TPS optimization.Results For tumor volume,the mean value of Dmean and D95 were ＜ 0.5％ and ＜ 1.3％ respectively,and the standard deviation were both ＜ 1.0％.For organs at risk (OAR),the max discrepancies were Dmean of femur with-6.7％,-7.0％,-8.0％,-5.8％,and V35 of rectum with 4.9％,-6.3％,-6.1％,-5.7％ in four types of VMAT plans.The γ (3％,3 am) rate of tumor volume was ＞95％ with standard deviation ＜ 2.5％,with no statistically significant difference among the four types of plans (P ＞0.05).The γ (3％,3 mm) rate of OARs were ＞ 98％ except femur (＞ 95％),and the standard deviations were within 1.9％-6.1％,with no statistical significance (P ＞0.05).The average γ rate of tumor volume and OARs were ＜ 0.4 except femur (＞ 0.4),with no statistically significant difference among four types of VMAT plans (P ＞ 0.05).Conclusions COMPASS pass rate was not influenced by the dose grid resolution between 0.2 ～ 0.5 cm for VMAT plan.
ABSTRACT
Objective To compare the whole-body equivalent doses from volumetric modulated arc therapy (VMAT) and static intensity-modulated radiotherapy (IMRT) for patients with cervical cancer.Methods Nine patients with cervical cancer admitted to our hospital in 2014 were included in this study.Both VMAT and IMRT were planned for each patient.Each patient's personal dose equivalent (Hp (10)) was measured using thermoluminescent dosimeters placed at the xiphoid process and glabella during IMRT and VMAT.The whole-body equivalent doses were estimated based on the results measured at the xiphoid process and compared between the VMAT and IMRT techniques.The paired t test was used for difference analysis.Results The Hp (10) values measured at the xiphoid process and glabella of every patient were lower for VMAT than for IMRT.At a prescribed dose of 50 Gy,if the mean Hp (10) values measured at the xiphoid process were considered to represent the whole-body equivalent doses,the whole-body equivalent doses for VMAT and IMRT were 364 mSv and 538 mSv,respectively.Conclusions VMAT results in a lower whole-body equivalent dose to patients compared with IMRT.The decreased whole-body equivalent dose delivered by VMAT may reduce the likelihood of a radiation-induced secondary malignancy.
ABSTRACT
Objective To evaluate the peripheral dose (PD) from megavoltage cone-beam CT (MVCBCT) imaging for head-and-neck region image-guided radiation therapy,to determine the correlation of PD with monitor unit (MU),and to investigate the impact of imaging field size on the PD.Methods Measurements of PD from MVCBCT were made with a 0.65 cm3 ionization chamber placed in a specially designed phantom at various depths and distances from the field edges.The PD at reference point inside the phantom was measured with the same ionization chamber to investigate the linearity between MU used for MVCBCT and the PD.The homogeneity of PD in the axial plane of the phantom were measured.Results PD from MVCBCT increased with increasing number of MU used for imaging and with increasing the field size.The measured PD in the phantom decreased exponentially as distance from the field edges increased.PD also decreased as the depth from the phantom surface increased.There was a strong linear relationship between PD and MUs used for MVCBCT.The PD was heterogeneous,with higher dose at the anterior than the posterior.Conclusions The PD from MVCBCT depend much on the MVCBCT delivery MU and the scan field size.In clinic,using the smallest number of MU allowable and reducing MVCBCT scanning field size without compromising acquired image quality is an effective method of reducing the PD.