ABSTRACT
Objective To simulate the possible systematic delivery errors introduced by monitor units ( MU) and multi-leaf collimator ( MLC) in radiotherapy plans for nasopharyngeal carcinoma ( NPC) , and to analyze the dosimetric sensitivity of static intensity-modulated radiotherapy ( IMRT ) and volumetric modulated arc therapy ( VMAT) with these errors. Methods Five IMRT plans were replanned using VMAT modality with the same physical parameters, and then MU errors of 125%, 250%, and 5. 00% were introduced into IMRT and VMAT plans. Meanwhile, to simulate leaf position errors during delivery, MLC position errors (025 mm, 050 mm, 100 mm, 150 mm, and 200 mm) were introduced by modifying the original plan documents. The types of MLC errors were as follows:( 1) the MLC banks moved in the same direction;( 2) the MLC banks moved in opposing directions ( expand or contract the MLC gaps ) . The differences in dosimetric sensitivity introduced by MU and MLC errors between IMRT and VMAT plans for NPC were calculated by linear regression analysis. Results With the increase in MU errors, the doses to target and organs at risk ( OARs) of IMRT and VMAT plans increased in a linear way, and met R2=0992-1000( P<005) . For MLC errors, the average dosimetric sensitivity for target and OARs of IMRT and VMAT were-026%/mm and-065%/mm in case of offset errors, 487%/mm and 868%/mm in case of expansion errors, and -604%/mm and -988%/mm in case of indentation errors. In addition, the dosimetric sensitivity with the three types of MLC errors was greater for VMAT plan than for IMRT plan. ConclusionsMU and MLC errors have a significant effect on the dose distribution of IMRT, and particularly VMAT, for NPC. It is important to execute routine quality assurance of MLC to ensure accurate radiotherapy.
ABSTRACT
Objective To simulate the possible systematic delivery errors introduced by monitor units ( MU) and multi-leaf collimator ( MLC) in radiotherapy plans for nasopharyngeal carcinoma ( NPC) , and to analyze the dosimetric sensitivity of static intensity-modulated radiotherapy ( IMRT ) and volumetric modulated arc therapy ( VMAT) with these errors. Methods Five IMRT plans were replanned using VMAT modality with the same physical parameters, and then MU errors of 125%, 250%, and 5. 00% were introduced into IMRT and VMAT plans. Meanwhile, to simulate leaf position errors during delivery, MLC position errors (025 mm, 050 mm, 100 mm, 150 mm, and 200 mm) were introduced by modifying the original plan documents. The types of MLC errors were as follows:( 1) the MLC banks moved in the same direction;( 2) the MLC banks moved in opposing directions ( expand or contract the MLC gaps ) . The differences in dosimetric sensitivity introduced by MU and MLC errors between IMRT and VMAT plans for NPC were calculated by linear regression analysis. Results With the increase in MU errors, the doses to target and organs at risk ( OARs) of IMRT and VMAT plans increased in a linear way, and met R2=0992-1000( P<005) . For MLC errors, the average dosimetric sensitivity for target and OARs of IMRT and VMAT were-026%/mm and-065%/mm in case of offset errors, 487%/mm and 868%/mm in case of expansion errors, and -604%/mm and -988%/mm in case of indentation errors. In addition, the dosimetric sensitivity with the three types of MLC errors was greater for VMAT plan than for IMRT plan. ConclusionsMU and MLC errors have a significant effect on the dose distribution of IMRT, and particularly VMAT, for NPC. It is important to execute routine quality assurance of MLC to ensure accurate radiotherapy.
ABSTRACT
[Abstra ct] Objective To investigate the long-term efficacy and adverse effects of intensity-modulated radiotherapy (IMRT) for nasopharyngeal carcinoma (NPC).Methods A total of 869 patients with biopsy-proven NPC without distant metastasis who underwent the whole course of IMRT from 2009 to 2010 were enrolled.Of all the patients, 84.8%received cisplatin-based chemotherapy.The prescribed dose to the primary lesion in the nasopharynx was 66-70Gy in 30-32 fractions, and the dose to the positive lymph nodes in the neck was 66 Gy in 30-32 fractions.The Kaplan-Meier method was used to calculate survival rates, the log-rank test was used for difference analysis and univariate prognostic analysis , and the Cox proportional hazards model was used for multivariate prognostic analysis .Rseu lts The 5-year overall survival( OS ) , local recurrence-free survival, regional recurrence-free survival, distant metastasis-free survival, and disease-free survival ( DFS ) were 84.0%, 89.7%, 94.5%, 85.6%, and 76.3%, respectively.In the patients with locally advanced NPC,concurrent chemotherapy tended to reduce distant metastasis (83.6%vs.75.7%, P=0.050) and improve OS (82.6%vs.77.0 %, P=0.082).Induction chemotherapy tended to improve OS ( 80.7% vs.71.4%, P=0.057 ) , and the induction chemotherapy containing docetaxel or gemcitabine tended to improve OS (83.3%vs.72.2%, P=0.058).The patients who received a boost after the initial radiotherapy had a significantly lower DFS rate than those who did not (52.2%vs.71.1%, P=0.004).The concurrent chemotherapy increased the incidence rates of long-term xerostomia and trismus, while a high dose of cisplatin increased the incidence rates of xerostomia and hearing impairment.Conclusions IMRT for NPC provides satisfactory long-term efficacy.Concurrent chemotherapy combined with IMRT tends to reduce the incidence of distant metastasis, and other values need further investigation.The boost therapy after radiotherapy may be associated with poor prognosis.Chemotherapy increases the incidence of long-term toxicities.
