Clinical practice and evaluation of electronic portal imaging device for VMAT quality assurance.

Volumetric-modulated arc therapy (VMAT) is a novel extension of the intensity-modulated radiation therapy (IMRT) technique, which has brought challenges to dose verification. To perform VMAT pretreatment quality assurance, an electronic portal imaging device (EPID) can be applied. This study's aim was to evaluate EPID performance for VMAT dose verification. First, dosimetric characteristics of EPID were investigated. Then 10 selected VMAT dose plans were measured by EPID with the rotational method. The overall variation of EPID dosimetric characteristics was within 1.4% for VMAT. The film system serving as a conventional tool for verification showed good agreement both with EPID measurements ([94.1 ± 1.5]% with 3 mm/3% criteria) and treatment planning system (TPS) calculations ([97.4 ± 2.8]% with 3 mm/3% criteria). In addition, EPID measurements for VMAT presented good agreement with TPS calculations ([99.1 ± 0.6]% with 3 mm/3% criteria). The EPID system performed the robustness of potential error findings in TPS calculations and the delivery system. This study demonstrated that an EPID system can be used as a reliable and efficient quality assurance tool for VMAT dose verification.

Lhermitte's sign among nasopharyngeal cancer patients after radiotherapy.

BACKGROUND: Lhermitte's sign (LS) is a side effect of radiotherapy (RT) on the spinal cord and typically occurs shortly after the procedure has been conducted. When treating patients with cancer of the head and neck region with irradiation, it remains difficult to avoid exposing the cervical spinal cord to unintended radiation. In this study, we focused on nasopharyngeal cancer (NPC) alone and looked for various parameters that might influence the occurrence of LS associated with this disease after RT. METHODS: From 1979 through 1990, 1171 patients with NPC completed RT either with or without chemotherapy at the Lin-Kou Medical Center, Chang Gung Memorial Hospital (CGMH), Tao-Yuan, Taiwan; the RT regimens for these treated patients were very similar. The nasopharyngeal tumor was treated to 75 Gy by photon teletherapy and after-loading brachytherapy. The neck lymphatics were irradiated with photon irradiation to 46.8 Gy and then boosted with electron beams to 10 to 30 Gy, in accordance with the patient's nodal status, either unilaterally or bilaterally. Every patient was followed monthly for the first 3 months after therapy and subsequently every 2 to 3 months for the next 2 years and, finally, every 6 months thereafter. At follow-up, a neurologic checkup of each patient was performed to determine whether any injury to the spinal cord or brain stem had arisen. RESULTS: LS was observed for 121 patients (10.3%). The median development time for such signs was 3.0 months after the completion of RT (range, 0.2-72 months), and the appearance of such a sign lasted 1 to 82 weeks (median, 17 weeks). No statistically significant differences between the sexes were noted in the development of such a sign (p = .5263),or among various T classifications (p = .0757) and N classifications (p = .4412). The incidence of LS was significantly lower for those patients who had also received chemotherapy than it was for those who had not (p = .003), and it was also lower for patients older than 60 years than for those younger than 60 years (p = .0061). Of the subjects who did not undergo neck-lymphatic boosting or who had undergone only unilateral neck-lymphatic boosting, 7.2% had LS develop, whereas 11.5% of patients who had been boosted bilaterally had LS develop (p = .0285). CONCLUSIONS: The incidence of LS associated with NPC and after RT was higher in patients who underwent bilateral neck-lymphatic boosting by electron beams than for those who underwent unilateral boosting or who did not undergo boosting. A correlation between increased incidence of LS and RT dose on the cervical spinal cord was noted when the cord dose exceeded 48.9 Gy. Therefore, wherever possible, a CT simulator and a three-dimensional treatment-planning system should necessarily be used to verify the dose distribution of electron-beam RT to diminish the chance of radiation overdose on the cervical cord.