OBJECTIVE To measure the imaging dose in the head phantom by different image-guided methods. METHODS The imaging dose was measured in a cylindrical phantom on a Varian Clinac iX linear accelerator equipped with EPID, OBI, CBCT using a PTW 0.6 cc ion chamber with UNIDOS E dosimeter. 2D images were acquired by two perpendicular fields (0 degrees & 270 degrees), and the 3D images by CBCT. RESULTS The 2D imaging average dose for OBI (KV X-ray) was 0.74 mGy in the head phantom. It was significantly lower than 90.93 mGy for EPID (MV X-ray) and the image quality was better. For a standard CBCT (KV X-ray), the imaging average dose was 4.77 mGy. For a low dose mode, the imaging dose was 50% of the standard mode. Moreover, 3D images could match accurately. CONCLUSION For the OBI system, the imaging dose is lower and image quality is better than EPID. The 3D image-guided method for CBCT is better than 2D and is also safe for daily position verification. Therefore, during treatment positioning, the appropriate image-guide methods and scanning parameters can effectively reduce the additional dose to the patients.
Objective To compare the dosimetric differences between pencil beam convolution (PBC) and anisotropic analytical algorithm (AAA) in Eclipse treatment planning system for intensitymodulated radiotherapy (IMRT) planning of lung cancer patients and dosimetric verification.Methods 10 IMRT plans of lung cancer patients were calculated using the PBC and AAA and the differences of dosimetric parameter were analyzed according to dose-volume histogram of planning target volume (PTV),lung and spinal cord.The verification measurements were performed on an inhomogeneous thorax phantom using a pinpoint ionization chamber.The agreement between calculated and measured doses was determined.The paired t test was used to compare the results.Results Compared with PBC,the AAA predicted higher maximum PTV dose (t =-4.03,P =0.010),lower minimum PTV dose (t =5.09,P =0.040),and a reduction of the volume of PTV covered by the prescribed dose.The AAA also predicted slightly increases than the PBC algorithm in the mean dose to the lung and the V20 as well as the maximum dose to the spinal cord,and the differences were statistically significant (t =-3.99,-2.79,-5.46,P =0.010,0.038,0.003).In the verification measurements,the agreement between the AAA and measurement was within 2％and superior to the PBC algorithm on isocenter (t =-3.82,P =0.012).Conclusions For IMRT treatment planning of lung cancer,the PBC algorithm overestimates the dose to the PTV and underestimates the dose to the lung and the spinal cord,so the AAA for treating planning in which the tissue inhomogeneous such as lung is present is recommended.
Key words:
Pencil beam convolution; Anisotropic analytical algorithm; Lung neoplasms/ intensity-modulated radiotherapy
Twenty five patients with brainstem area tumor were treated by means of stereotactic conformal radiotherapy (SCRT) between Oct. 1997 and Apr. 2002. In these patients, 6 accepted conventional radiotherapy, 7 accepted operation before SCRT and 12 accepted only SCRT. In results, the alleviation rate of patients’ symptoms was 100.0%, and the complication rate was 16.0%. This study demonstrats that SCRT is an effective treatment method on brainstem area tumors and has remarkable effect on alleviating clinical symptoms.
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