ABSTRACT
Objective To study the dynamic changes of anatomy and the dosimeter distribution those changes influenced. Methods Initially simplified intensity modulated radiation therapy (sIMRT)were performed to twenty-nine patients with phase Ⅲ - Ⅳa esophageal carcinoma from January 2007 to March 2009. The target volumes and involving organs were contoured on the primary spiral CT pictures.After sIMRT planning being finished, secondary CT scan was acquired to rectify the treatment center. For eleven patients at every other week and eighteen patients at the fourth week, spiral CT images were acquired according to the same treatment center, and thereafter fused with the first CT images. Firstly, the law of change and the best time of replanning were searched:the changed gross tumor volume (GTV), gross node volume (GTVnd), plan target volume (PTV) and normal organs (lung, spinal cord, heart and outline) on the fusion interface were modified by a single physician, the changes for each structure throughout treatment were measured by system software. Secondly, dose distributions were computed and evaluated for replanning CT using the same beams arrangement as the initial plan. Cumulative dose was estimated using weighted average and compared with the original plan. Results For eleven patients, The law of change:the volume of outlines and GTV gradually decreased, and the change come to peak in the fourth week. The conformal index for PTV gradually decreased, whereas the heterogeneous index gradually increased. For twenty-nine patients on the fourth week, the dose of GTV were more than 60 Gy. The dose of PTV-D95 and CTV-D99 decreased ( t = 1.49, P = 0. 147 and t = 2. 07, P = 0. 048 respectively). The dose of CTV-D99 in two patients deceased to 54 Gy or less. The cord-Dmax and lung V30 increased significantly ( t = - 2. 42, P = 0. 022 and t = -2. 26,P =0. 032). Conclusions During the course of sIMRT for esophageal cancer, the volume of GTV decreased and the change come to peak in the fourth week. It is the best time for evaluating the change of dose of target volume using CT-CT fusion. For some patients, revise of the treatment plan is needed to ensure adequate target volume dosage and safety of normal tissues.
ABSTRACT
Objective To study the dynamic change of anatomy and dosimetry distribution and its influence during intensity modulated radiation therapy(IMRT) for nasopharyngeal carcinoma(NPC). Methods From June 2006 to August 2007,12 patients with stage Ⅲ-Ⅳ NPC receiving initial IMRT concurrently combined with chemotherapy were included in the study. The target volumes and,involving organs were contoured on the first set of spiral CT images. When IMRT planning was finished,a second CT scans was acquired to rectify the treatment center. Weekly spiral CT images acquired during the treatment period according to the same treatment center were fused with the first CT images. In order to determine the best time of replanning, modified plan target volume( PTV1 ) and normal organs( parotids and outline) were contoured on the fusion interface by a single physician. Changes of each structure throughout treatment period were measured by a system software. Then the dose distributions were computed and evaluated for replanning CT using the same beams arrangement in the original plan. Cumulative dose was estimated compared with the original plan. Results The volume of outlines and parotids decreased gradually, and the change came to peak in the fifth week. So the fifth and first week CT scans were selected as research objectives. No significant changes in maximal and mean dose was observed in the brainstem, spinal cord or mandible. Despite volume changes,D99 and D95 of the PTV1 did not change siguificantly(P＞0.05). D95 of the bilateral parotids increased significantly ( PL = 0.03,PR = 0.01 ). Conclusions During IMRT for NPC, the volume of PTV1, the outlines and parotids decrease,and the change come to peak in the fifth week. Comparing to the first treatment plan, the dose parameters of the parotids increase significantly in the second plan based on the fifth week CT,but those of the brainstem,cord,mandible and PTV1 change slightly.