Volumetric And Dosimetric Variations In Radiation Treatment Planning Using Ct Images Based On Different Reconstructed Field-of-view

Objective:To analyze the volumetric and dosimetric variations in radiation treatment planning (RTP) using CT images based on normal and extended reconstructed field-of-view (EFOV).Methods:Raw data of CT scans from 16 cases of nasopharyngeal carcinomas were reconstructed to form 2 sets of CT images with normal (45 cm) and EFOV (65 cm), which were then exported to RTP. Contouring of targets/OAR including GTV (Gross Tumor Volume), CTV (Clinical Target Volume, CTV), brain stem, lens, parotids and cord was made on normal FOV CT set. A 7-field equi-angular IMRT (Intensity Modulated Radiation Therapy) plan was generated with prescribed GTV dose of 70 Gy. Two sets of CT images were fused in DICOM coordinate system and targets/OARs on normal FOV CT were copied to EFOV CT. IMRT plans were then transplanted from normal FOV to EFOV CT, with the same isocenter on DICOM coordinates. Volumetric and dosimetric variations including GTV, CTV brain stem, lens, parotids and cord were calculated on dose-volume-histogram (DVH) and paired-t test was used for statistical analysis. For dosimrtric verification, IMRT plans were inputted into fluence maps of Mapcheck 1175 phantom based on normal FOV and EFOV, and DTA (Distance to Agreement) was used to analyze the passing rate of calculated/measured absolute doses at 5 cm depth. Paired-t test was used to compare the passing rate of field 1-7 of IMRT plans based on 2 CT sets.Results:Volumes of targets and OARs on 2 CT sets of different FOVs were statistically different, with larger calculated volume on normal FOV in all cases. There was no statistic difference in the maximal (max) doses received by all targets and OARs except the small-volume lens, in which dose was higher on normal than EFOV CT (t=-3.14, P<0.007). The mean doses (Dmean) to the CTV (clinical target volume) and GTV (gross tumor volume) were higher on EFOV than normal FOV CT (CTV:t=-6.45, P< 0.001; GTV:t=-5.65; P<0.001). There was no statistic difference in Dmean received by OARs and the minimal dose (Dmin) by all targets and OARs (P>0.05). There was also no statistic difference in the passing rate of field 1-7 of IMRT plans based on 2 CT sets.Conclusion:There were volumetric and dosimetric variations as evaluated on DVH using different reconstructed FOV during CT simulation, though the difference between the passing rates as verified in 2 dimensional fluence map was not significant.