Influence of MRI on target volume delineation and IMRT planning in nasopharyngeal carcinoma.

Abstract

To compare CT and MRI target volumes for nasopharyngeal carcinoma (NPC) and evaluate the role of intensity-modulated radiotherapy (IMRT) in treating composite CT+MRI targets. CT and T(1)/T(2)-weighted MRI scans were obtained for 8 consecutive NPC patients. Using CT, MRI, and fused CT/MRI, various target volumes (gross target volume, clinical target volume, and planning target volume [PTV]) and critical structures were outlined. For each patient, three treatment plans were developed: (1) a three-dimensional conformal RT (3D-CRT) plan using CT-based targets; (2) a 3D-CRT plan using composite CT+MRI targets; and (3) a IMRT plan using CT+MRI targets. The prescription dose was 57.6 Gy and 70.2 Gy to the initial and boost PTV, respectively. Treatment plans were compared using the PTV dose to 95% volume (D(95)), critical structure dose to 5% organ volume (D(5)), and mean dose. Compared with CT, the MRI-based targets were 74% larger, more irregularly shaped, and did not always include the CT targets. For CT-based targets, 3D-CRT plans, in general, achieved adequate target coverage and sparing of critical structures. However, when these plans were evaluated using CT+MRI targets, the average PTV D(95) was approximately 60 Gy (14% underdosing), and critical structure doses were significantly worse. The use of IMRT for CT+MRI targets resulted in marked improvement in the PTV coverage and critical structure sparing: average PTV D(95) improved to 69.3 Gy, brainstem D(5) to <43 Gy (19% reduction), spinal cord D(5) to <37 Gy (19% reduction), and the mean dose to the parotids and cochlea reduced to below tolerance (23.7 Gy and 35.6 Gy, respectively). CT/MRI fusion improved the determination of target volumes in NPC. In contrast to 3D-CRT, IMRT planning resulted in significantly improved coverage of composite CT+MRI targets and sparing of critical structures.