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

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.