Potential Utility of Image-guidance to Reduce PTV Margins for Patients Treated by IMRT for Head and Neck Cancer: Evaluation of Temporal-based Protocols

Abstract

Although image-guided radiotherapy (IGRT) has been proposed as a means of potentially reducing planning target volume (PTV) margins, there is limited data on how to best utilize this technology to accomplish this for head and neck cancer patients. The aim of this study was to determine the optimal PTV margins required for patients treated by IMRT for head and neck cancer using commonly employed IGRT strategies. Positional alignment data was collected from 114 patients who underwent volumetric imaging with on-board kV cone beam CT (CBCT) prior to each daily IMRT fraction, resulting in 3, 575 image data sets. Assuming zero residual error after IGRT correction, we estimated the per fraction geometric error that would have occurred in the absence of daily IGRT correction. Population error parameters were calculated according to Van Herk's formula, and margin values were subsequently calculated for PTV expansion. Eight temporal IGRT protocols were evaluated with this method: (1) No IGRT correction; (2) IGRT correction for the first fraction; (3) IGRT correction for the first three fractions; (4) IGRT for the first five fractions; (5) IGRT for the first seven fractions; (6) IGRT weekly; (7) IGRT for the first five fractions then weekly; and (8) IGRT every other day with a running mean. In the absence of any on-board correction, the mean shifts were 0.6 ±1.9 mm, 2.0 ± 6.1 mm, 5.8 ± 5.4 mm in the primary translational axes (anteroposterior, superoinferior, and right-left directions), respectively. These positional would require PTV margins of 5.9 mm, 16.9 mm, and 14.8 mm, respectively. of the 8 IGRT protocols evaluated, the optimal less-than-daily IGRT protocol requiring the lowest hypothetical margins (1.8 mm, 2.8 mm, 1.9 mm) to ensure accurate delivery in 95% of the treated fractions was protocol 8 (IGRT on alternating days with running mean). Protocol 6 (weekly IGRT) the next optimal less-than-daily protocol, with corresponding margins of 2.7 mm, 3.9 mm, and 2.8 mm, respectively. The use of IGRT can safely allow for the reduction of PTV margins in the treatment of head and neck cancer patients using IMRT. This strategy is possible even in the setting of less-than-daily IGRT. Clinical implications will be discussed.