Emphasizing conformal avoidance versus target definition for IMRT planning in H&N cancer

Abstract

Purpose/Objective: One of the significant hurdles facing the widespread implementation of IMRT for H&N cancer involves the complexity of target definition. Although precise contouring of primary H&N tumors alone is often difficult, the accurate, reproducible and time-efficient contouring of elective nodal risk regions represents an even greater challenge. Experienced H&N cancer specialists commonly consume several hours to fully contour and refine desired targets for a single H&N IMRT case. Methods that simplify and increase uniformity of this process will be valuable to facilitate the safe and effective transfer of H&N IMRT into routine community practice with the goal of high treatment quality. Conformal avoidance IMRT planning represents one such method for planning simplification. Materials/Methods: Twenty patients receiving high dose IMRT for H&N cancer underwent comprehensive treatment planning using three distinct design techniques (60 total treatment plans). Physician-contoured plans were designed for each patient including conventional three-field design, target definition IMRT and conformal avoidance IMRT. For each patient, the conventional three-field design was created first, thereby providing “outermost boundaries” for subsequent IMRT design. Briefly, target definition IMRT involved physician contouring of a GTV, CTV1, CTV2 and normal tissue avoidance structures on consecutive 2.5-mm CT images. Conformal avoidance IMRT involved physician contouring of a GTV and normal tissue avoidance structures only. The overall physician time for each approach was monitored and the resultant plans were rigorously compared. Results: The average physician working time for design of respective H&N treatment contours was 0.3 hours for the conventional three field plan, 2.7 hours for the target definition IMRT approach and 0.9 hours for the conformal avoidance IMRT approach. DVH analysis confirmed that the largest volume of tissue treated to intermediate (50 Gy) and high dose (66 Gy) occurred with the conventional three-field design followed by conformal avoidance IMRT and then target definition IMRT. However, DVH analysis established comparable results when evaluating dose distributions for the normal tissue avoidance structures using the two IMRT approaches. Conclusions: The conformal avoidance planning approach for H&N IMRT offers an attractive alternative to the target definition approach. The process is considerably more time-efficient for the physician in that normal tissue structures such as parotid gland and spinal cord are more clearly defined on imaging studies and therefore easier to contour reproducibly than less well-defined elective nodal or “at risk” regions. The overall time for physician contouring is substantially reduced (∼3 fold) and the resultant DVH analysis shows comparable salivary gland and spinal cord protection to that achieved with the target definition approach. The conformal avoidance approach may ultimately prove a safer and more reliable method to export to practitioners who either cannot devote the necessary hours for complex target contouring, or do not feel they have the anatomic expertise to confidently accomplish complex target definition for their H&N cancer patients.