The potential for sparing of parotids and escalation of biologically effective dose with intensity-modulated radiation treatments of head and neck cancers: a treatment design study

Abstract

Purpose: Conventional radiotherapy for cancers of the head and neck (HN) can yield acceptable locoregional tumor control rates, but toxicity of many normal tissues limits our ability to escalate dose. Xerostomia represents one of the most common complications. The purpose of this study is to investigate the potential of intensity-modulated radiotherapy (IMRT) to achieve adequate sparing of parotids and to escalate nominal and/or biologically-effective dose to achieve higher tumor control without exceeding normal tissue tolerances.

Methods and materials: An IMRT optimization system, developed at our institution for research and clinical purposes, and coupled to a commercial radiation treatment planning system, has been applied to a number of cases of HN carcinomas. IMRT plans were designed using dose- and dose-volume-based criteria for 4 and 6 MV coplanar but non-collinear beams ranging in number from 5 to 15 placed at equi-angular steps. Detailed analysis of one of the cases is presented, while the results of the other cases are summarized. For the first case, the IMRT plans are compared with the standard 3D conformal radiation treatment (3DCRT) plan actually used to treat the patient, and with each other. The aim of the 3DCRT plan for this particular case was to deliver 73 Gy to the tumor volume in 5 fractions of 2 Gy and 28 fractions of 2.25 Gy/fx; and 46 Gy to the nodes in 2 Gy/fx while maintaining critical normal tissues to below specified tolerances. The IMRT plans were designed to be delivered as a "simultaneous integrated boost" (SIB) using the "sweeping window" technique with a dynamic MLC. The simultaneous integrated boost strategy was chosen, partly for reasons of efficiency in planning and delivery of IMRT treatments, and partly with the assumption that dose distributions in such treatments are more conformal and spare normal tissues to a greater extent than those with sequential boost strategy. Biologically equivalent dose normalized to 2 Gy/fx, termed here as normalized total dose (NTD), for this strategy was calculated using published head and neck fractionation data.

Results: IMRT plans were more conformal than the 3DCRT plans. For equivalent coverage of the tumor and the nodes, and for the dose to the spinal cord and the brainstem maintained within tolerance limits, the dose to parotids was greatly reduced. For the detailed example presented, it was shown that the tumor and the nodes in the 3DCRT plan receive NTDs of 78 and 46 Gy, respectively. For the IMRT plan, a nominal dose of 70 Gy could be delivered to the tumor in 28 fractions of 2.5 Gy each, simultaneously with 50.4 Gy to nodes with 1.8 Gy/fx. The two are biologically equivalent to 82 and 46 Gy, respectively, if delivered in 2 Gy/fx. Similar computations were carried out for other cases as well. The quality of IMRT plans was found to improve with increasing number of beams, up to 9 beams. Dose-volume-based criteria led to a modest improvement in IMRT plans and required less trial and error.

Conclusion: IMRT has the potential to significantly improve radiotherapy of HN cancers by reducing normal tissue dose and simultaneously allowing escalation of dose. SIB strategy is not only more efficient and yields better dose distributions, but may also be biologically more effective. Dose-volume-based criteria is better than purely dose-based criteria. The quality of plans improves with number of beams, reaching a saturation level for a certain number of beams, which for the plans studied was found to be 9.