Clinical Investigation
Evaluation of the Planning Target Volume in the Treatment of Head and Neck Cancer With Intensity-Modulated Radiotherapy: What Is the Appropriate Expansion Margin in the Setting of Daily Image Guidance?

https://doi.org/10.1016/j.ijrobp.2010.07.017Get rights and content

Purpose

To compare patterns of disease failure among patients treated with intensity-modulated radiotherapy (IMRT) in conjunction with daily image-guided radiotherapy (IGRT) for head and neck cancer, according to the margins used to expand the clinical target volume (CTV) to create a planning target volume (PTV).

Methods and Materials

Two-hundred and twenty-five patients were treated with IMRT for squamous cell carcinoma of the head and neck. Daily IGRT scans were acquired using either kilovoltage or megavoltage volumetric imaging prior to each delivered fraction. The first 95 patients were treated with IMRT with 5-mm CTV-to-PTV margins. The subsequent 130 patients were treated using 3-mm PTV expansion margins.

Results

Two-year estimates of overall survival, local-regional control, and distant metastasis-free survival were 76%, 78%, and 81%, respectively. There were no differences with respect to any of these endpoints among patients treated with 5-mm and 3-mm PTV expansion margins (p > 0.05, all). The 2-year local-regional control rate for patients treated with IMRT with 5-mm and 3-mm PTV margins was 78% and 78%, respectively (p = 0.96). Spatial evaluation revealed no differences in the incidences of marginal failures among those treated with 5-mm and 3-mm PTV margins.

Conclusions

The use of 3-mm PTV expansion margins appears adequate and did not increase local-regional failures among patients treated with IMRT for head and neck cancer. These data demonstrate the safety of PTV reduction of less than 5 mm and support current protocols recommending this approach in the setting of daily IGRT.

Introduction

Given its ability to achieve a highly conformal dose distribution, intensity-modulated radiotherapy (IMRT) has become widely adopted in the radiotherapeutic management of head and neck cancer. However, since the treatment plans generated by this sophisticated technology are characterized by steep gradients between high- and low-dose regions, the precise targeting and delivering of radiation therapy on a daily basis is imperative. While the clinical target volume (CTV) is defined based on patterns of tumor spread, an additional geometric expansion is typically utilized to create the planning target volume (PTV), which is then used for treatment planning. According International Commission on Radiation Units and Measurements (ICRU) Report 50, the PTV is thus defined “to select the appropriate beam sizes and beam arrangements, taking into consideration the net effect of all possible geometrical variations, in order to ensure that the prescribed dose is actually absorbed in the CTV (1).” From a realistic standpoint, the goal of CTV-to-PTV margins is to compensate for the variability of treatment setup and internal organ motion. However, with improved methods of target localization, specifically with the advent of image-guided radiotherapy (IGRT), the concept of PTV continues to be evaluated 2, 3, 4, 5. At our institution, the standard PTV margins for all patients treated with IMRT for head and neck cancer was 5 mm from April 2005 until August 2007. At that time, an unplanned interim review of our data for all patients treated using daily IGRT was conducted, and this PTV margin was changed to 3 mm. The purpose of this study was to compare patterns of failure, specifically local-regional recurrence (LRR), among patients treated with IMRT for head and neck cancer with the use of 3-mm and 5-mm PTV margins in the setting of daily IGRT.

Section snippets

Study population

This study was approved by the institutional review board at the University of California, Davis School of Medicine, prior to the retrospective collection of all patient information. The medical records of 225 consecutive patients treated with IMRT for squamous cell carcinoma of the oropharynx, oral cavity, nasopharynx, larynx, and hypopharynx between April 2005 and January 2010 formed the subject population for this study. All patients were retrospectively staged in accordance with 2002

Patients

Table 2 outlines the clinical and disease characteristics of the patient population treated with IMRT according to CTV-to-PTV margin size. The median age was 60 years (range, 22-93 years). Concurrent chemotherapy was administered to 129 patients (57%). For patients treated with definitive IMRT, the distribution of T stage was as follows: 15% of patients were T stage 1, 22% were T2, 25% were T3, and 38% were T4. Clinical N stage was as follows: 17% of patients were N stage 0, 14% were N1, 49%

Discussion

A lack of consensus currently exists regarding the optimal CTV-to-PTV expansion margins to be used in the treatment of head and neck cancer with IMRT. Recently developed RTOG trials exemplify this discordance with protocol 0615 (nasopharynx), stating that “a minimum of 5 mm margin around the CTV is required in all directions to define each respective PTV,” and protocol 0522 (oropharynx) specifically states that “a minimum margin of 3 mm can be used in all directions as long as an institution

Conclusions

In conclusion, our results, demonstrating no difference with respect to any of the clinical endpoints studied, suggest that CTV-to-PTV expansion margins can safely be reduced from 5 mm to 3 mm when daily IGRT is used to guide dose delivery. As the toxicity of IMRT from dose to normal structures becomes increasingly apparent, it is likely that a move toward even narrower PTV margins will become popularized, as evident by current RTOG protocols. Further studies analyzing geographical patterns of

References (25)

Cited by (0)

Conflict of interest: none.

View full text