International Journal of Radiation Oncology*Biology*Physics
Clinical InvestigationEvaluation 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?
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)
- et al.
Analysis of interfractional set-up errors and intrafractional organ motions during IMRT for head and neck tumors to define an appropriate planning target volume (PTV)- and planning organs at risk volume (PRV)-margins
Radiother Oncol
(2006) - et al.
Quantifying appropriate PTV setup margins: analysis of patient setup fidelity and intrafraction motion using post-treatment megavoltage computed tomography scans
Int J Radiat Oncol Biol Phys
(2007) - et al.
Evaluation of MVCT protocols for brain and head and neck tumor patients treated with helical tomotherapy
Radiother Oncol
(2009) - et al.
Comparison of daily versus less-than-daily image-guided radiotherapy protocols in the treatment of head and neck cancer
Int J Radiat Oncol Biol Phys
(2008) - et al.
Vector analysis of patient setup with image guided radiation therapy (IGRT) via Kv cone beam CT (CBCT)
Int J Radiat Oncol Biol Phys
(2008) - et al.
Toxicity criteria of the Radiation Therapy Oncology Group (RTOG) and the European Organization for Research and Treatment of Cancer (EORTC)
Int J Radiat Oncol Biol Phys
(1995) - et al.
The impact of daily setup variations on head-and-neck intensity-modulated radiation therapy
Int J Radiat Oncol Biol Phys
(2005) - et al.
Effect of patient setup errors on simultaneously integrated boost head and neck IMRT treatment plans
Int J Radiat Oncol Biol Phys
(2005) - et al.
American Society of Radiation Oncology recommendations for documenting intensity-modulated radiation therapy treatments
Int J Radiat Oncol Biol Phys
(2009) - et al.
Assessment of a customized immobilization system for head and neck IMRT using electronic portal imaging
Radiother Oncol
(2005)
A comprehensive assessment of tumor site of patient setup using daily MVCT imaging from more than 3800 helical tomotherapy treatments
Int J Radiat Oncol Biol Phys
The dose to the parotid glands with IMRT for oropharyngeal tumors: The effect of reduction of positioning margins
Radiother Oncol
Cited by (0)
Conflict of interest: none.