Clinical investigation
Head and neck
Repeat CT imaging and replanning during the course of IMRT for head-and-neck cancer

The paper was presented orally at the 46th Annual Meeting of the American Society for Therapeutic Radiology and Oncology (ASTRO), Atlanta, GA, October 4, 2004.
https://doi.org/10.1016/j.ijrobp.2005.07.957Get rights and content

Purpose: Many patients with head-and-neck (H&N) cancer have tumor shrinkage and/or weight loss during the course of radiotherapy. We conducted this retrospective study to determine the dosimetric effects of repeat computed tomography (CT) imaging and replanning during the course of intensity-modulated radiotherapy (IMRT) on both normal tissues and target volumes.

Methods and Materials: A retrospective chart review identified 13 patients with H&N cancer treated with IMRT who had repeat CT imaging and replanning during the course of radiotherapy. The first IMRT plan for each patient was generated based on the original planning CT scan acquired before the start of treatment. Because of tumor shrinkage or weight loss during radiotherapy, a second CT scan was acquired, and a new plan was generated and used to complete the course of IMRT. CT–CT fusion was used to correct patient positioning differences between the scans. By using a commercial inverse IMRT planning system, a hybrid IMRT plan was generated for each patient by applying the beam configurations of the first IMRT plan (including the intensity profile of each beam) to the anatomy of the second CT scan. The dose–volume histograms of the actual and hybrid IMRT plans were compared using analysis of variance methods for repeated measures.

Results: All patients had locally advanced, nonmetastatic Stage III or IV disease, including 6 nasopharynx, 6 oropharynx, and 1 unknown primary site. All patients were treated with concurrent platinum-based chemotherapy. When replanning vs. not replanning was compared, the hybrid IMRT plans (without replanning) demonstrated reduced doses to target volumes and increased doses to critical structures. The doses to 95% (D95) of the planning target volumes of the gross tumor volume (PTVGTV) and the clinical target volume (PTVCTV) were reduced in 92% of patients, by 0.8–6.3 Gy (p = 0.02) and 0.2–7.4 Gy (p = 0.003), respectively. The maximum dose (Dmax) to the spinal cord increased in all patients (range, 0.2–15.4 Gy; p = 0.003) and the brainstem Dmax increased in 85% of patients without replanning (range, 0.6–8.1 Gy; p = 0.007).

Conclusions: Repeat CT imaging and replanning during the course of IMRT for selected patients with H&N cancer is essential to identify dosimetric changes and to ensure adequate doses to target volumes and safe doses to normal tissues. Future prospective studies with larger sample sizes will help to determine criteria for repeat CT imaging and IMRT replanning for H&N cancer patients undergoing radiotherapy.

Introduction

Radiotherapy plays a critical role in the management of many patients with head-and-neck (H&N) cancer. During the course of radiation treatment, many patients develop significant anatomic changes that may be related to multiple factors, including shrinkage of the tumor and/or nodal masses, weight loss, and resolution of postoperative changes (1, 2, 3). In a recent study of 14 patients with tumors or lymph nodes measuring ≥4 cm in diameter, the gross tumor volume decreased throughout the course of radiotherapy at a median rate of 1.7%–1.8% per treatment day, and the parotid glands also decreased in volume at a median rate of 0.6% per treatment day (1). Moreover, the volume loss of these structures was frequently asymmetric. It may thus be hypothesized that the doses to target volumes and normal structures during the course of fractionated radiotherapy may significantly differ from the planning doses based on the computed tomography (CT) images obtained before treatment.

Four-dimensional radiotherapy has been described as “the explicit inclusion of the temporal changes in anatomy during the imaging, planning, and delivery of radiotherapy” (4, 5). Temporal changes in anatomy may occur for many reasons as a result of interfraction or intrafraction motion or deformation. Examples of interfraction changes include daily changes in bowel or bladder filling, daily positioning or setup errors, or tumor shrinkage or weight loss over time. Intrafraction changes may occur during sessions of radiotherapy as a result of processes such as breathing, swallowing, and cardiac motion, or patient movement due to discomfort. Temporal changes in anatomy are not usually accounted for with conventional three-dimensional conformal radiotherapy or intensity-modulated radiotherapy (IMRT) plans that use a planning CT scan acquired before treatment.

With the advent of IMRT, highly conformal treatments are designed to maximize tumor coverage and spare normal tissues (6, 7, 8). In the setting of IMRT, the dosimetric changes that occur during treatment may be even more drastic than in conventional treatments, due to the sharp dose gradients between the boundary of target volumes and critical normal tissues (9). The clinical consequences of these changes may be underdosage of tumor volumes and/or overdosage of critical normal structures. We therefore initiated this retrospective study to investigate the dosimetric consequences of repeat CT imaging and IMRT replanning during the course of IMRT for patients with H&N cancer.

Section snippets

Patients

A retrospective chart review at the University of California, San Francisco (UCSF) identified 13 patients with head-and-neck cancer who were treated with IMRT between November 2000 and February 2004 who had repeat CT imaging and replanning during their course of treatment. The UCSF Committee on Human Resources approved this study to investigate the use of reimaging and replanning during the course of IMRT.

Treatment and imaging

Before treatment, all patients underwent head-and-neck immobilization with a thermoplastic

Patient characteristics

Of the 13 patients, 6 had nasopharynx cancer, 6 had oropharynx cancer, and 1 had carcinoma of unknown primary site. All patients had locally advanced, nonmetastatic Stage III–IV disease (Table 1). All patients were treated with concurrent platinum-based chemotherapy. The indications for reimaging and replanning were tumor and/or nodal shrinkage (5 patients), weight loss (3 patients), or both (5 patients). The degree of weight loss among the 3 patients replanned for this reason only was 7%, 12%,

Discussion

This retrospective study demonstrates the importance of repeat CT imaging and IMRT replanning during the course of IMRT for selected patients with H&N cancer. Our results suggest that repeat CT imaging and IMRT replanning help to ensure adequate doses to target volumes and safe doses to normal structures for patients who have clinically identified anatomic changes during the course of IMRT. The key findings of this investigation relate to the dosimetric changes that were observed without IMRT

Conclusion

Repeat CT imaging and replanning during the course of IMRT for selected patients with H&N cancer is essential to identify dosimetric changes and to ensure adequate doses to target volumes and safe doses to normal tissues. Future prospective studies with larger sample sizes will help to determine criteria for repeat CT imaging and IMRT replanning for H&N cancer patients undergoing radiotherapy.

Acknowledgments

We thank Leah Ezzell, A.B. and Jeffrey Bellerose, B.S., for their assistance in treatment planning in this study.

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