International Journal of Radiation Oncology*Biology*Physics
Clinical investigation: head and neckQuantification of volumetric and geometric changes occurring during fractionated radiotherapy for head-and-neck cancer using an integrated CT/linear accelerator system
Introduction
Radiotherapy (RT) has been used in the treatment of patients with head-and-neck cancer for several decades. However, whether RT is used definitively in an organ-preserving strategy or postoperatively to enhance locoregional control, it is fundamental that an adequate radiation dose be accurately delivered to the desired target volume throughout the treatment course. The clinical consequences for inaccuracy include both potential underdosage of the target volumes (with a resultant increased risk of tumor recurrence) and potential overdosage of normal tissues (with a resultant increased risk of complications).
Recent advances in image-based treatment planning have improved the clinician's ability to design conformal treatment plans that maximize both tumor coverage and normal tissue sparing 1, 2, 3, 4, 5. However, several variables could theoretically cause deviations in radiation dose delivery from the thoughtful initial treatment plan. Aside from variations in the initial target volume and normal tissue delineation, these uncertainties include external daily setup variations and internal geometric and volumetric changes occurring throughout the 6–7-week RT course.
It has long been recognized that some patients receiving RT to the head-and-neck will have significant anatomic changes during their treatment course, including shrinking primary tumors or nodal masses, resolving postoperative changes/edema, and changes in overall body habitus/weight loss 6, 7, 8, 9. An example of such a case is shown in Fig. 1. However, the methods available for quantification of such changes have been rather crude. The studies of radiation effect on the gross tumor volume (GTV) and position, for instance, have been limited to recording the gross disease characteristics by methodical physical examinations before, during, and at the completion of RT 6, 7, 8, 9. These studies did not adequately describe the ongoing geometric changes in tumors and normal tissues during a several-week course of treatment.
Despite this limited understanding of the anatomic changes occurring during RT, clinicians have nonetheless recognized these changes as clinically significant. For example, ongoing weight loss can cause imprecise daily repositioning if an immobilization device loosens over time; this could potentially result in underdosage of the tumor/target tissues (persistence/recurrence risk) or overdosage of normal tissues (toxicity risk). Although at times these are clinically relevant even with conventional RT techniques, concern is heightened that geometric changes have even greater significance when using the more modern, highly conformal RT delivery methods.
Currently, RT is planned on the basis of the acquisition of a single set of CT scans before the start of treatment. This treatment plan includes a margin for potential microscopic spread (i.e., clinical target volume), along with the anticipated uncertainties related to daily setup variation and intrafractional organ motion (i.e., planning target volume) 10, 11. This margin does not specifically address ongoing alterations in anatomy during the next 6–7 weeks of therapy. It is our hypothesis that anatomic changes throughout fractionated RT could have significant dosimetric effects in the setting of highly conformal treatment approaches, such as intensity-modulated RT (IMRT). We, therefore, conducted a pilot study to investigate this question with frequent CT imaging throughout the RT course. With the recent availability of in-room CT scanners or integrated CT/linear accelerator combinations, patient position and internal anatomy can be imaged frequently with the patient in the treatment position 12, 13, 14, 15. In this investigation, we concentrated on quantifying these geometric and volumetric changes during the course of RT.
Section snippets
Patient eligibility
We designed a pilot study, approved by the institutional review board (IDO2-617), to enroll 15 patients at The University of Texas M.D. Anderson Cancer Center. Eligible patients were newly diagnosed with head-and-neck cancer and had gross primary and/or cervical nodal disease measuring at least 4 cm in maximal dimension that was visible by CT. Patients were treated with definitive external beam RT using either conventional or conformal treatment techniques. The use of concurrent chemotherapy
Patient accrual and clinical characteristics
Between December 31, 2002 and March 3, 2003, 15 patients were enrolled in a pilot study at The University of Texas M.D. Anderson Cancer Center. The patient characteristics are shown in Table 1. The planned CT scans could not be acquired in 1 patient because the body habitus was incompatible with the bore of the treatment CT scanner. The remaining 14 patients were treated with definitive RT or chemoradiotherapy and successfully underwent additional CT scanning according to the protocol. Most
Discussion
In this study, we attempted to specifically quantify the progressive geometric/anatomic changes occurring in patients treated with RT for head-and-neck cancer. In this study, we used high-definition imaging studies throughout the treatment course to quantify the magnitudes and rates of three-dimensional anatomic changes occurring gradually over time. Significant volumetric and positional changes occurred for gross disease in these patients with locally advanced head-and-neck carcinoma. Over
Conclusion
In this investigation, we quantified the geometric and volumetric changes in anatomy for 14 patients who underwent fractionated external beam RT for head-and-neck cancer. Measurable anatomic changes occurred throughout the course of treatment. For both primary tumor and involved lymph nodes, the rate of volume regression was approximately 1.8%/d, relative to the initially defined volume. The parotid volume also reduced at a rate of approximately 0.6%/d. These changes in anatomy resulted in
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