International Journal of Radiation Oncology*Biology*Physics
Physics ContributionAssessment of Planning Target Volume Margins for Intensity-Modulated Radiotherapy of the Prostate Gland: Role of Daily Inter- and Intrafraction Motion
Introduction
Radiotherapy for localized prostate cancer currently utilizes conformal techniques to improve survival rates, local control rates, and toxicity rates. The use of intensity-modulated radiotherapy, in particular, may enable even greater sparing of organs at risk, making dose escalation a realistic option or permitting a further reduction of treatment-related side effects. Nevertheless, accurate target (prostate) localization remains a crucial factor for optimal target dosing and normal tissue avoidance.
Traditionally, target localization has relied on skin marks to infer prostate position, in conjunction with periodic pelvic bony anatomy portal imaging for verification. However, this technique neither takes into account the fact that bony anatomy and skin marks are not reproducibly related, nor does it take into account the fact that the prostate gland moves relative to both skin marks and bony anatomy (1). Although interfraction motion can be reduced using daily image guidance and custom immobilization devices, intrafraction motion continues to occur, and its mitigation has proven quite difficult to quantify. Nonetheless, systems have been used to quantify intrafraction motion, including megavoltage portal imaging (2), magnetic resonance imaging 3, 4, kilovoltage radiographs (5), transabdominal ultrasound (6), and electromagnetic tracking systems (7). Currently, a technique of growing interest is the use of intraprostatic fiducial markers 8, 9 to serve as a surrogate of prostate position. With two-dimensional and three-dimensional (3D) imaging now an integral component of contemporary linear accelerators, fiducial-based image guidance has become a well-established technique not only for patient positioning and repositioning but also for target motion assessment during the course of treatment, albeit snapshots in time.
In the present study, the magnitude of interfraction motion was assessed using four daily localization techniques: three-point skin mark alignment, volumetric imaging with bony landmark registration, volumetric imaging with fiducial marker registration, and patient repositioning with implanted electromagnetic transponders. Intrafraction motion was also assessed by real-time motion tracking using the Calypso System (10) (Calypso Medical Technologies, Inc., Seattle, WA). The planning target volume (PTV) margins needed to deliver 95% of the prescription dose to 95% of the clinical target volume (CTV) for 90% of the patients (11) were computed for all four alignment techniques.
Section snippets
Patient cohort and treatment planning
A cohort of 14 patients with histologically confirmed clinical Stage I–III adenocarcinoma of the prostate gland formed the basis of the present retrospective study. Each patient had three electromagnetic transponders implanted within the prostate gland under transrectal ultrasound guidance at least 1 week before treatment planning simulation (12). With the patient in the supine position, simulation computed tomographic imaging (CT) was performed on a dedicated 16-slice helical big-bore
Quality assurance
For the 14 phantom measurements, the average differences between the measured Calypso offset and the calculated CBCT shift were 0.4 ± 0.4 mm, 0.2 ± 0.3 mm, and 0.4 ± 0.3 mm in the LR, SI, and AP directions, respectively. The accuracy of the bony anatomy registration algorithm (VelocityAI) was better than 0.5 mm (0.07 ± 0.41 mm), a value that included intrauser variability and the effects of fusion CT images of different thickness.
Interfraction prostate motion
Table 1 summarizes interfraction motion as a function of three
Discussion
Linear accelerator and multileaf collimator technology has evolved to the point that radiation doses can be delivered to target volumes with high accuracy and precision. However, the accuracy of these technologies is limited by uncertainty in treatment parameters, including organ motion and setup errors. Therefore, knowledge of these treatment errors, their characteristics and causes, in addition to techniques necessary for their control or mitigation, is an increasingly important component of
Conclusions
Increasing both the prescribed radiation dose and the PTV margins yields the risk of increased treatment-related toxicity. Small increases in PTV margin expansions may greatly increase the volume of tissue irradiated to the prescription dose. Clinically feasible, rapid, and reliable tools to monitor target location during a radiotherapy treatment, like the Calypso System, allow clinicians to reduce the irradiated volume and facilitate safe dose escalation, where appropriate.
Acknowledgments
The authors thank Janet R. Garrett, R.T.(T)., Scott A. Madsen, R.T.(T)., and James O. Price, R.T.(T)., for their dedication to ensuring the implementation of real-time motion assessment for this study; and Charles R. Thomas, Jr., M.D., for providing additional support for the accomplishment of this project.
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Supported in part by generous gifts from Deanne and Dick Rubinstein, the Medical Research Foundation of Oregon, and the Oregon Health and Science University Partnership for Scientific Inquiry program.
Conflict of interest: M. Fuss and J. A. Tanyi receive research support from Calypso Medical Technologies. M. Fuss is a speaker for Varian Medical Systems and receives research support from Varian Medical Systems.