Physics Contribution
Assessment of Planning Target Volume Margins for Intensity-Modulated Radiotherapy of the Prostate Gland: Role of Daily Inter- and Intrafraction Motion

Presented in part at the 50th Annual Meeting of the American Society for Therapeutic Radiology and Oncology, September 21–25, 2008, Boston, MA, and the American Association for Physicists in Medicine, July 27–31, 2008, Houston, TX.
https://doi.org/10.1016/j.ijrobp.2010.02.001Get rights and content

Purpose

To determine planning target volume margins for prostate intensity-modulated radiotherapy based on inter- and intrafraction motion using four daily localization techniques: three-point skin mark alignment, volumetric imaging with bony landmark registration, volumetric imaging with implanted fiducial marker registration, and implanted electromagnetic transponders (beacons) detection.

Methods and Materials

Fourteen patients who underwent definitive intensity-modulated radiotherapy for prostate cancer formed the basis of this study. Each patient was implanted with three electromagnetic transponders and underwent a course of 39 treatment fractions. Daily localization was based on three-point skin mark alignment followed by transponder detection and patient repositioning. Transponder positioning was verified by volumetric imaging with cone-beam computed tomography of the pelvis. Relative motion between the prostate gland and bony anatomy was quantified by offline analyses of daily cone-beam computed tomography. Intratreatment organ motion was monitored continuously by the Calypso® System for quantification of intrafraction setup error.

Results

As expected, setup error (that is, inter- plus intrafraction motion, unless otherwise stated) was largest with skin mark alignment, requiring margins of 7.5 mm, 11.4 mm, and 16.3 mm, in the lateral (LR), longitudinal (SI), and vertical (AP) directions, respectively. Margin requirements accounting for intrafraction motion were smallest for transponder detection localization techniques, requiring margins of 1.4 mm (LR), 2.6 mm (SI), and 2.3 mm (AP). Bony anatomy alignment required 2.1 mm (LR), 9.4 mm (SI), and 10.5 mm (AP), whereas image-guided marker alignment required 2.8 mm (LR), 3.7 mm (SI), and 3.2 mm (AP). No marker migration was observed in the cohort.

Conclusion

Clinically feasible, rapid, and reliable tools such as the electromagnetic transponder detection system for pretreatment target localization and, subsequently, intratreatment target location monitoring allow clinicians to reduce irradiated volumes and facilitate safe dose escalation, where appropriate.

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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      Additionally, although the bone-based registration was performed in our study to obtain weekly pseudo dose distributions, TBPP approaches with implanted fiducial markers and implanted electromagnetic transponders have recently become common practice in radiotherapy. Other researchers have reported that TBPP approaches resulted in reduced margins [31,32]. Therefore, organ motion errors in this study would become smaller than the current results in the TBPP approaches.

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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.

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