Practical aspects of inverse-planned intensity-modulated radiation therapy for prostate cancer: a radiation treatment planner's perspective.

INTRODUCTION

From a radiation treatment planner perspective, in the treatment of prostate cancer, inverse-planned intensity-modulated radiation therapy (IMRT) differs considerably from conventional, conformal, and forward-planned IMRT. In this work we aim to discuss the rationale behind the use of inverse-planned IMRT for the treatment of prostate cancer, as well as some of the practical aspects, including the differences in planning strategies, dose fractionation and issues in plan evaluation.

DISCUSSION

The primary motivation behind the use of inverse-planned IMRT for prostate cancer radiotherapy is to attempt further dose escalation while maintaining critical structure and healthy tissue sparing at an acceptable level. The sparing of normal tissues is largely dependent on the size of the planning target volume (PTV) defined, and if the PTV overlaps critical structures. Depending on how the PTV is defined it may be impossible to achieve the desired healthy tissue sparing even with IMRT. A second role for the use of IMRT in the treatment of prostate cancer may be to conform the isodose distribution to a complex PTV, such as one that includes the seminal vesicles or the pelvic lymph nodes in the treatment volume. Finally, inverse planned IMRT may be useful in the planning and delivery of simultaneous integrated boosts where different parts of the target structures receive different daily doses. This again has applications for the simultaneous treatment of pelvic lymph nodes with the prostate treatment volume, and presents interesting opportunities for hypo-fractionation. All of these options of course require careful plan evaluation with respect to isodose distributions and dose-volume constraints as well as the radiobiological consequences of using unconventional fractionation.

CONCLUSION

IMRT seems to be the most effective modality for treating complex target geometries and for delivering simultaneous integrated boosts. In particular for prostate cancer, the simultaneous treatment of the prostate and pelvic lymph nodes lends itself perfectly to IMRT, allowing the prostate to receive a higher daily dose per fraction, as well as minimizing the amount of small bowel in the field, while at the same time sparing the rectum and bladder adequately. Inverse-planned IMRT is, however, a complex procedure, and to safely implement it, an extensive patient- and machine-specific quality assurance program is required.