Are Current IMRT Plans Superior to 3D Conformal Therapy for Prostate Cancer? A Dosimetric Comparison

Abstract

Historically, dose escalation in prostate cancer has been limited by normal tissue toxicity, primarily in the rectum and to a lesser degree in the bladder. The introduction of CT based 3D conformal radiation therapy (3DCRT) with blocks or multileaf collimators (MLCs) provided for a significant improvement in dose conformality. With the subsequent development and introduction of intensity modulated radiation therapy (IMRT), it is now possible to further conform the delivered dose to the outlined planning target volume (PTV), while at the same time maximize the sparing of the critical structures. This has enabled attempts to escalate the prescribed dose with the goal of not increasing toxicity. With the combination of IMRT and the inverse treatment planning algorithms currently available, dose constraints to critical structures such as the rectum and bladder can be directly included in the optimization calculation of the optimal solution. As of today, there are no standard criteria for absolute dose limits to critical structures, nor is there even a customary system to describe these criteria in the treatment of the prostate cancer. The Radiation Therapy Oncology Group (RTOG) has used data from past published works in an effort to define some of the prescription parameters for 3D-CRT and IMRT treatments in prostate cancer. Early investigations, based on results from a randomized study at MD Anderson Hospital, showed the benefits of higher radiation doses to the prostate. Evaluating toxicity, they found that if no more than 25% of the rectum received 7000 cGy, the risk of rectal bleeding decreased from 46% to 16%. Those have been the parameters for subsequent studies, including the current high priority study RTOG 0415. Some cancer centers have elected to create their own criteria, which is the case at Fox Chase Cancer Center (FCCC) in Philadelphia, PA. The strict criteria they use was also reviewed in this study, to compare with the 3D and RTOG 0415 criteria. Given the significant difference in complexity between the planning and delivery of a 3DCRTplan as compared to an IMRT treatment, the scope of this present study is to investigate whether there are dosimetric advantages of employing IMRT instead of 3D-CRT, utilizing the current constraints as described in RTOG 0415. Methods and Materials