An off-line strategy for constructing a patient-specific planning target volume in adaptive treatment process for prostate cancer

Purpose: To improve the efficacy of dose delivery and dose escalation for e xternal beam radiotherapy of prostate cancer, an off-line strategy for cons tructing a patient-specific planning target volume is developed in the adap tive radiotherapy process using image feedback of target location and patie nt setup position. Materials and Methods: We hypothesize that a patient-specific confidence-li mited planning target volume (cl-PTV), constructed using an initial sequenc e of daily measurements of internal target motion and patient setup error, exists and ensures that the clinical target volume (CTV) in the prostate ca ncer patient receives the prescribed dose within a predefined dose toleranc e. A patient-specific bounding volume to correct for target location and co mpensate for target random motion was first constructed using the convex hu ll of the first k days of CT measurements. The bounding volume and the init ial days of CT measurements were minimized based on a predefined dosimetric criterion. The hypothesis was tested using multiple daily CT images by mim icking the actual treatment of both conventional 4-field-box and intensity- modulated radiotherapy (IMRT) on each of 30 patients with prostate cancer. For each patient, a patient-specific setup margin was also applied to the b ounding volume to form the final cl-PTV. This margin was determined using t he random setup error predicted from the initial days of portal imaging mea surements and the residuals after correcting for the systematic setup error . Results: The bounding volume constructed using daily CT measurements in the first week of treatment are adequate for the conventional beam delivery to achieve maximum dose reduction in the CTV of 2% or less of the prescriptio n dose, for at least 80% of patients (p = 0.08), and 4.5% or less for 95% o f patients (p = 0.1). However, for IMRT delivery, 2 weeks of daily CT measu rements are required to achieve a similar level of the dosimetric criterion , otherwise the maximum dose reduction of 7%, on average, in the CTV is exp ected. Furthermore, the patient-specific setup margin required for the IMRT treatment is at least twice larger than that for the conventional treatmen t, to maintain the same dosimetric criterion. As compared to the convention al PTV, the volume of cl-PTV is significantly reduced, while maintaining th e same dosimetric criterion. Conclusion: The cl-PTV for prostate treatment can be constructed within the first week of treatment using the feedback of imaging measurements. The cl -PTV has the capability to exclude the systematic variation and compensate for the patient-specific random variation on target location and patient se tup position. This implies that in the current off-line image feedback adap tive treatment process, a single plan modification can be performed within the second week of treatment to improve the efficacy of dose delivery and d ose escalation for external beam therapy of prostate cancer. (C) 2000 Elsev ier Science Inc.