Comparison of three strategies in management of independent movement of the prostate and pelvic lymph nodes.

PURPOSE: Concurrent irradiation of the prostate and pelvic lymph nodes is technically challenging due to treating one moving target and one immobile target. The purposes of this article are to propose a new management strategy and to compare this strategy to the conventional isocenter shift method and the previously proposed MLC-shifting method. METHODS: To cope with two target volumes (one moving and one immobile), the authors propose a new management strategy referred to as multiple adaptive plans (MAPs). This strategy involves the creation of a pool of plans for a number of potential prostate locations. Without requiring any additional hardware or software, the MAP strategy is to choose a plan from the pool that most closely matches the "prostate position of the day." This position can be determined by dual image registrations: One aligned to the implant markers in the prostate and the other aligned to the pelvic bones. This strategy was clinically implemented for a special patient with high risk prostate cancer and pathologically confirmed positive pelvic lymph nodes, requiring concurrent IMRT treatment of the prostate and pelvic lymph nodes. Because this patient had an abdominal kidney, small planning margins around the both targets were desired. Using 17 daily acquired megavoltage cone beam CTs (CBCTs), three sets of validation plans were calculated to retrospectively evaluate the MAP strategy as well as the isoshifting and MLC-shifting strategies. RESULTS: According to the validation plans, MAP, isoshifting, and MLC-shifting strategies resulted in D95 of the prostate > 95% of the daily dose on 65%, 100%, and 100% treatment days, respectively. Similarly, D95 of the pelvic lymph nodal was > 95% of the daily dose on 100%, 75%, and 94% of treatment days, respectively. CONCLUSIONS: None of the above strategies simultaneously achieved all treatment goals. Among the three strategies, the MLC shifting was most successful. Validation plans based on daily CBCTs are useful to evaluate the effectiveness of the motion management strategies and to provide additional dose guidance if further dose compensation is needed.

Investigation of the dosimetric accuracy of the isocenter shifting method in prostate cancer patients with and without hip prostheses.

PURPOSE: The use of image guided radiation therapy (IGRT) enables compensation for prostate movement by shifting the treatment isocenter to track the prostate on a daily basis. Although shifting the isocenter can alter the source to skin distances (SSDs) and the effective depth of the target volume, it is commonly assumed that these changes have a negligible dosimetric effect, and therefore, the number of monitor units delivered is usually not adjusted. However, it is unknown whether or not this assumption is valid for patient with hip prostheses, which frequently contain high density materials. METHODS: The authors conducted a retrospective study to investigate dosimetric effect of the isocenter shifting method for prostate patients with and without hip prostheses. For each patient, copies of the prostate volume were shifted by up to 1.5 cm from the original position to simulate prostate movement in 0.5 cm increments. Subsequently, 12 plans were created for each patient by creating a copy of the original plan for each prostate position with the isocenter shifted to track the position of the shifted prostate. The dose to the prostate was then recalculated for each plan. For patients with hip prostheses, plans were created both with and without lateral beam angles entering through the prostheses. RESULTS: Without isocenter shifting to compensate for prostate motion of 1.5 cm, the dose to the 95% of the prostate (D-95%) changed by an average of 30% and by up to 64%. This was reduced to less than 3% with the isocenter shifting method. It was found that for patients with hip prostheses, this technique worked best for treatment plans that avoided beam angles passing through the prostheses. CONCLUSIONS: The results demonstrated that the isocenter shifting method can accurately deliver dose to the prostate even in patients with hip prostheses.