A Fixed Jaw Method to Protect Critical Organs in Intensity-Modulated Radiotherapy
J Chen*, J Dai, Cancer Institute Hospital, CAMSBeijingSU-E-T-631 Sunday 3:00PM - 6:00PM Room: Exhibit Hall
Purpose:For some patients to be treated with IMRT, extremely low tolerances of critical organs(such as lens, ovaries and testicles) could not be met during treatment planning. The aim of this work is to introduce a new planning method to overcome this problem.
Methods:In current planning practice, jaw positions are automatically set to cover all target volumes by planning system(eg, Pinnacle3 system). Because of such setting, critical organs may be fully blocked by MLC, but sit in the field shaped by collimator jaws. These critical organs receive doses from MLC leaf transmission and leakage. To reduce their doses further, we manually fixed jaw positions to block them. This method has been used for different treatment sites in our clinic. Here we take 8 patients with radical hysterectomy plus ovarian transposition as example, to demonstrate its performance. For each patient, two treatment plans were generated with the same optimization parameters: the original plan(called O-plan), and the plan with fixed jaw positions(named F-plan). In F-plan, the jaws were manually fixed to block the ovaries.
Results:For target coverage, the mean CI of F-plan was remarkable lower than O-plan(p<0.05). Meanwhile, for the critical organ sparing, the mean dose of left and of right ovary were much lower in F-plan than O-plan(p<0.05). V20, V30 and V40 of bladder were also lower with the F-plan (p<0.05). The maximum dose to the spinal cord PRV of the O-plan and the F-plan were with significant differences(p<0.01). But there were 1.4 times MUs in F-plan compared with the O-plan, thus the treatment time could be longer by using the F-plan.
Conclusion:A fixed-jaw method was developed to protect critical organs. Since it results in better plan quality but more MUs, it is recommended to be used only clinical requirements to critical organs cannot be met with regular method.