A novel IMRT planning study by using the fixed-jaw method in the treatment of peripheral lung cancer with mediastinal lymph node metastasis.

Intensity-modulated radiotherapy (IMRT) is an important technology in cancer radiotherapy. In the current planning system, such as in the Pinnacle3 system, jaw positions are automatically set to cover all target volumes, and many medical centers in developing countries are not equipped with linear accelerators with a jaw tracing function. As solitary lesions are often located in patients, the resulting radiation leakage and transmission increase the dose exposure in surrounding critical organs, although blocked by multileaf collimator (MLC) leaves. We therefore designed a method to manually fix jaw positions, which further reduces doses. We particularly focused on the patients of peripheral lung cancer combined with mediastinal lymph node metastasis, as our medical center mainly targets lung cancer. We designed 2 treatment plans for each patient with the same optimization parameters, i.e., the plan of automatically chosen jaw positions (jaw auto-chosen plan) and the plan of fixed-jaw positions (fixed-jaw plan). In the latter plan, jaws were manually fixed for tumors in lung and in mediastinal lymph node metastases, respectively. We found that both plans met the clinical requirements, and the D2, D98, conformation number (CN), and homogeneity index (HI) for planning target volume (PTV) had no significant differences between the 2 plans. Importantly, the machine units (MUs) for fixed-jaw plans were 50%~60% more than routine jaw auto-chosen plans, whereas the V5, V10, V20, V30, and the mean dose in the total lung and the ipsilateral lung were less than the routine jaw auto-chosen plans. Dose-volume values D1 for the spinal cord and D2, V40, V60 for the heart existed no significant differences for 2 plans. In the fixed-jaw method, the total lung TLV5-△Vab and TLV10-△Vab values had a moderate positive correlation with the lung radiation leakage and the transmission area reduction. We concluded that the fixed-jaw plan is superior to the routine jaw auto-chosen plan in reducing the radiation exposure of surrounding critical organs, which will benefit the IMRT application.