Kinetics of myxoid liposarcoma radiation response and effects on radiation dose delivery

Recent reports have described the radioresponsiveness of myxoid liposarcomas (MLS). We sought to describe the kinetics of these changes during image guided radiation therapy and determine the effect of these changes in volume on the dose distribution.

We reviewed the images of all patients with the diagnosis of MLS who had undergone preoperative radiation therapy using image guidance at our institution. All patients received a dose of 50 Gy in 25 fractions. Tumor volume was measured on each patient's computed tomographic (CT) simulation scan, on weekly CT images performed for image guidance, as well as on postradiation imaging. Dose distributions were recalculated using the last treatment CT images.

Fifteen patients, 9 men and 6 women with a median age of 48 years, were identified in our database. All tumors were located in the lower extremity. During treatment, all tumors decreased in volume (P = .001). The median change in volume from CT simulation to the last image guided radiation therapy image set was -42% (-142 cm(3) median absolute reduction). Volume reduction ranged from -8% to -69%. The median rate of volume change was -5.5 cm(3)/day (range, -0.4 to -14.1). Five patients were clinically judged to require replanning during the course of treatment, all of them during the first 3 weeks. Shrinkage was typically greatest in the transverse direction. The dose distribution did not change significantly when the volumetric changes were taken into account, except for a higher maximal dose to adjacent bone.

Our results confirm that MLS shrink significantly during radiation treatment. In spite of the large volumetric changes, we found little change in the dose to the tumor and surrounding structures. Routine treatment replanning may not be required in these patients but the interval from CT simulation to first treatment should be minimized to avoid the need for early replanning due to tumor progression.