2967 - Volumetric Change of Myxoid Liposarcoma During Preoperative Radiotherapy

P. DUrso¹, M. G. Petrongari¹, V. Ferraresi², R. Biagini³, and G. Sanguineti¹; ¹Department of Radiation Oncology, IRCCS Regina Elena National Cancer Institute, Rome, Italy, ²Sarcomas and Rare Tumors Unit, IRCCS Regina Elena National Cancer Institute, Rome, Italy, ³Orthopaedic Oncology Unit, IRCCS Regina Elena National Cancer Institute, Rome, Italy

Purpose/Objective(s): Soft tissue sarcomas are considered radioresistant tumors and few data are available on their response during chemoradiotherapy (CRT). The purpose of the present study is to assess the volumetric change of myxoid liposarcoma during preoperative CRT.

Materials/

Methods: Patients with pathologically confirmed myxoid liposarcoma undergoing preoperative CRT at our Institution between 2019 and 2023 were considered. Per internal guidelines all patients underwent 50 Gy at 2 Gy fraction. Chemotherapy regimens used were epirubicin and ifosfamide in 7 patients, trabectedine in 2 patients and doxorubicin in monotherapy in 1 patient. Patients underwent daily cone beam CT (CBCT) prior to every RT treatment. The gross tumor volume (GTV) was contoured on each daily CBCT taken during RT. To address the uncertainty of the volume encompassed by each CBCT as well as the identification of the disease at its edge, a margin of 3 cm starting from the isocenter both cranially and caudally was given, aiming to define a core GTV (cGTV), which was then limited to 6 cm craniocaudally. Each cGTV volume was retrospectively contoured by a single observer. The absolute and relative volumetric changes of the cGTV were calculated with respect to the 1^st day and the 1^st week of treatment. For statistical analysis, the linearity of volumetric change was assessed using linear regression, while means were compared with the t test.

Results: Ten patients were identified. All tumors were localized in the lower limb. All patients completed the planned RT course and chemotherapy. By the end of the RT course, all cGTVs experienced volumetric reduction. The median absolute cGTV volumes were 185.9 cc (IQR: 114.9-392.6 cc) at the first day of treatment and 136.2 (IQR: 51.2-236.7 cc) at the last day of treatment, for a median absolute reduction of 61.1 cc (IQR: 36.4-68.3 cc). Linear regression showed a highly significant direct correlation (R²=0.90, p<0.0001) between the volumes at the beginning and the end of RT. Compared to the median volume during the first week of treatment, the median volumes during each subsequent week of treatment fit a linear regression line almost perfectly (R²=0.97, p=0.0013). We observed a median volumetric shrinkage of 40.1% (IQR: 18.8-54.2%) from the first to the last day of treatment. The relative median average reductions during each week of treatment were 6.9% (IQR: 4.7-9.6%), 8.8% (2.6-17.1%), 12.5% (3.3-18.7%) and 8.0% (IQR: 3.0-14.5%) from week 1 to week 2, 2 to 3, 3 to 4 and 4 to 5, respectively (p always >0.5).

Conclusion: All lower extremity myxoid liposarcomas shrank during radiotherapy course and the pattern of their volume reduction was, on average, highly predictable. The rate of shrinkage was not dissimilar during each week of treatment suggesting that there is no specific time point to consider replanning.