2380 - Quantitative Dosimetric Analysis of Patterns of Local Recurrence (LR) after IMRT for Primary Undifferentiated Pleomorphic Sarcoma (UPS) of the Extremity
Presenter(s)
J. Y. Shin1, S. Shen2, S. L. Berry3, M. Zhang4, C. A. Barker1, B. A. Mueller1, and K. M. Alektiar2; 1Department of Radiation Oncology, Memorial Sloan Kettering Cancer Center, New York, NY, 2Memorial Sloan Kettering Cancer Center, New York, NY, 3Department of Medical Physics, Memorial Sloan Kettering Cancer Center, New York, NY, 4MSKCC, New York, NY
Purpose/Objective(s): UPS is one of the most common histologies in soft tissue sarcoma (STS) of the extremity with high risk for recurrence. The aim of this study is to apply a quantitative dosimetric method of characterizing and analyzing LR in patients with primary extremity STS treated with IMRT and limited to UPS histology.
Materials/Methods: Data were extracted from an institutional tumor registry for primary UPS treated with limb-sparing surgery and IMRT. The CTV for the primary was defined as the GTV/tumor bed plus a 4 cm superior/inferior margin, and 1-1.5 cm radial margin, excluding bone and skin. For post-op cases, the sequential cone-down CTV was defined as the tumor bed plus a 1 cm uniform expansion. A PTV was generated using a 0.5-1 cm uniform expansion. Kaplan-Meier and log-rank test were used to estimate LR. Patterns of LR were defined as follows: central as =95% of gross tumor volume recurrence (rGTV) within the prior IMRT course 95% isodose line (D95), in-field as =80%-95% of rGTV within the prior D95, marginal as 20-80% of rGTV within the prior D95, and outside as <20% of rGTV within the prior D95.
Results: 134 patients with primary non-metastatic UPS who were treated with oncologic resection and IMRT from 9/2004 to 12/2020 were examined. The sequencing of IMRT was preoperative (median dose 50 Gy) in 27% (n=36) of patients and postoperative (median dose 63 Gy) in 73% (n=98). With a median follow-up of 74 months, there were 19 (14%) LRs (5-yr LR 12.5%). Of which, 31% (6) were central, 21% (4) in-field, 26% (5) marginal, and 21% (4) were outside. The 5-year LR rate as well the median time to LR is shown in the Table. Factors including age, site of disease, depth of disease, tumor size, margin status, IMRT sequencing, and chemotherapy administration were not predictors for LR.
Conclusion: Based on this quantitative dosimetric analysis, the 5-year marginal LR rate was only 3%. Furthermore, within the marginal LR category, 4/5 of the 5 LRs had <37% of the rGTV within the prior D95. This suggests that modifications of the RT volumes, beyond an already large margins, are unlikely to impact most of LRs in UPS. Search for an answer may have to come from better understanding of its tumor biology.
| Pattern of LR | #LR / N | Volume rGTV within Prior D95 | #Positive margins/LR | 5-year LR (95% CI) | Median time to LR (mo) |
| Central | 6/134 | >97% in 6 pts | 3/6 | 5% (1-9%) | 33 (range: 13 - 111) |
| In-field | 4/134 | >92% in 3 pts, 81% in 1 pt | 1/4 | 2.5% (1-5%) | 15 (range: 13 – 81) |
| Marginal | 5/134 | 74% 1 pt, <37% in 4 pts | 1/5 | 3% (1-6%) | 17 (range: 10 - 111) |
| Outside | 4/134 | 6.7% 1 pt, 0% in 3 pts | 1/4 | 2% (1-5%) | 17 (range: 4-88) |