Treatment of Thymic Oligometastastic or Oligoprogressive Lesions with Hypofractionated Radiation Therapy or Stereotactic Body Radiation Therapy

Purpose/Objective(s)

Recent large clinical trials have demonstrated an improvement in local failure-free survival (LFFS) and progression free survival (PFS) with hypofractionated radiation therapy (HFRT) and stereotactic body radiation therapy (SBRT) in select patients with oligometastatic (OM) and oligoprogressive (OP) disease. However, little is known about the outcomes of HFRT and SBRT in patients with OM or OP disease from thymic malignancies. Therefore, the purpose of this analysis was to characterize the outcomes of such patients treated at our institution.

Materials/Methods

We retrospectively reviewed patients with OM or OP thymic malignancies treated with HFRT or SBRT between 2009-2021. We defined OM as 5 or fewer sites of metastatic disease at the time of RT and OP as 5 or fewer sites of metastatic disease increasing in radiological size. Patients treated with 20 or fewer fractions of radiation were included. Survival outcomes were defined from the end of RT. Analyses were done using the lifelines module in Python.

Results

We analyzed 48 patients treated to 81 lesions. The median age was 54 years old (range: 24-82 years), 63% were male and 79% were white. Patients had thymoma (54%), thymic carcinoma (38%), or atypical thymic carcinoid (8%). At the time of initial diagnosis, patients were stage I-II (21%), III (12.5%), IVA (23%) or IVB (42%). The majority of patients (65%) received systemic therapy prior to radiation therapy. Of the 81 lesions treated, 67 (83%) were OM lesions and 14 (17%) were polymetastatic, but OP lesions. Treatment sites included bone (37%), pleura (25%), lymph nodes (20%), lung (10%), brain (5%) and other (4%). Most patients were treated in 3 fractions (N=31; 8-10 Gy x 3 fractions), 5 fractions (N=28; 5-7 Gy x 5 fractions (N=20) or 8-10 Gy x 5 fractions (N=8)), or 15 fractions (N=11; 2.5-4 Gy x 15 fractions). The median biological effective dose (BED) was 51.3 Gy. Median follow up was 16 months. The 2-year/5-year rates of LFFS and PFS were 83%/76% and 32%/17%, respectively. The 2-year/5-year/10-year rates of overall survival (OS) were 92%/80%/61%. Patients with thymic carcinoma or atypical carcinoid had similar LFFS vs. those with thymoma (2-year LFFS: 85% vs. 82%, p=0.31), but they had shorter PFS (2-year PFS: 5% vs. 39%, p=0.01), and OS was not significantly different (2-year OS: 86% vs. 96%, p=0.25). Patients (N=14 patients treated to 19 lesions) treated with a BED > 60 Gy did not experience any local failure at 2 years (2-yr LFFS=100%).

Conclusion

Patients with OM or OP thymic malignancies treated with HFRT or SBRT had high rates of LFFS, PFS and OS. Despite having a more aggressive disease, patients with thymic carcinoma still experienced favorable LFFS with HFRT or SBRT. Patients treated with regimens with BED greater than 60 Gy had excellent LFFS. These results suggest that with long OS and PFS times, consideration should be given to SBRT/HFRT regimens to maximize local control in these patients with favorable prognoses.