P05: Low-dose radiotherapy for the treatment of pleural recurrence of thymic carcinoma

Background: Pleural recurrence is a common type of failure in some late-staged thymic carcinomas even after primary mediastinal mass is completely removed or well controlled. Generally, systemic chemotherapy would be firstly chosen by most oncologists. But it becomes challenging when chemotherapy shows no efficacy or the tumor relapses after response. In most cases, this group of patients has received high-dose radiotherapy to the mediastinal tumor bed before the pleural recurrence is confirmed. Due to normal tissue (lung, heart, etc.) dose restriction, high dose radiotherapy to the pleural lesions would be highly risky. From Nov 2011, we started trying low-dose intensity modulated radiotherapy (IMRT) as a palliative treatment to these patients.

Methods: Patients included in this trial should meet the following criteria: (I) pathologically diagnosed thymic carcinoma; (II) primary mediastinal tumor was resected or well controlled by other means; (III) new pleural lesions were detected during the follow-up; (IV) chemotherapy failed or tumor relapsed after response; (V) adequate pulmonary and cardiac function. The radiotherapy was conducted using IMRT technique and the dose was prescribed below 40 Gy. The gross tumor volume (GTV) should cover all visible lesions.

Results: From Nov 2011 to Feb 2015, there have been 14 patients (male:female =9:5) included in this study. The pathological subtype distribution is squamous carcinoma in 12 patients, lymphoepithelioa-like carcinoma in 1 and thyroid carcinoma showing thymus-like differentiation (CASTLE) in 1. According to Masaoka staging definition, three, seven and four patients are classified as stage III, IVa and IVb respectively when initially diagnosed. Surgery was conducted as first-line treatment in twelve patients to macroscopically remove their intrathoracic tumors, and another two patients received chemoradiotherapy to control mediastinal tumors. The median time between the first-line treatment and pleural recurrence is 18 [7–55] months. There are two patients who received 3 times of IMRT for the recurrence on the different parts of the pleura. There are also three patients who received 2 times. Totally there are 21 courses of IMRT completed in this study. The median dose is 30 [20–40] Gy. The median tumor volume is 27 [8–190] mL. The response rate is 100% (CR:PR =11:10). No tumor progression inside the radiation field is discovered until now. The median local control time is 10 [4–44] months. There are two patients who developed grade 3 pneumonitis induced by radiation. Another 58-year-old male patient died of grade 5 pneumonitis because he suffered a right phrenic nerve injury in the surgery which resulted in the lifting of right diaphragm and increased the proportion of lung volume exposing to X-ray.

Conclusions: IMRT showed excellent local control effect in the management of pleural recurrence of thymic carcinoma even with a dose as low as 20 Gy. At the same time, the in-field progression free survival time is also encouraging. But for cases with phrenic nerve injury, the potential high risk should not be neglected.

Keywords: Thymic carcinoma; pleural recurrence; radiotherapy