Abstract
Simple SummaryThymic tumors, i.e., thymomas and thymic carcinomas, are rare tumors that derive from the remnant of the thymus gland. Although surgery is the first treatment of choice, some patients will be treated with radiotherapy. For many patients the prognosis is good, hence it is important to avoid treatment related complications such as radiation-induced secondary malignancies. Radiotherapy can be delivered with different techniques and with different particles. In the present study, we compare the calculated (estimated) risks for secondary malignancies after treatment of thymic tumors with two different photon (x-ray) radiotherapy techniques or with proton beam therapy. We use a commonly used radiobiological model to calculate the risks for radiation induced secondary malignancies for each treatment modality. In conclusion, proton beam therapy was shown to provide the potential for reducing the risk of secondary malignancies, compared to photon radiotherapy, after treatment of thymic tumors.We compared the calculated risks of radiation-induced secondary malignant neoplasms (SMNs) for patients treated for thymic tumors with 3D-CRT, IMRT, or single-field uniform dose (SFUD) proton beam therapy (PBT) using the pencil beam scanning (PBS) technique. A cancer-induction model based on the organ equivalent dose (OED) concept was used. For twelve patients, treated with 3D-CRT for thymic tumors, alternative IMRT and SFUD plans were retrospectively prepared. The resulting DVHs for organs at risk (OARs) were extracted and used to estimate the risk of SMNs. The OED was calculated using a mechanistic model for carcinoma induction. Two limit cases were considered; the linear-exponential model, in which the repopulation/repair of the cells is neglected, and the plateau model, in which full repopulation/repair of the irradiated cells is assumed. The calculated risks for SMNs for the different radiation modalities and dose-relation models were used to calculate relative risks, which were compared pairwise. The risks for developing SMNs were reduced for all OARs, and for both dose-relation models, if SFUD was used, compared to 3D-CRT and IMRT. In conclusion, PBS shows a potential benefit to reduce the risk of SMNs compared to 3D-CRT and IMRT in the treatment of thymic tumors.
Highlights
Thymomas, i.e., tumors that derive from the epithelium of the thymus gland, are the most common tumor of the anterior mediastinum [1]
We focused on the use of active-scanning proton beam therapy (PBT) to reduce secondary malignant neoplasms (SMNs), compared to photon beam RT techniques, i.e., 3D-CRT and intensity-modulated RT (IMRT)
Doses given to all organs at risk (OARs) were significantly reduced in the pencil beam scanning (PBS) plans compared to the 3D-CRT and IMRT plans, with the exceptions of esophagus (Dmax), breast (Dmax), and skin (Dmax)
Summary
I.e., tumors that derive from the epithelium of the thymus gland, are the most common tumor of the anterior mediastinum [1]. The tumor location adjacent to vital organs at risk (OARs), e.g., lungs, heart, and esophagus, makes it a rather challenging diagnosis in RT. Most patients treated for thymoma have a good long-term prognosis [6], it is vital to avoid late radiation-induced toxicity and secondary malignant neoplasms (SMNs) as much as possible. Most patients are treated with photon beam RT delivered by three-dimensional conformal RT (3D-CRT) or intensity-modulated RT (IMRT) [7]. Due to better target dose conformity obtained with IMRT, the higher doses given to the OARs are usually given to smaller volumes compared to 3D-CRT. The integral dose given to the entire body is often significantly higher in IMRT, and this may cause more SMNs compared to 3D-CRT [8,9]
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