Dose enhancement in radiotherapy of small lung tumors using inline magnetic fields: A Monte Carlo based planning study

Purpose: To report on significant dose enhancement effects caused by magnetic fields aligned parallel to 6 MV photon beam radiotherapy of small lungtumors. Findings are applicable to future inline MRI-guided radiotherapy systems. Methods: A total of eight clinical lungtumor cases were recalculated using Monte Carlo methods, and external magnetic fields of 0.5, 1.0, and 3 T were included to observe the impact on dose to the planning target volume (PTV) and gross tumor volume (GTV). Three plans were 6 MV 3D-CRT plans while 6 were 6 MV IMRT. The GTV’s ranged from 0.8 to 16 cm3, while the PTV’s ranged from 1 to 59 cm3. In addition, the dose changes in a 30 cm diameter cylindrical water phantom were investigated for small beams. The central 20 cm of this phantom contained either water or lung density insert. Results: For single beams, an inline magnetic field of 1 T has a small impact in lungdose distributions by reducing the lateral scatter of secondary electrons, resulting in a small dose increase along the beam. Superposition of multiple small beams leads to significant dose enhancements. Clinically, this process occurs in the lungtissue typically surrounding the GTV, resulting in increases to the D98% (PTV). Two isolated tumors with very small PTVs (3 and 6 cm3) showed increases in D98% of 23% and 22%. Larger PTVs of 13, 26, and 59 cm3 had increases of 9%, 6%, and 4%, describing a natural fall-off in enhancement with increasing PTV size. However, three PTVs bounded to the lung wall showed no significant increase, due to lack of dose enhancement in the denser PTV volume. In general, at 0.5 T, the GTV mean dose enhancement is around 60% lower than that at 1 T, while at 3 T, it is 5%–60% higher than 1 T. Conclusions: Monte Carlo methods have described significant and predictable dose enhancement effects in small lungtumor plans for 6 MV radiotherapy when an external inline magnetic field is included. Results of this study indicate that future clinical inline MRI-guided radiotherapy systems will be able to deliver a dosimetrically superior treatment to small (PTV < 15 cm3), isolated lungtumors over non-MRI-Linac systems. This increased efficacy coincides with the reimbursement in the United States of lungCT screening and the likely rapid growth in the number of patients with small lungtumors to be treated with radiotherapy.