Purpose: Megavoltage cone-beam computed tomography (MVCBCT) has been proposed for treatment verification in conformal radiotherapy. However, the doses required for such imaging may compromise the quality of the delivered dose distribution. The present paper explores the effect of cone-beam imaging on dose homogeneity and critical organ dose and the use of our new tool, adapted intensity-modulated radiation therapy (AIMRT).
Methods and materials: Three types of treatment plans were devised (3D-CRT [three-dimensional conformal radiotherapy], IMRT [intensity-modulated radiotherapy], and AIMRT) based on 4 patients with thoracic malignancies. MVCBCT fields were then integrated into the plans. The MVCBCT technique used 21 imaging portals at 10 degrees intervals. The MVCBCT apertures were shaped to conform to the planning target volume with a 6-mm margin. In a second set of plans, the field size was expanded by a further 2 cm. The unoptimized MVCBCT dose distribution was incorporated into the IMRT plan using AIMRT.
Results: Normal-tissue complication probability with MVCBCT is acceptable for all plans at the 66.6 Gy level, but exceeds tolerance for both 3D-CRT alone and 3D-CRT with MVCBCT at higher doses. In contrast, the use of AIMRT planning with MVCBCT allowed safe dose escalation to 85 Gy. Expanding the MVCBCT aperture provided better anatomic visibility with an acceptable lung dose. The results using IMRT with MVCBCT fell between the values measured for 3D-CRT and AIMRT with MVCBCT.
Conclusion: The present study is the first to demonstrate that MVCBCT can be incorporated into 3D-CRT and IMRT planning with minimal effect on planning target volume homogeneity and dose to critical structures. This paves the way for highly conformal radiotherapy at greater doses delivered with increased confidence and safety.