Megavoltage CT imaging as a by-product of multileaf collimator leakage

Abstract

In addition to their potential for the delivery of highly conformal radiation therapy treatments, tomotherapeutic treatments also feature increased potential for verification. For example, megavoltage CT allows one to use the megavoltage linac to generate tomographic images of the patient in the treatment position. This is typically done before or after radiation therapy treatments. However, it is also possible to collect MVCT images entirely during the treatment itself. This process utilizes the leakage radiation through the closed leaves of the Nomos MIMiC MLC, along with slight inefficiencies in treatment delivery, to generate MVCT images during treatment that require neither additional time nor dose. The image quality is limited, yet sufficient to see a patient's external boundary, density differences over 8% for 25.0 mm objects and resolutions of 3.0 mm for high-contrast objects. Such images can potentially be viewed during treatment, used to flag additional CT immediately after the treatment and provide a representation of the patient's exact position during treatment for use with dose reconstruction.

A feasible method for clinical delivery verification and dose reconstruction in tomotherapy

“…Using a MVCT at the time of treatment, [17][18][19][20] the RP and DPD values were computed for both the simulated prostate and canine nasopharyngeal deliveries ͑Sec. III D͒.…”

“…This integration of equipment on a single gantry allows for a host of verification processes. Specifically, patient registration in projection space, 14,15 delivery modification without the need for reoptimization, 16 megavoltage CT imaging ͑MVCT͒, [17][18][19][20] DV, 1,2,7 and DR 4,7-10 will be possible. The transfer matrix approach to DV and DR described previously are verification techniques for general radiotherapy and dynamic intensity modulated radiation therapy as well as tomotherapy: In principle the processes and that described in this manuscript can be used for other delivery strategies ͑e.g., those using dynamic MLCs͒.…”

“…These values are obtained for each projection from a CT of the patient/phantom in the treatment position. [17][18][19][20]43 The consequences of not having a CT that accurately represents the patient/phantom in treatment position are discussed elsewhere. [44][45][46] It will be shown that this database approach to DV provides results that are nearly identical to those with a measured matrix, which in principle is the most accurate technique and is thus used as a benchmark.…”

Towards online patient imaging during helical radiotherapy

“…Their method is highly dependent on the MLC motion, and may be compromised by the irreproducibility of the MLC pattern on the Benchtop machines. Another study [12] explored a method of image reconstruction by utilising leakage radiation through closed leaves. However to achieve this all leaves have to be closed.…”

A Feasibility Study of a Tilted Head Position in Helical Tomotherapy for Fractionated Stereotactic Radiotherapy of Intracranial Malignancies

“…HT offers several unique features including thousands of targeted beamlets, 3608 delivery, and megavoltage computed tomography (MVCT) image guidance (3,4). Thus, HT holds some advantages for irradiating intracranial tumors by: 1) providing accurate radiotherapy using…”