Patient-specific Dose From Megavoltage CT Imaging With a Helical Tomotherapy Unit

Article Outline

• Purpose/Objective(s)
• Materials/Methods
• Results
• Conclusions
• Copyright

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Purpose/Objective(s) 

Megavoltage CT is used daily for imaging with a helical tomotherapy unit to check patient alignment prior to treatment delivery. Daily imaging reduces geometric uncertainties in treatment delivery, but with it increases in-room time and patient dose. The patient dose from imaging varies with different IGRT systems. The purpose of this work was to report the dose received by patients due to megavoltage imaging on a helical tomotherapy unit (TomoTherapy, Inc., Madison, WI).

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Materials/Methods 

A MVCT beam model was commissioned and verified through a series of absorbed dose measurements in phantoms. The computed versus measured dose in phantom was within 5.0%. The beam model was then used to calculate the imaging dose to a variety of clinical scenarios presented in our clinic. MVCT dose and dose-volume histograms were computed for five patients previously treated with helical tomotherapy (prostate, breast, head/neck, lung, and pediatric craniospinal irradiation). In order to report the highest possible imaging dose, the largest MVCT scan region was chosen for dose computation. The computation was then repeated with variations to the CT pitch, as this is the only adjustable parameter during image acquisition (Figure).

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Results 

For each clinical case, the computed dose was inversely proportional to the CT pitch. Relative to a normal pitch of 2.0, the organ doses can be scaled down by 0.33 for coarse (pitch = 3.0) scans and up by 2.0 for fine (pitch = 1.0) scans. For example, the maximum imaging dose to the spinal cord was 1.67 cGy per fraction for the head/neck patient scanned with a pitch of 2.0. When imaged with a pitch of 1.0 and 3.0, the max dose is 3.34 cGy and 0.56 cGy, respectively. In the case of the breast patient, the average breast dose from imaging (pitch of 2.0) totaled 29 cGy over 28 fractions, which is 0.6% of the prescription dose. In the craniospinal case (pitch = 1.0), the maximum total dose to the normal tissues from imaging (20 fractions) was 72 cGy, which is 2.0% of the max dose delivered to the PTV during treatment. To estimate carcinogenesis from imaging doses (pitch = 2.0), an increased excess relative risk of thyroid cancer induction of 2.4 was estimated from the calculated thyroid dose in the craniospinal case.

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Conclusions 

Calculation of patient-specific MVCT doses has shown that the dose from imaging is relatively low. Typical doses range from 1–2 cGy with higher doses in head/neck cases of 3.5 cGy, when imaged with a fine pitch. The uniform MVCT dose delivered via helical tomotherapy was highest when the anatomical thickness was the smallest and the pitch was the lowest. An increased risk of carcinogenesis from daily imaging during radiation therapy treatment is estimated to be minimal.

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