Abstract
We studied the feasibility of evaluating tumor control probability (TCP) reductions for tumor motion beyond planned gated radiotherapy margins. Tumor motion was determined from cone‐beam CT projections acquired for patient setup, intrafraction respiratory traces, and 4D CTs for five non‐small cell lung cancer (NSCLC) patients treated with gated radiotherapy. Tumors were subdivided into 1 mm sections whose positions and doses were determined for each beam‐on time point. (The dose calculation model was verified with motion phantom measurements.) The calculated dose distributions were used to generate the treatment TCPs for each patient. The plan TCPs were calculated from the treatment planning dose distributions. The treatment TCPs were compared to the plan TCPs for various models and parameters. Calculated doses matched phantom measurements within 0.3% for up to 3 cm of motion. TCP reductions for excess motion greater than 5 mm ranged from 1.7% to 11.9%, depending on model parameters, and were as high as 48.6% for model parameters that simulated an individual patient. Repeating the worst case motion for all fractions increased TCP reductions by a factor of 2 to 3, while hypofractionation decreased these reductions by as much as a factor of 3. Treatment motion exceeding gating margins by more than 5 mm can lead to considerable TCP reductions. Appropriate margins for excess motion are recommended, unless applying daily tumor motion verification and adjusting the gating window.PACS numbers: 87.55.dk, 87.57.Q‐
Highlights
Respiratory motion can cause lung tumor underdoses and healthy tissue overdoses when not accounted for in radiotherapy (RT) planning
We evaluate the feasibility of estimating the delivered doses using cone-beam CT (CBCT) and strain gauge data recorded during treatment delivery, calculate the tumor control probability (TCP), and compare the results against TCPs estimated with current models for local progression-free survival
Dose calculation model In the phantom studies, we experimented with a number of parameters to adjust the length of the uniform central field, the length of the penumbra, and the slope of the penumbra
Summary
Respiratory motion can cause lung tumor underdoses and healthy tissue overdoses when not accounted for in radiotherapy (RT) planning. One technique for managing tumor motion is gated RT. In gated RT, the beam is turned on once the tumor is completely inside the opening that defines the radiation field. As soon as any portion of the tumor goes outside the field, radiation is turned off. Fields are designed to contain minimal (< 1 cm) tumor motion. Monitors used as signals to control irradiation may not measure tumor motion, but rather some surrogate, such as marker block motion or strain gauge (SG) pressure. Tumor motion, and their relationships may vary between fractions. Tumors could move beyond the motion margins included in the treatment fields.[1] Resulting cold spots can reduce the tumor control probability TCP.[2]
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