Practical method of adaptive radiotherapy for prostate cancer using real-time electromagnetic tracking

Abstract

Purpose: We have created an automated process using real-time tracking data to evaluate the adequacy of planning target volume (PTV) margins in prostate cancer, allowing a process of adaptive radiotherapy with minimal physician workload. We present an analysis of PTV adequacy and a proposed adaptive process.

Methods and materials: Tracking data were analyzed for 15 patients who underwent step-and-shoot multi-leaf collimation (SMLC) intensity-modulated radiation therapy (IMRT) with uniform 5-mm PTV margins for prostate cancer using the Calypso® Localization System. Additional plans were generated with 0- and 3-mm margins. A custom software application using the planned dose distribution and structure location from computed tomography (CT) simulation was developed to evaluate the dosimetric impact to the target due to motion. The dose delivered to the prostate was calculated for the initial three, five, and 10 fractions, and for the entire treatment. Treatment was accepted as adequate if the minimum delivered prostate dose (D(min)) was at least 98% of the planned D(min).

Results: For 0-, 3-, and 5-mm PTV margins, adequate treatment was obtained in 3 of 15, 12 of 15, and 15 of 15 patients, and the delivered D(min) ranged from 78% to 99%, 96% to 100%, and 99% to 100% of the planned D(min). Changes in D(min) did not correlate with magnitude of prostate motion. Treatment adequacy during the first 10 fractions predicted sufficient dose delivery for the entire treatment for all patients and margins.

Conclusions: Our adaptive process successfully used real-time tracking data to predict the need for PTV modifications, without the added burden of physician contouring and image analysis. Our methods are applicable to other uses of real-time tracking, including hypofractionated treatment.

Figure 1

(A) Checkerboard axial image of the fused MR and CT scans, and the contoured prostate, illustrated for one patient. The final contoured prostate (CTV) is shown in red. A fiducial beacon placed at the right base of the prostate is seen on this CT slice (arrow). (B) Isodose distribution of a 5-field IMRT prostate plan, showing the 50% (purple), 95% (green), 100% (yellow), and 105% (red) isodose lines.

Figure 2

Real-time tracking data for one treatment, including right/left (top), superior/inferior (middle), and anterior/posterior (bottom) tracking data. The shaded regions illustrate the time when the treatment beam was turned on for each of the 5 planned fields. Following delivery of the second field, treatment was paused due to a persistent shift in the superior and anterior directions which exceeded tolerance. This was corrected with intra-fraction repositioning 270 seconds into treatment. Additional repositioning was utilized at 380 seconds following delivery of the 3rd field, due to motion in the right, superior, and anterior directions which exceeded tolerance.

Figure 3

Illustration of the impact of translational and rotational motion for a prostate (red) plan using a 3mm PTV margin (grey), with prostate rotation of 20 degrees pitch, and translational shifts of 3 mm in the anterior and inferior directions. Real-time tracking translational and rotational data was used to virtually move the prostate within the treatment dose cloud, in order to estimate the ‘actual’ dose delivered for each patients’ motion.

Figure 4

Histogram of the intra-fraction intertransponder distance variation from mean distance (mm) observed throughout treatment for all patients and fractions.

Figure 5

Sagittal image showing 95% (orange) and 50% (green) isodose lines for a patient planned using a tissue density (left) compared to air-equivalent density (right) rectum structure. The rectum-prostate interface structure is shown in red.

Figure 6

Dose Volume Histogram (DVH) curves for patient #4 (a) and patient #6 (b), illustrating “actual” (dotted) and planned (solid) dose for the CTV (see inset) and rectum, for 0 mm (red), 3 mm (green), and 5 mm (blue) PTV margins.

Figure 7

Left: Dose decrement as a function of mean isocenter displacement (left) along the x (left-right), y (superior-inferior), z (anterior-posterior) axes, and composite displacement calculated using the distance formula. Right: Dose decrement as a function of mean intra-fraction pitch, roll, and yaw of the prostate for all patients based on 0 mm PTV plans.

Figure 8

Schematic to illustrate the variable effect of transponder geometry as a gauge for rotation measurements. The fiducial markers are depicted by yellow circles, within the prostate (red) and PTV (orange). 30 degree rotations are shown about the centroid of the three beacons for idealized (top) and skewed (bottom) fiducial geometry. A rotation of a given magnitude (30 degrees in this example) will carry different implications depending on the reference axis of rotation.