The virtual cone: A novel technique to generate spherical dose distributions using a multileaf collimator and standardized control-point sequence for small target radiation surgery

Abstract

Purpose: The study aimed to develop and demonstrate a standardized linear accelerator multileaf collimator-based method of delivering small, spherical dose distributions suitable for radiosurgical treatment of small targets such as the trigeminal nerve.

Methods and materials: The virtual cone is composed of a multileaf collimator-defined field with the central 2 leaves set to a small gap. For 5 table positions, clockwise and counter-clockwise arcs were used with collimator angles of 45 and 135 degrees, respectively. The dose per degree was proportional to the sine of the gantry angle. The dose distribution was calculated by the treatment planning system and measured using radiochromic film in a skull phantom for leaf gaps of 1.6, 2.1, and 2.6 mm. Cones with a diameter of 4 mm and 5 mm were measured for comparison. Output factor constancy was investigated using a parallel-plate chamber.

Results: The mean ratio of the measured-to-calculated dose was 0.99, 1.03, and 1.05 for 1.6, 2.1, and 2.6 mm leaf gaps, respectively. The diameter of the measured (calculated) 50% isodose line was 4.9 (4.6) mm, 5.2 (5.1) mm, and 5.5 (5.5) mm for the 1.6, 2.1, and 2.6 mm leaf gap, respectively. The measured diameter of the 50% isodose line was 4.5 and 5.7 mm for the 4 mm and 5 mm cones, respectively. The standard deviation of the parallel-plate chamber signal relative to a 10 cm × 10 cm field was less than 0.4%. The relative signal changed 32% per millimeter change in leaf gap, indicating that the parallel-plate chamber is sensitive to changes in gap width.

Conclusions: The virtual cone is an efficient technique for treatment of small spherical targets. Patient-specific quality assurance measurements will not be necessary in routine clinical use. Integration directly into the treatment planning system will make planning using this technique extremely efficient.

Figure 1

Beam geometry for table angles 0, 36, 72, 288, and 324 degrees and leaf gap 2.1 mm. The beams-eye-view of the multileaf collimators aperture is shown for (A) clockwise and (B) counterclockwise arcs at table angles 0, 36, 72, 288, and 324 degrees. Arc geometry is shown in (C) the coronal and (D) sagittal planes.

Figure 2

Profiles measured using radiochromic film in an anthropomorphic skull phantom for the virtual cone with a 2.1 mm leaf gap, 4 mm cone, and 5 mm cone.

Figure 3

Profiles calculated using Eclipse AAA version 13.6 and measured using radiochromic film in an anthropomorphic skull phantom.

Figure 4

Electronic portal imaging device images for a 2.1 mm × 5 mm multileaf collimator aperture at collimator angles (A) 45 and (B) 135 degrees. The sum of the images (A) and (B) is shown in (C). The centroid of the high dose region in (A) to (C) is shown with a red x. An electronic portal imaging device image of a 3 cm × 3 cm multileaf collimator aperture is shown in (D). The centroid of the target ball in (D) is shown with a red +.

Figure 5

Dose distribution of virtual cone treatment targeting the root entry zone of the trigeminal nerve. Displayed in the transverse and coronal planes of the treatment planning CT and 3-dimensional constructive interference in steady-state (3D-CISS) magnetic resonance images are isodose lines corresponding to 50% (yellow), 25% (green), and 10% (cyan) of the maximum dose.

Figure 6

Dose distribution of virtual cone treatment targeting the ventral intermediate nucleus of the hypothalamus. Displayed in the principle planes of a T1 magnetic resonance imaging scan obtained 11 weeks after treatment are the isodose lines corresponding to 50% (yellow), 25% (green), and 10% (cyan) of the maximum dose. Note the enhancement encompassed by the 50% isodose volume.

Figure 7

Dose distribution for a virtual cone treatment plan with a dose per degree (A) proportional to the sine of the gantry angle and (B) constant. The displayed isodose lines correspond to 50% (yellow), 25% (green), and 10% (cyan) of the maximum dose.