A novel, yet simple MLC-based 3D-crossfire technique for spatially fractionated GRID therapy treatment of deep-seated bulky tumors

Abstract

Purpose: Treating deep-seated bulky tumors with traditional single-field Cerrobend GRID-blocks has many limitations such as suboptimal target coverage and excessive skin toxicity. Heavy traditional GRID-blocks are a concern for patient safety at various gantry-angles and dosimetric detail is not always available without a GRID template in user's treatment planning system. Herein, we propose a simple, yet clinically useful multileaf collimator (MLC)-based three-dimensional (3D)-crossfire technique to provide sufficient target coverage, reduce skin dose, and potentially escalate tumor dose to deep-seated bulky tumors.

Materials/methods: Thirteen patients (multiple sites) who underwent conventional single-field cerrobend GRID-block therapy (maximum, 15 Gy in 1 fraction) were re-planned using an MLC-based 3D-crossfire method. Gross tumor volume (GTV) was used to generate a lattice pattern of 10 mm diameter and 20 mm center-to-center mimicking conventional GRID-block using an in-house MATLAB program. For the same prescription, MLC-based 3D-crossfire grid plans were generated using 6-gantry positions (clockwise) at 60° spacing (210°, 270°, 330°, 30°, 90°, 150°, therefore, each gantry angle associated with a complement angle at 180° apart) with differentially-weighted 6 or 18 MV beams in Eclipse. For each gantry, standard Millenium120 (Varian) 5 mm MLC leaves were fit to the grid-pattern with 90° collimator rotation, so that the tunneling dose distribution was achieved. Acuros-based dose was calculated for heterogeneity corrections. Dosimetric parameters evaluated include: mean GTV dose, GTV dose heterogeneities (peak-to-valley dose ratio, PVDR), skin dose and dose to other adjacent critical structures. Additionally, planning time and delivery efficiency was recorded. With 3D-MLC, dose escalation up to 23 Gy was simulated for all patient's plans.

Results: All 3D-MLC crossfire GRID plans exhibited excellent target coverage with mean GTV dose of 13.4 ± 0.5 Gy (range: 12.43-14.24 Gy) and mean PVDR of 2.0 ± 0.3 (range: 1.7-2.4). Maximal and dose to 5 cc of skin were 9.7 ± 2.7 Gy (range: 5.4-14.0 Gy) and 6.3 ± 1.8 Gy (range: 4.1-11.1 Gy), on average respectively. Three-dimensional-MLC treatment planning time was about an hour or less. Compared to traditional GRID-block, average beam on time was 20% less, while providing similar overall treatment time. With 3D-MLC plans, tumor dose can be escalated up to 23 Gy while respecting skin dose tolerances.

Conclusion: The simple MLC-based 3D-crossfire GRID-therapy technique resulted in enhanced target coverage for de-bulking deep-seated bulky tumors, reduced skin toxicity and spare adjacent critical structures. This simple MLC-based approach can be easily adopted by any radiotherapy center. It provides detailed dosimetry and a safe and effective treatment by eliminating the heavy physical GRID-block and could potentially provide same day treatment. Prospective clinical trial with higher tumor-dose to bulky deep-seated tumors is anticipated.

Keywords: 3D-MLC Crossfire; Bulky-tumors; cerrobend GRID-block; dose-escalation.

Figure 1

Demonstration of the three‐dimensional‐multileaf collimator (3D‐MLC) fit to the grid pattern for each gantry angle used (example case #12, right adrenal). The original gross tumor volume (GTV) contour is shown in red with orange showing the grid‐pattern generated within the original GTV contour for MLC‐based 3D‐crossfire planning.

Figure 2

The isodose colorwash in the axial, coronal and sagittal views is shown for a three‐dimensional‐multileaf collimator (3D‐MLC) GRID plan of example patient #12. The original GTV size was 15 cm (in diameter) in the right abdomen. The prescription was 15 Gy in 1 fraction, allowing maximum point dose of 110% at the tumor center. Utilizing the 3D‐MLC cross‐fire technique, skin was spared dramatically (see all three views) while also respecting dose tolerances of the other internal structures such as large bowel (blue), liver (purple) and right kidney (dark green). Yellow color ring was contoured to calculate D2cm (%) for GRID target.

Figure 3

The beam‐on time for GRID‐block vs three‐dimensional‐multileaf collimator (3D‐MLC) plans for all 13 patients. Mean values of beam‐on time for GRID‐block and 3D‐MLC plans were 4.6 ± 0.2 min (ranged, 4.33–4.94 min), 5.6 ± 0.4 min (ranged, 5.0–6.2 min) with 400 MU/min and 3.7 ± 0.2 min (ranged, 3.33–4.16 min) while re‐calculating 3D‐MLC plans with 600 MU/min, respectively; with 3D‐MLC plans consistently improving the beam‐on time. However, due to gantry rotation time in the 3D‐MLC plans the overall treatment time would be similar.

Figure 4

Calculation of predicted average skin doses (maximal and dose to 5 cc of skin) as a function of escalated prescription doses (Dp) for all 13 GRID therapy patients. A simple three‐dimensional‐multileaf collimator crossfire GRID planning technique allowed for escalation of tumor doses up to 23 Gy while maintaining the skin toxicity.