Conebeam CT-Guided MLC-Based GRID Therapy for Very Large Bulky Masses

Abstract

To evaluate the feasibility, safety and efficacy of conebeam CT-guided MLC-based GRID therapy as a same day treatment procedure for large bulky masses.Twenty-eight patients (12 head and neck, 6 pelvis muscle, 3 breast, 2 chest, 2 liver, 2 para-spinal mass and 1 right arm) with large tumors (range 6.5-13.0 cm) tumors were planned and treated using MLC-based 3D-GRID therapy on the same day as CT-simulation. Utilizing an in-house MLC algorithm, 120 Millennium leaves were fitted to gross tumor volume (GTV) to generate 1 cm diameter holes and 2 cm center-to-center distance (at isocenter). For a single-dose of 15 Gy, 3D MLC-based GRID plans were generated using 4 to 6 coplanar crossfire gantry angles at 60o spacing for differentially-weighted 6 and/or 10MV beams with advanced dose calculation algorithm, generating sieve-like dose channeling without post-processing the GRID-GTV contour-allowing for same day treatment. Dosimetric parameters evaluated included: peak-to-valley dose ratio (PVDR) = GTVD10%/GTVD90%, GTVD50%, mean GTV dose, skin dose and maximal dose to immediately adjacent critical organs. Treatment planning and delivery times and patient set up accuracy was recorded.MLC-GRID plans showed excellent dose parameters with mean GTVD50% and mean GTV of 15 Gy being 7.6 ± 0.5 Gy (range 6.6-8.5 Gy) and 7.8 ± 0.4 Gy (range 7.2-8.9 Gy), respectively. Average PVDR was 3.1 ± 0.6 (range 2.7-4.6). Maximum and D5 cc of skin were 10.0 ± 3.2 Gy (range 5.5-13.1 Gy) and 6.6 ± 2.2 Gy (range 1.3-9.6 Gy), respectively. Maximal doses immediately adjacent critical organs were as follows: spinal cord < 3.5 Gy, larynx < 6.7 Gy, humeral head < 6.3 Gy, femoral head < 6.5 Gy, small bowel < 6.9 Gy. Planning time was typically an hour. Mean beam-on time was 3.31 ± 0.16 min (range 3.11-3.54 min) and mean treatment time, including patient set up and conebeam CT imaging to beam-off, was 11.35 ± 0.21 min (range 11.11-11.56 min). Average 3-dimentional vector couch correction was (from the skin-marker) 0.49 ± 0.22 cm (range 0.18-0.88 cm).The 3D MLC-based forward planned GRID therapy enhanced target dose for bulky masses including deep-seated large tumors while protecting skin and adjacent critical organs. This same day MLC-GRID treatment provides safe, effective, and convenient treatment by eliminating the risk to therapists and patient of heavy gantry-mounted physical GRID-block. This rapid 3D MLC-GRID planning and treatment can be easily adopted by any radiation clinics on the same day eliminating the need of longer plan optimization and patient-specific quality assurance. MLC-GRID provides all the dosimetry information in the planning system and potentially allow escalating dose to the bulky masses. It may allow for debulking of unresectable large tumors-opening an avenue for a neoadjuvant treatment option. Clinical follow up of these patients is ongoing.