Conebeam CT-guided 3D MLC-based spatially fractionated radiation therapy for bulky masses.

For fast, safe, and effective management of large and bulky (≥8 cm) non-resectable tumors, we have developed a conebeam CT-guided three-dimensional (3D)-conformal MLC-based spatially fractionated radiation therapy (SFRT) treatment. Using an in-house MLC-fitting algorithm, Millennium 120 leaves were fitted to the gross tumor volume (GTV) generating 1-cm diameter holes at 2-cm center-to-center distance at isocenter. SFRT plans of 15 Gy were generated using four to six coplanar crossfire gantry angles 60° apart with a 90° collimator, differentially weighted with 6- or 10-MV beams. A dose was calculated using AcurosXB algorithm, generating sieve-like dose channels without post-processing the physician-drawn GTV contour within an hour of CT simulation allowing for the same day treatment. In total, 50 extracranial patients have been planned and treated using this method, comprising multiple treatment sites. This novel MLC-fitting algorithm provided excellent dose parameters with mean GTV (V7.5 Gy) and mean GTV doses of 53.2% and 7.9 Gy, respectively, for 15 Gy plans. Average peak-to-valley dose ratio was 3.2. Mean beam-on time was 3.32 min, and treatment time, including patient setup and CBCT to beam-off, was within 15 min. Average 3D couch correction from original skin-markers was <1.0 cm. 3D MLC-based SFRT plans enhanced target dose for bulky masses, including deep-seated large tumors while protecting skin and adjacent critical organs. Additionally, it provides the same day, safe, effective, and convenient treatment by eliminating the risk to therapists and patients from heavy gantry-mounted physical GRID-block-we recommend other centers to use this simple and clinically useful method. This rapid SFRT planning technique is easily adoptable in any radiation oncology clinic by eliminating the need for plan optimization and patient-specific quality assurance times while providing dosimetry information in the treatment planning system. This potentially allows for dose-escalation to deep-seated masses to debulk unresectable large tumors providing an option for neoadjuvant treatment. An outcome study of clinical trial is underway.