Comparison of treatment planning approaches for spatially fractionated irradiation of deep tumors

Abstract

Purpose The purpose of this work was to compare the dosimetry and delivery times of 3D‐conformal (3DCRT)‐, volumetric modulated arc therapy (VMAT)‐, and tomotherapy‐based approaches for spatially fractionated radiation therapy for deep tumor targets. Methods Two virtual GRID phantoms were created consisting of 7 “target” cylinders (1‐cm diameter) aligned longitudinally along the tumor in a honey‐comb pattern, mimicking a conventional GRID block, with 2‐cm center‐to‐center spacing (GRID2 cm) and 3‐cm center‐to‐center spacing (GRID3 cm), all contained within a larger cylinder (8 and 10 cm in diameter for the GRID2 cm and GRID3 cm, respectively). In a single patient, a GRID3 cm structure was created within the gross tumor volume (GTV). Tomotherapy, VMAT (6 MV + 6 MV‐flattening‐filter‐free) and multi‐leaf collimator segment 3DCRT (6 MV) plans were created using commercially available software. Two tomotherapy plans were created with field widths (TOMO2.5 cm) 2.5 cm and (TOMO5 cm) 5 cm. Prescriptions for all plans were set to deliver a mean dose of 15 Gy to the GRID targets in one fraction. The mean dose to the GRID target and the heterogeneity of the dose distribution (peak‐to‐valley and peak‐to‐edge dose ratios) inside the GRID target were obtained. The volume of normal tissue receiving 7.5 Gy was determined. Results The peak‐to‐valley ratios for GRID2 cm/GRID3 cm/Patient were 2.1/2.3/2.8, 1.7/1.5/2.8, 1.7/1.9/2.4, and 1.8/2.0/2.8 for the 3DCRT, VMAT, TOMO5 cm, and TOMO2.5 cm plans, respectively. The peak‐to‐edge ratios for GRID2 cm/GRID3 cm/Patient were 2.8/3.2/5.4, 2.1/1.8/5.4, 2.0/2.2/3.9, 2.1/2.7/5.2 and for the 3DCRT, VMAT, TOMO5 cm, and TOMO2.5 cm plans, respectively. The volume of normal tissue receiving 7.5 Gy was lowest in the TOMO2.5 cm plan (GRID2 cm/GRID3 cm/Patient = 54 cm3/19 cm3/10 cm3). The VMAT plans had the lowest delivery times (GRID2 cm/GRID3 cm/Patient = 17 min/8 min/9 min). Conclusion Our results present, for the first time, preliminary evidence comparing IMRT‐GRID approaches which result in high‐dose “islands” within a target, mimicking what is achieved with a conventional GRID block but without high‐dose “tail” regions outside of the target. These approaches differ modestly in their ability to achieve high peak‐to‐edge ratios and also differ in delivery times.

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

“…Currently, MV GRID therapy treatments are delivered using a high attenuation GRID-block with divergent holes, with step and shoot multileaf collimator (MLC) control points, and/or Tomotherapy machines. [4][5][6][7][8][9][10][11][12][13][14][15] Although, the treatment planning studies for GRID therapy using tomotherapy and step and shoot MLCs are evolving, these techniques require longer treatment times due to beam modulation and need patient-specific quality assurance (QA). Moreover, noninterdigitating MLCs potentially may not allow an efficient implementation of this method.…”

“…3. A major difference of our study from the previous two-dimensional-GRID therapy approach, 1-3,16,22 tomotherapy or MLC-based studies [4][5][6][7][8][9][10][11][12][13][14][15] was that our treatment planning approach uses an MLCbased, 3D-conformal forward planning technique with no beam modulation. Therefore, this MLC cross-firing procedure preserves the characteristics of 3D-conformal radiation therapy and provides all dosimetry information without the need for patient-specific QA.…”

Spatially fractionated stereotactic body radiation therapy (Lattice) for large tumors

“…VMAT was used to achieve the internal high dose gradients, target coverages, and OAR objectives. VMAT plans are hypothesized to offer superior target coverage, reduce high-dose spill, and better spare OARs as compared with 3D conformal radiotherapy (3DCRT) for Lattice radiotherapy 14,15 . After an initial attempt to generate both a 3DCRT and VMAT comparison plans for Lattice SBRT, we did not pursue 3DCRT plans further as it became clear that, regardless of the number of beam angles, we would not achieve OAR dose constraints while After treatment planning, physician and physicist review, and plan approval, plan integrity and deliverability was evaluated following the standard clinical SBRT QA protocol.…”

“…VMAT plans are hypothesized to offer superior target coverage, reduce high-dose spill, and better spare OARs compared with 3-dimensional conformal radiation therapy (CRT) for Lattice radiation therapy. 14 , 15 After an initial attempt to generate both 3-dimensional CRT and VMAT comparison plans for Lattice SBRT, we did not further pursue 3-dimensional CRT plans because, regardless of the number of beam angles, we would not achieve OAR dose constraints while attaining our desired dose gradient. All plans were delivered on a Varian Truebeam using standard Millennium 120 MLC (5 mm/10 mm MLC widths).…”

Spatially fractionated stereotactic body radiotherapy (Lattice SBRT) for large tumors

“…VMAT was used to achieve the internal high dose gradients, target coverages, and OAR objectives. VMAT plans are hypothesized to offer superior target coverage, reduce high-dose spill, and better spare OARs as compared with 3D conformal radiotherapy (3DCRT) for Lattice radiotherapy 14,15 . After an initial attempt to generate both a 3DCRT and VMAT comparison plans for Lattice SBRT, we did not pursue 3DCRT plans further as it became clear that, regardless of the number of beam angles, we would not achieve OAR dose constraints while attaining our desired dose gradient.…”