Dose Validation of GRID Block Treatment Applicator within the RayStation Treatment Planning System

This paper presents the first implementation and comprehensive dose validation of the .decimal GRID block applicator within the RayStation Treatment Planning System, establishing a robust protocol and novel standardization approach for Spatially Fractionated Radiation Therapy (GRID therapy) that achieved a 98% agreement between planned and measured doses.

The Gist

The Big Problem: The "Too Big to Fit" Tumor

Imagine a patient has a very large, bulky tumor. In radiation therapy, doctors usually try to blast the whole tumor with a uniform beam of energy, like shining a giant flashlight on a wall. But if the tumor is too big, shining that bright light on the whole thing at once would burn the healthy skin and organs surrounding it. It's like trying to cook a massive steak on a tiny pan; the outside burns before the inside is done.

The Old Solution: The "Swiss Cheese" Approach

About a century ago, doctors came up with a clever trick called GRID Therapy. Instead of a solid beam of light, they used a metal block with holes in it (like a colander or a piece of Swiss cheese).

• How it works: The beam shoots through the holes, creating a pattern of tiny, intense "pencil beams" of radiation hitting the tumor, while the metal parts block the radiation in between.
• The Magic: This creates a checkerboard pattern of high-dose and low-dose areas. The high-dose spots kill the cancer cells, while the low-dose "valleys" let the healthy tissue recover. It's like a gardener using a sprinkler that only waters specific spots, letting the grass in between dry out and recover, rather than flooding the whole garden.

The New Challenge: The Missing Software

For a long time, this "Swiss cheese" method was hard to do with modern machines. Some computer programs used by hospitals (like Eclipse or Monaco) had the instructions built-in to handle this tricky pattern. However, the hospital in this study uses a different, very popular computer program called RayStation, and it didn't have the instructions for GRID therapy.

It was like having a high-tech kitchen with a fancy oven, but the recipe book didn't include instructions for baking a specific type of cake.

What This Paper Did: Writing the Recipe

The team of scientists (Blessing, Edwin, Gene, and their colleagues) decided to write the missing instructions themselves. Here is how they did it, step-by-step:

1. Building the Tool: They used a custom-made brass block with 149 holes (the "Swiss cheese").
2. Teaching the Computer: They wrote a special computer script (a set of digital instructions) to tell the RayStation software how to calculate the radiation passing through those specific holes.
3. The "Test Kitchen" (Measurements): Before they could treat a real person, they had to make sure the computer's math matched reality.
   • They shot radiation through the block into water (which acts like a human body).
   • They used special film and sensors to measure exactly how much radiation got through the holes and how much was blocked.
   • They checked this at different depths and energies to ensure the "pencil beams" were hitting the right spots.
4. The Final Exam (QA): They ran a test plan on a phantom (a fake patient made of plastic) and compared the computer's plan to what the machine actually delivered.
   • The Result: The computer and the machine agreed 98% of the time. This is like a student getting an A+ on a very difficult exam.

The Analogy: The Stencil and the Spray Paint

Think of the radiation beam as a can of spray paint.

• Without the GRID: You spray the whole wall. If the wall is huge, you might run out of paint or damage the floor.
• With the GRID: You hold up a heavy metal stencil with holes in it. You spray through the holes. You get perfect dots of paint on the wall.
• The Paper's Achievement: The RayStation software didn't know how to calculate the physics of the spray paint going through the stencil. These scientists figured out the math, taught the software, and proved it works safely.

Why This Matters

• Safety: They proved that they can treat these massive, difficult tumors without burning the healthy skin or organs nearby.
• Standardization: They created a "rulebook" that other hospitals using RayStation can follow. Now, doctors across the country (and the world) can use this method safely.
• The Future: The team is already working on the next step. Instead of using a heavy, physical brass block that has to be moved around, they want to program the machine's moving leaves (MLC) to create the holes digitally. This would be like replacing the heavy metal stencil with a digital projector that draws the holes in the air—faster, cheaper, and easier to use.

The Bottom Line

This paper is a success story of medical physics. The team took an old, powerful idea (GRID therapy), figured out how to make it work on modern, popular software (RayStation), and proved it is safe and accurate. They are essentially handing the keys to a new, safer way of treating large tumors to hospitals everywhere.

Technical Summary

1. Problem Statement

• Clinical Challenge: Conventional External Beam Radiation Therapy (EBRT) often struggles to control bulky tumors (typically >8 cm) without exceeding the tolerance limits of surrounding healthy tissues (Organs at Risk - OARs).
• Therapeutic Gap: Spatially Fractionated Radiation Therapy (SFRT), or GRID therapy, is a proven technique for treating bulky tumors by delivering alternating high-dose (peaks) and low-dose (valleys) radiation. This induces a "bystander effect," damaging non-irradiated tumor cells while sparing healthy tissue.
• Technical Limitation: While GRID therapy protocols are available in major Treatment Planning Systems (TPS) like Elekta's Monaco and Varian's Eclipse, RayStation TPS lacked a native protocol for implementing GRID therapy. This prevented centers using RayStation from utilizing this effective treatment modality.

2. Methodology

The study aimed to develop, model, and validate a custom GRID therapy protocol within the RayStation TPS using a physical .decimal GRID block applicator.

• Apparatus:
  • Applicator: A custom-made brass GRID block (30cm x 30cm total) featuring 149 holes arranged in a specific pattern (11 holes × 7 rows and 12 holes × 6 rows). At the isocenter, the field is 25cm x 25cm with a 50:50 open-to-closed area ratio. Hole diameter is 1.43 cm with 2.11 cm spacing.
  • Hardware: Varian 21_IX and TrueBeam linear accelerators (LINAC).
  • Detectors: EBT3 GafChromic film, Markus ionization chamber, 3-D water tank, and MapCheck2 diode array.
• Software & Modeling:
  • A custom script was developed within RayStation to define the .decimal block as a single opening containing sub-millimeter channels between holes.
  • Beam Modeling: Comprehensive dosimetry data was collected for 6 MV, 10 MV, and 15 MV energies to match the TPS Monte Carlo simulations with physical measurements.
    • Measurements: Percentage Depth Dose (PDD), beam profiles (in-plane and cross-plane), output factors (open field vs. GRID field), and GRID transmission factors.
    • Setup: Measurements were taken at 100 cm SSD, with specific depths ($d_{max}$) of 1.5 cm (6 MV), 2.5 cm (10 MV), and 3.0 cm (15 MV).
• Validation Process:
  • Film Analysis: EBT3 film was used to measure absolute dose and transmission through the blocked areas. The transmission factor in RayStation was scaled from 0.01 to 0.025 to match measurements.
  • QA Verification: A Quality Assurance (QA) plan was delivered to a MapCheck2 phantom. Dose distribution was evaluated using gamma analysis criteria of 3%/3mm at a 10% dose threshold.

3. Key Contributions

• First Implementation: This is the first reported implementation of GRID therapy protocols specifically within the RayStation TPS environment.
• Robust Validation Framework: The authors established a rigorous workflow for validating GRID doses, integrating script variables (aperture size, hole spacing) with physical measurements (MapCheck2 and film).
• Standardization Protocol: A novel, standardized clinical implementation protocol for GRID blocks in RayStation was proposed, making the technique accessible to other institutions using this software.
• Future Roadmap: The study outlines a path toward replacing physical GRID blocks with a Multi-Leaf Collimator (MLC) solution scripted within RayStation, which would offer cost efficiency and convenience despite a slightly higher valley-to-peak ratio.

4. Results

• Dose Distribution: The GRID openings successfully covered the Planning Target Volume (PTV). The "hot spots" (peak doses) were contained within the PTV, while "cold spots" (low doses) were minimized within the target but sufficient to spare OARs.
• Beam Modeling Accuracy:
  • Measured data (PDD, profiles, output factors) showed strong agreement with RayStation's Monte Carlo simulations.
  • Output factors and GRID factors increased with field size due to increased scattering.
  • Transmission through the blocked area was successfully modeled and scaled.
• QA Performance: The gamma analysis on the MapCheck2 phantom yielded a 98% passing rate (3%/3mm, 10% threshold) for 6 MV, 10 MV, and 15 MV energies. This confirms the LINAC's ability to deliver the planned GRID dose with high accuracy.
• Penumbra: The use of EBT3 film allowed for precise evaluation of the beam penumbra, which is critical for the small hole sizes of the GRID block.

5. Significance

• Clinical Impact: This work enables hospitals using RayStation TPS to treat bulky tumors with GRID therapy, potentially achieving complete response rates (previously reported at 62.5%) without the acute or late toxicity associated with conventional high-dose EBRT.
• Radiobiological Advantage: By validating the "bystander effect" delivery mechanism in a modern TPS, the study supports the safe administration of high doses (10–15 Gy) to massive tumors that were previously difficult to manage.
• Scalability: The proposed protocol allows for immediate clinical adoption by other centers. Furthermore, the development of an MLC-based script solution promises to democratize GRID therapy by removing the need for expensive, custom-made physical brass blocks.
• Software Integration: The collaboration with RaySearch Laboratory ensures that this protocol will be integrated into future RayStation software releases (targeted for 2024), standardizing the technique globally.