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6CCVD Research

Feasibility study for dose calculation with a radiation treatment planning system using a fixed-size electron cone applicator for small electron fields

At a Glance

Section titled “At a Glance”
MetadataDetails
Publication Date2025-08-14
JournalPLoS ONE
AuthorsSu Chul Han, Min Cheol Han, Jihun Kim, Heerim Nam, Jin Sung Kim
InstitutionsSungkyunkwan University, Kangbuk Samsung Hospital
AnalysisFull AI Review Included

Executive Summary

Section titled “Executive Summary”

This study validates an indirect method for integrating non-standard, fixed-size electron cone applicators into commercial Treatment Planning Systems (TPS) for small-field electron radiotherapy.

  • Core Value Proposition: Enables efficient and reliable Monitor Unit (MU) calculation and dose visualization for specialized electron applicators not natively supported by the TPS, eliminating time-consuming manual measurements.
  • Methodology: An indirect calculation approach was developed, leveraging the relative output factor (OFrelative) between the standard cerrobend cutout applicator and the fixed-size cone as a scaling factor.
  • MU Accuracy: The difference between the indirectly calculated TPS Monitor Units (MUFC for TPS) and the directly measured MUs (MUFC) was confirmed to be within 1.0% across all small electron fields (2-5 cm).
  • Dosimetric Validation (PDD): Depth dose comparisons achieved a gamma pass rate of >95% using stringent 1%/1 mm criteria, confirming beam quality consistency.
  • Dosimetric Validation (Profiles): Beam profile comparisons achieved a gamma pass rate of >99% using 2%/2 mm criteria.
  • 2D Dose Agreement: Two-dimensional dose distributions (TPS vs. Gafchromic film measurements) showed >99% agreement using 3%/3 mm criteria.
  • Scope Limitation: The study focused exclusively on a single energy level (6 MeV) but provides a robust framework for clinical implementation.

Technical Specifications

Section titled “Technical Specifications”
ParameterValueUnitContext
Electron Beam Energy6MeVUsed for all measurements and TPS modeling.
Small Field Diameters2, 3, 4, 5cmFixed-size electron cone applicator dimensions.
Reference Field Size10 x 10cm2Used for Output Factor (OF) normalization (TG-70).
TPS Software/VersionRayStationv5.0Commercial Treatment Planning System used.
Dose Calculation AlgorithmElectron Monte Carlo (eMC)N/AUsed for high-accuracy small-field dose calculation.
Dose Grid Resolution2mmResolution used in the TPS calculation.
MU Difference (Standard Applicator)< 2.0%MUC vs. MUC from TPS comparison.
MU Difference (Fixed Cone, Indirect)< 1.0%MUFC vs. MUFC for TPS comparison.
PDD Gamma Pass Rate> 95%Achieved using 1%/1 mm criteria.
Beam Profile Gamma Pass Rate> 99%Achieved using 2%/2 mm criteria.
2D Dose Distribution Gamma Pass Rate> 99%Achieved using 3%/3 mm criteria (Gafchromic film).
OF Difference (Max, 2 cm field)38.4%Maximum difference in OF between applicators (at 105 cm SSD).
Detector TypeMicro-diamond detectorPTW 60019Used for PDD, profile, and OF measurements.

Key Methodologies

Section titled “Key Methodologies”

The feasibility study was conducted in two primary steps to compare dosimetric characteristics and validate the indirect MU calculation method:

  1. Dosimetric Characterization (Step 1):

    • Equipment: 6 MeV electron beam, Beamscan 3D water phantom, micro-diamond detector.
    • Comparison: Measured Percentage Depth Dose (PDD) curves, beam profiles (at R100, R90, R70, R50), and Output Factors (OFs) for both the standard Circular Cerrobend Cutout (C) applicator and the Fixed-Size Cone (FC) applicator.
    • Field Sizes/SSDs: Measurements performed for 2, 3, 4, and 5 cm fields at 95 cm, 100 cm, and 105 cm Source-to-Surface Distances (SSDs).
    • Consistency Check: PDD and profile differences were analyzed using gamma evaluation (1%/1 mm and 2%/2 mm criteria, respectively).

  2. Indirect MU Calculation and Validation (Step 2):

    • TPS Modeling: The standard cerrobend cutout applicator (C) was modeled in RayStation TPS (eMC algorithm) to calculate MUC from TPS for a prescribed dose (500 cGy at 1.4 cm depth).
    • Relative OF Determination: The relative output factor (OFrelative) was calculated as the ratio of the measured OFs (OFFC / OFC).
    • Indirect MU Calculation: The MU for the fixed-size cone (MUFC for TPS) was calculated indirectly by scaling the TPS-derived standard MU: MUFC for TPS = MUC from TPS / OFrelative.
    • MU Verification: The calculated MUFC for TPS was compared against the MUFC derived from direct measurements using TG 71 protocol (Equation 3).
    • 2D Dose Validation: Two-dimensional dose distributions calculated by the TPS (using the standard applicator model) were compared against Gafchromic film measurements taken using the fixed-size cone applicator (FC). Gamma evaluation (3%/3 mm) was used for validation.

Commercial Applications

Section titled “Commercial Applications”

This methodology provides a critical workflow enhancement for clinical environments utilizing specialized electron beam radiotherapy equipment.

  • Radiation Oncology & Medical Physics:
    • Small-Field Electron Therapy: Directly supports specialized treatments requiring small, circular electron fields (2-5 cm), such as skin cancer, superficial tumors, or Intraoperative Radiation Therapy (IORT) boosts.
    • TPS Integration: Provides a validated, systematic approach to integrate non-standard vendor applicators (which lack native TPS support) into routine clinical planning workflows.
    • Workflow Efficiency: Eliminates the need for extensive, time-consuming manual MU determination for every non-standard applicator size, improving throughput and reducing human error.
  • Medical Device Manufacturing (LINAC Vendors):
    • Dosimetry Validation: Offers a robust method for validating dose delivery accuracy when using proprietary fixed-size cones, particularly when relying on Monte Carlo (eMC) algorithms for small-field calculations.
  • Quality Assurance (QA) & Compliance:
    • Accuracy Assurance: The high agreement rates (MU difference < 1.0%) ensure dose delivery accuracy meets clinical standards, crucial for patient safety and regulatory compliance.
View Original Abstract

Objective This study aims to evaluate the feasibility of radiation treatment planning using a commercial treatment planning system (TPS) for small fixed-size electron cone electron applicators not natively supported by the TPS. Methods Dosimetric characteristics, including beam profiles and output factors (OFs), were compared between a 6 MeV electron beam collimated by a small fixed-size electron cone applicator and a cerrobend cutout-based general applicator. Measurements were performed using a micro-diamond detector in a water phantom for field sizes of 2, 3, 4, and 5 cm. The monitor units (MUs) from the TPS were compared with direct measurements. To estimate the MU for the fixed-size electron cone applicator using the TPS, the relative OFs were defined as the ratio of the OFs for the fixed-size electron cone and general applicators. Dose distributions obtained from the TPS were validated against measurements using Gafchromic films, ensuring accuracy. Results Gamma analysis showed a passing rate >95% with 1%/1 mm criteria for depth dose comparisons and >99% with 2%/2 mm criteria for beam profiles. The general applicator’s OFs were consistently higher across all measured field sizes. The MU difference between the TPS and measurements was within 2.0%, while the difference between indirect TPS calculations and direct measurements for the fixed-size electron cone applicator remained within 1.0%. Dose distribution analysis showed >99% agreement (3%/3 mm) between the 2D dose distribution obtained using film in the fixed-size electron cone applicator and that calculated by the TPS of cerrobend cutout-based applicator. Conclusion The results demonstrate the feasibility of calculating monitor units and dose distributions for small fixed-size electron cone applicators using a commercial TPS combined with relative output factors. This approach offers a reliable method for dose calculation in specialized electron therapy applications.

Tech Support

Section titled “Tech Support”

Original Source

Section titled “Original Source”

References

Section titled “References”
  1. 2006 - Review of electron beam therapy physics [Crossref]
  2. 2015 - Assessment of clinically relevant dose distributions in pelvic IOERT using Gafchromic EBT3 films [Crossref]
  3. 2015 - Characterization of a microDiamond detector in high-dose-per-pulse electron beams for intra operative radiation therapy [Crossref]
  4. 2018 - Evaluation of prototype of improved electron collimation system for Elekta linear accelerators [Crossref]
  5. 2015 - AAPM Medical Physics Practice Guideline 5.a.: Commissioning and QA of Treatment Planning Dose Calculations - Megavoltage Photon and Electron Beams [Crossref]
  6. 2015 - Comparison between small radiation therapy electron beams collimated by Cerrobend and tubular applicators [Crossref]
  7. 2009 - Recommendations for clinical electron beam dosimetry: supplement to the recommendations of Task Group 25 [Crossref]
  8. 2020 - Report of AAPM Task Group 235 Radiochromic Film Dosimetry: An Update to TG-55 [Crossref]
  9. 2014 - Monitor unit calculations for external photon and electron beams: Report of the AAPM Therapy Physics Committee Task Group No. 71 [Crossref]
  10. 2013 - Radiotherapy electron beams collimated by small tubular applicators: characterization by silicon and diamond diodes [Crossref]

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