Magnetic fields with photon beams: Monte Carlo calculations for a model magnetic field.

Medical physics · 2000 · Mixed · Insufficient evidence

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MixedInsufficient evidencePaper 3990

Study snapshot

PublicationMedical physics · 2000

Study typeother

Reported directionMixed

Evidence ratingInsufficient

Abstract

Strong transverse magnetic fields can produce very large dose enhancements and reductions in localized regions of a patient under irradiation by a photon beam. We have suggested a model magnetic field which can be expected to produce such large dose enhancements and reductions, and we have carried out EGS4 Monte Carlo calculations to examine this effect for a 6x6 cm2 photon beam of energy 15, 30, or 45 MV penetrating a water phantom. Our model magnetic field has a nominal (center) strength B0 ranging between 1 and 5 T, and a maximum strength within the geometric beam which is 2.2xB0. For all three beam energies, there is significant dose enhancement for B0 = 2 T which increases greatly for B0 = 3 T, but stronger magnetic fields increase the enhancement further only for the 45-MV beam. Correspondingly, there is major reduction in the dose just distal to this region of large dose enhancement, resulting from secondary electrons and positrons originating upstream which are depositing energy in the dose-enhancement region rather than continuing further into the patient. The dose peak region is fairly narrow (in depth), but the magnetic field can be shifted along the longitudinal axis to produce a flat peak region of medium width (approximately 2 cm) or of large width (approximately 4 cm), with rapid dose dropoffs on either side. For the 30-MV beam with B0 = 3 T, we found a dose enhancement of 55% for the narrow-width configuration, 32% for the medium-width configuration, and 23% for the large-width configuration; for the 45-MV beam with B0 = 3 T, the enhancements were quite similar, but for the 15-MV beam they were considerably less. For all of these 30-MV configurations, the dose reductions were approximately 30%, and they were approximately 40% for the 45-MV configurations.

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Study typeother

Effect directionmixed

PopulationNot supplied

Sample sizeNot supplied

ExposureNot supplied

Evidence strengthInsufficientConfidence: 78% · Peer reviewed: yes

Main findings

EGS4 Monte Carlo calculations in a water phantom suggest strong transverse magnetic fields (nominal center strength B0 1–5 T; max within beam 2.2×B0) can produce localized dose enhancements and corresponding distal dose reductions for 15, 30, and 45 MV photon beams. Significant enhancement occurred at B0=2 T and increased greatly at B0=3 T; stronger fields further increased enhancement only for the 45 MV beam. Example results: for 30 MV with B0=3 T, dose enhancement was 55% (narrow), 32% (medium ~2 cm), and 23% (large ~4 cm) peak-width configurations; dose reductions were ~30% for 30 MV configurations and ~40% for 45 MV configurations.

Outcomes measured

Radiation dose enhancement in water phantom under photon beam irradiation
Radiation dose reduction distal to enhancement region
Depth/width characteristics of dose peak and dropoffs under shifted magnetic field configurations

Limitations

Monte Carlo simulation/modeling study in a water phantom rather than measurements in patients
Magnetic field is a suggested model field; generalizability to real clinical magnetic field configurations is not established in the abstract
No biological/health outcomes assessed; outcomes are dosimetric changes only

View raw extracted JSON

{
  "study_type": "other",
  "exposure": {
    "band": null,
    "source": "model magnetic field during photon beam irradiation (radiotherapy context)",
    "frequency_mhz": null,
    "sar_wkg": null,
    "duration": null
  },
  "population": null,
  "sample_size": null,
  "outcomes": [
    "Radiation dose enhancement in water phantom under photon beam irradiation",
    "Radiation dose reduction distal to enhancement region",
    "Depth/width characteristics of dose peak and dropoffs under shifted magnetic field configurations"
  ],
  "main_findings": "EGS4 Monte Carlo calculations in a water phantom suggest strong transverse magnetic fields (nominal center strength B0 1–5 T; max within beam 2.2×B0) can produce localized dose enhancements and corresponding distal dose reductions for 15, 30, and 45 MV photon beams. Significant enhancement occurred at B0=2 T and increased greatly at B0=3 T; stronger fields further increased enhancement only for the 45 MV beam. Example results: for 30 MV with B0=3 T, dose enhancement was 55% (narrow), 32% (medium ~2 cm), and 23% (large ~4 cm) peak-width configurations; dose reductions were ~30% for 30 MV configurations and ~40% for 45 MV configurations.",
  "effect_direction": "mixed",
  "limitations": [
    "Monte Carlo simulation/modeling study in a water phantom rather than measurements in patients",
    "Magnetic field is a suggested model field; generalizability to real clinical magnetic field configurations is not established in the abstract",
    "No biological/health outcomes assessed; outcomes are dosimetric changes only"
  ],
  "evidence_strength": "insufficient",
  "confidence": 0.78,
  "peer_reviewed_likely": "yes",
  "keywords": [
    "transverse magnetic field",
    "photon beam",
    "radiotherapy",
    "dose enhancement",
    "dose reduction",
    "Monte Carlo",
    "EGS4",
    "water phantom",
    "tesla"
  ],
  "suggested_hubs": []
}

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AI-extracted fields are generated from the abstract and metadata and may be incomplete or incorrect. This material is for informational purposes and is not medical advice.

Identifiers and source

DOI: 10.1118/1.1326447
PubMed: 11190956

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