The clinical impact of uncertainties in the mean excitation energy of human tissues during proton therapy

Abigail Besemer, Harald Paganetti, Bryan Bednarz

Research output: Contribution to journalArticle

30 Citations (Scopus)

Abstract

Uncertainties in the estimated mean excitation energies (I-values) needed for calculating proton stopping powers can be in the order of 10-15%, which introduces a fundamental limitation in the accuracy of proton range determination. Previous efforts have quantified shifts in proton depth dose distributions due to I-value uncertainties in water and homogenous tissue phantoms. This study is the first to quantify the clinical impact of I-value uncertainties on proton dose distributions within patient geometries. A previously developed Geant4 based Monte Carlo code was used to simulate a proton treatment plan for three patients (prostate, pancreases, and liver) with varying tissue I-values. A uniform variation study was conducted in which the tissue I-values were varied by ±5% and ±10% of the nominal values as well as a probabilistic variation study in which the I-values were randomly sampled according to a normal distribution with the mean equal to the nominal I-value and a standard deviation of 5 and 10% of the nominal values. Modification of tissue I-values impacted both the proton range and SOBP width. R90 range shifts up to 7.7 mm (4.4.%) and R80 range shifts up to 4.8 mm (1.9%) from the nominal range were recorded. Modulating the tissue I-values by 10% the nominal value resulted in up to a 3.5% difference mean dose in the target volumes and organs at risk compared to the nominal case. The range and dose differences were the largest for the deeper-seated prostate and pancreas cases. The treatments that were simulated with randomly sampled I-values resulted in range and dose differences that were generally within the upper and lower bounds set by the 10% uniform variations. This study demonstrated the impact of I-value uncertainties on patient dose distributions. Clearly, sub-millimeter precision in proton therapy would necessitate a reduction in I-value uncertainties to ensure an efficacious clinical outcome.

Original languageEnglish (US)
Pages (from-to)887-902
Number of pages16
JournalPhysics in medicine and biology
Volume58
Issue number4
DOIs
StatePublished - Feb 21 2013

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Proton Therapy
Uncertainty
Protons
Prostate
Pancreas
Organs at Risk
Normal Distribution
Water
Liver
Therapeutics

ASJC Scopus subject areas

  • Radiological and Ultrasound Technology
  • Radiology Nuclear Medicine and imaging

Cite this

The clinical impact of uncertainties in the mean excitation energy of human tissues during proton therapy. / Besemer, Abigail; Paganetti, Harald; Bednarz, Bryan.

In: Physics in medicine and biology, Vol. 58, No. 4, 21.02.2013, p. 887-902.

Research output: Contribution to journalArticle

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abstract = "Uncertainties in the estimated mean excitation energies (I-values) needed for calculating proton stopping powers can be in the order of 10-15{\%}, which introduces a fundamental limitation in the accuracy of proton range determination. Previous efforts have quantified shifts in proton depth dose distributions due to I-value uncertainties in water and homogenous tissue phantoms. This study is the first to quantify the clinical impact of I-value uncertainties on proton dose distributions within patient geometries. A previously developed Geant4 based Monte Carlo code was used to simulate a proton treatment plan for three patients (prostate, pancreases, and liver) with varying tissue I-values. A uniform variation study was conducted in which the tissue I-values were varied by ±5{\%} and ±10{\%} of the nominal values as well as a probabilistic variation study in which the I-values were randomly sampled according to a normal distribution with the mean equal to the nominal I-value and a standard deviation of 5 and 10{\%} of the nominal values. Modification of tissue I-values impacted both the proton range and SOBP width. R90 range shifts up to 7.7 mm (4.4.{\%}) and R80 range shifts up to 4.8 mm (1.9{\%}) from the nominal range were recorded. Modulating the tissue I-values by 10{\%} the nominal value resulted in up to a 3.5{\%} difference mean dose in the target volumes and organs at risk compared to the nominal case. The range and dose differences were the largest for the deeper-seated prostate and pancreas cases. The treatments that were simulated with randomly sampled I-values resulted in range and dose differences that were generally within the upper and lower bounds set by the 10{\%} uniform variations. This study demonstrated the impact of I-value uncertainties on patient dose distributions. Clearly, sub-millimeter precision in proton therapy would necessitate a reduction in I-value uncertainties to ensure an efficacious clinical outcome.",
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