Can SPECT functional lung dose improve radiation pneumonitis prediction?

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BOSTON—Utilizing a SPECT intensity-weighted dose appears to only provide a small, although significant, improvement in predicting radiation pneumonitis (RP) and SPECT weighting implicates functional lung above high doses as critical in causing RP, according to study results presented Tuesday during a scientific session at the 50th annual meeting of the American Society for Therapeutic Radiology and Oncology (ASTRO).

SPECT imaging of the lung provides an intensity map of the spatial distribution of perfusion, often considered to be a surrogate for function. Consequently, radiotherapy (RT) dose distributions with a larger incidental dose in well-perfused lung regions could result in greater toxicity.

“The rationale is that SPECT gives you a map of perfusion distribution in the lung and if you make the leap of faith that somehow perfusion is related to function, then it could be that areas of high function are more important than dosage,” according to Shiva Das, PhD, Duke University Medical Center in Durham, N.C., who presented the data. “Can we actually separate those to see if areas of high function versus low function make a difference?”

Das and colleagues investigated whether SPECT intensity-weighted radiotherapy dose distribution (in combination with other non-dose variables) was more predictive for the occurrence of radiation-induced grade 2+ pneumonitis (RP), compared to traditional anatomic/volume-based dosimetric parameters.

The database consisted of 105 patients irradiated for lung cancer, all with pre-RT SPECT scans (23/105 diagnosed with RP at post-RT follow-up). They built two self-organizing map (SOM) models to predict for radiation pneumonitis from the database. The SOM technique groups patients with similar features into categories, with each category assigned a probability of pneumonitis. One model, SOMDVH, selected features from patient non-dose factors and the lung dose-volume histogram (DVH). For comparison, the other model, SOMDFH, selected input features from non-dose factors and the lung dose-function histogram (DFH), which quantifies the fraction of total SPECT intensity above dose levels, according to the authors.

To gauge the impact of SPECT, Das and colleagues made repeated blind predictions for each patient using SOMDVH and SOMDFH models built from varying subsets of the remaining patients. The predictions from the models with and without SPECT-weighting were compared via receiver operating characteristic (ROC) curves.

The area under the ROC curve for SOMDFH was 0.740 (sensitivity 68.8 percent, specificity 67.5 percent), compared to the corresponding SOMDVH area of 0.712 (sensitivity 62.6 percent, specificity 60.3 percent), they reported.

“The increase in ROC area from utilizing the SPECT weighted dose, albeit small, was statistically significant. This increase is also reflected in the sensitivity/specificity,” Das and colleagues wrote.