The lung is highly sensitive to radiation and clinical outcomes are affected by initial lung functions. Diseases of the lung associated with tobacco and minerals often coexist with or are associated with cancer inception and multi-factorial gas exchange defects, volume and elasticity. Radiation injury to the lung includes fibrosis and pneumonitis. Radiation pneumonitis is clinically manifest by cough, low grade fever, dyspnea beginning weeks to months after radiation is complete. Long term pulmonary fibrosis can lead to respiratory insufficiency, often exacerbating prior respiratory insufficiency. Baseline PFTS are important to determine whether or not radiation is safe or not.
There are several parameters associated with the risk of pneumonitis. Multiple institutions have demonstrated various parameters of the dose-volume histogram, mean lung dose, and spatial region of the lungs treated are associated with the risk of developing pneumonitis. There are various correlations and inter-relations that appear to predict increasng risk of pneumonitis and the relationships demonstrated to date are increasingly complex, making it difficult to accept any one parameter as a definitive cut off. A list of potential parameters include:
A Dutch study looked at the distribution of radiation in regions of the lung. This study analyzed the dose to the following regions:
The mean radiation doses to the posterior, caudal, ipsilateral central and peripheral lung volumes were significantly correlated with the incidence of steroid requiring radiation pneumonitis. A Washington University study found the inferior tumor location was a significant predictor of radiation pneumonititis. RTOG 9311 on the other hand, found no such correlation. Combining the two studies, tumor location and mean lung dose were the most significant predictors of pneumonitis.
IMRT tends to have a higher dose heterogeneity and more low dose regions over a larger volume. M.D. Anderson described a reduction in toxicity with IMRT over 3D conformal techniques. a V5 > 70; was associated with a grade 3 or higher radiation pneumonitis risk of 21%, v. a 2% risk with V5 ≤ 70%.
SBRT treats a small volume of lung in 3 to 5 large fractions delivered between 5 and 20 days. The dose-volume distributions are significantly different from conventionally fractionated radiation and likely the radiobiology is different as well. Radiation pneumonitis is uncommon after SBRT. Bronchial stenosis and fatal bleeding has been associated with SBRT and endobronchial HDR to central/perihilar tumors.
QUANTEC using pooled data recommends the following limits based on gradually increasing risk of pneumonitis :