Abstract
Purpose/ObjectivesFor lung stereotactic body radiation therapy (SBRT), real‐time tumor tracking (RTT) allows for less radiation to normal lung compared to the internal target volume (ITV) method of respiratory motion management. To quantify the advantage of RTT, we examined the difference in radiation pneumonitis risk between these two techniques using a normal tissue complication probability (NTCP) model.Materials/Method20 lung SBRT treatment plans using RTT were replanned with the ITV method using respiratory motion information from a 4D‐CT image acquired at the original simulation. Risk of symptomatic radiation pneumonitis was calculated for both plans using a previously derived NTCP model. Features available before treatment planning that identified significant increase in NTCP with ITV versus RTT plans were identified.ResultsPrescription dose to the planning target volume (PTV) ranged from 22 to 60 Gy in 1–5 fractions. The median tumor diameter was 3.5 cm (range 2.1–5.5 cm) with a median volume of 14.5 mL (range 3.6–59.9 mL). The median increase in PTV volume from RTT to ITV plans was 17.1 mL (range 3.5–72.4 mL), and the median increase in PTV/lung volume ratio was 0.46% (range 0.13–1.98%). Mean lung dose and percentage dose–volumes were significantly higher in ITV plans at all levels tested. The median NTCP was 5.1% for RTT plans and 8.9% for ITV plans, with a median difference of 1.9% (range 0.4–25.5%, pairwise P < 0.001). Increases in NTCP between plans were best predicted by increases in PTV volume and PTV/lung volume ratio.ConclusionsThe use of RTT decreased the risk of radiation pneumonitis in all plans. However, for most patients the risk reduction was minimal. Differences in plan PTV volume and PTV/lung volume ratio may identify patients who would benefit from RTT technique before completing treatment planning.
Highlights
“Stereotactic body radiation therapy” (SBRT) or “stereotactic ablative body radiotherapy” (SABR) refers to highly spatially precise radiation therapy with steep dose gradients delivered to an extracranial target, typically completed in 1–5 fractions with higher doses per fraction than conventional radiation therapy
Compensation may be achieved with larger planning target volume (PTV) margins, but this further increases the dose to normal lung tissue.[4]
Normal tissue complication probability (NTCP) models based on mean lung dose have been derived to predict risk of symptomatic radiation pneumonitis from lung stereotactic body radiation therapy (SBRT).[16,17,24]
Summary
“Stereotactic body radiation therapy” (SBRT) or “stereotactic ablative body radiotherapy” (SABR) refers to highly spatially precise radiation therapy with steep dose gradients delivered to an extracranial target, typically completed in 1–5 fractions with higher doses per fraction than conventional radiation therapy. Symptoms usually resolve with corticosteroids, permanent pulmonary dysfunction can occur.[12] The lungs function as a classic radiobiological “parallel organ”[23] and symptomatic radiation pneumonitis is generally correlated with critical dose–volumes rather than maximum dose to lung tissue.[14,18,19,22] Normal tissue complication probability (NTCP) models based on mean lung dose have been derived to predict risk of symptomatic radiation pneumonitis from lung SBRT.[16,17,24] At our institution, it is standard for all lung SBRT patients to undergo respiratory 4D-CT imaging at simulation regardless of the intended motion management technique. Classifications based on predosimetric characteristics were performed to generate practical guidelines predicting which patients could substantially benefit from real-time tracking, without needing to perform time-consuming replanning and NTCP calculation
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