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  MO17 - Radiotherapy I: Stereotactic Ablative Body Radiotherapy (ID 106)

  • Event: WCLC 2013
  • Type: Mini Oral Abstract Session
  • Track: Radiation Oncology + Radiotherapy
  • Presentations: 1
  • Moderators:M. Zwitter, S.K. Vinod
  • Coordinates: 10/29/2013, 16:15 - 17:45, Bayside 204 A+B, Level 2

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    MO17.08 - TCP modeling in Stereotactic Body Radiotherapy for early stage non small cell lung cancer: is a dose-volume effect present? (ID 2205)

    16:15 - 17:45  |  Author(s): I. Grills
    • Abstract
    • Presentation
    • Slides

    Background
    In early stage non-small cell lung cancer (NSCLC) stereotactic body radiotherapy (SBRT) has become standard of care for inoperable patients. Tumor size >3cm was reported to be a predictor of local recurrence (LR), suggesting a dose-volume effect. Recently, the dose effect relation was questioned[1]. We used a Tumor-Control-Probability (TCP) model on a large pooled multi-center cohort to test this.

    Methods
    850 patients were analyzed from our five institutes. Patients received a 4D CT-scan and plans were inversely optimized using advanced dose calculation algorithms. Treatment was delivered using online cone-beam CT guidance. Immobilization, margins, dose prescription and treatment planning was performed according to institute specific protocols. Median tumor diameter was 2.2 cm (range:0.7-8.0), median prescribed dose was 54 Gy (range:18-64) and median number of fractions were 3 (range:1-10). LRs were either biopsy proven or defined as a FDG-PET positive growing mass on CT-scan. The Web-Nahum TCP-model[2] was fitted to LR-data using maximum-likelihood estimation by optimizing its parameters: α representing the population-average radio-sensitivity, σ~α~ representing the population-variation in α and ρ the clonogen density. Input variables were the patient specific Gross Tumor Volume (estimated from the tumor diameter), for the dosimetric parameter PTV-D~min~, D~max~, D~mean~, D~1~, D~99~ were evaluated after conversion to Biological-Effective-Dose (BED) using the LQ-model with α/β=10Gy. We tested the optimized TCP model against a random model in which TCP was fixed independent of dose and volume. The optimal model was selected based on the Akaike-Information-Criterion (AIC).

    Results
    After a median follow up (FU) of 17 months (range:0-93), 43 LRs (5%) were diagnosed at 14 months FU (range:2-56), of which 25 tumors were biopsy proven and 18 recurrences diagnosed on PET-CT. The PTV-BED~mean~ based TCP model showed the best fit with parameters α=0.43Gy[-1] (CI:0.33–0.75) and σ~α~=0.17 Gy[-1] (CI:0.11–0.37). The model-fit was insensitive to ρ and set to literature values: 10[7]/cm[3]. The AIC of the optimal model was 12 units higher than the random model indicating a clear dose-volume-effect. At high PTV~mean~-BEDs, however, the volume effect is modest. Additionally, the AIC of the BED corrected model was 9.4 units higher than the BED uncorrected model. Figure 1

    Conclusion
    A dose-volume-effect relation in SBRT for early stage NSCLC for local control was derived in a large cohort of patients. This dose-effect relation requires validation in independent datasets and prospective trials. 1.van Baardwijk,Rad.Onc.,2012. 2.Web&Nahum,PMB,1993.

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