International Journal of Radiation Oncology*Biology*Physics
Clinical InvestigationDose–Response Relationship for Image-Guided Stereotactic Body Radiotherapy of Pulmonary Tumors: Relevance of 4D Dose Calculation
Introduction
Stereotactic body radiotherapy (SBRT) is considered as the treatment of choice for patients with early stage non–small-cell lung cancer (NSCLC), who are inoperable because of medical comorbidities or who refuse surgical resection. However, there is still controversy about the SBRT treatment dose, which is sufficient to achieve local control. Multiple institutions published similar local control rates between 80% and >90% for a large range of treatment doses 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11. Differences in patient and tumor characteristics, length of follow-up, and definition of local failure may partially explain this inconsistent dose–response relationship.
Additional facts make a true comparison of data from the literature difficult. Uncertainties in treatment planning and delivery may result in large discrepancies between the prescribed/reported dose and the dose, which was actually delivered to the pulmonary tumor. A well-known uncertainty is breathing-induced tumor motion: gated treatment delivery, irradiation in breath hold, real-time tumor tracking, or addition of safety margins in free breathing are methods for compensation of this uncertainty (12). Because of large differences in density between the tumor and the surrounding lung tissue, the dose calculation algorithm is considered highly relevant in treatment planning (13). Internal variability of the tumor position independent from the bony anatomy and the stereotactic system was shown to result in reduced doses to the target unless these setup errors are corrected by image guidance (14).
This study describes a single institution experience of 124 patients treated with SBRT for early-stage NSCLC and pulmonary metastases. Dose response was calculated with consideration of the uncertainties breathing motion, dose calculation, and patient setup: the effects of tumor motion were estimated by four-dimensional (4D) dose calculation, heterogeneity correction with collapsed cone dose calculation algorithm was standard protocol, and setup errors at the time of treatment were minimized by routine computed tomography (CT)-based image guidance.
Section snippets
Methods and Materials
This retrospective single institution analysis is based on 124 consecutive patients treated for 159 pulmonary target volumes between 1997 and 2007. Patient and treatment characteristics are summarized in Table 1.
Local control
After mean/median follow-up of 18 months/14 months, actuarial local control rate was 83% at 36 months for the whole patient group. Local control was not different between early stage NSCLC and pulmonary metastases: 3-year actuarial local control rate was 84% and 82% for early-stage NSCLC and pulmonary metastases, respectively.
Table 2 shows all fractionation schedules with BED doses to the PTV based on 3D dose calculation and BED doses to the CTV based on 4D dose calculation. Treatment dose
Discussion
This retrospective single-institution study reports outcome of SBRT for early-stage NSCLC and pulmonary metastases. We correlated clinical outcome not only with prescribed doses based on 3D treatment planning, but also with the estimated doses that were actually delivered to the mobile pulmonary targets.
The collapsed cone algorithm was shown to be accurate for dose calculation in the thoracic region; differences between calculated and measured doses are acceptable 13, 20. Monte Carlo dose
Conclusions
Doses of >100 Gy BED to the CTV based on 4D dose calculation resulted in excellent local control rates for image-guided SBRT of primary early-stage NSCLC and pulmonary metastases. This cutoff dose is not specific to the treatment technique and protocol of our study, but may serve as a general recommendation.
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Conflict of interest: none.