(P009) Monte Carlo Dosimetry Evaluation of Lung Stereotactic Body Radiosurgery

April 30, 2015

Without the use of MC planning, target structures were substantially underdosed. Local failures were associated with PTVmicro undercoverage, which suggests that delivering a therapeutic dose to this expanded microscopic disease target volume is beneficial. MC dosimetry is preferable for lung SBRT, while the PB algorithm is adequate for predicting pulmonary toxicity.

Colbert A. Parker, BS, Roger Ove, MD, PhD, Madhu B. Chilikuri, PhD, Suzanne M. Russo, MD;University of South Alabama School of Medicine; Mitchell Cancer Institute

BACKGROUND: Promising results have been obtained using stereotactic body radiosurgery (SBRT) for early-stage lung cancer. The calculation of dose in pulmonary parenchyma can be inaccurate.

MATERIALS AND METHODS: We retrospectively analyzed 47 cases treated over a 2-year period with CyberKnife SBRT, planned with the standard pencil beam (PB) algorithm. Cases were a mixture of early-stage lung cancer and oligometastatic cases. The median prescribed dose was 50 Gy in four or five fractions. We compared the planned dose with the dose actually delivered, as estimated with Monte Carlo (MC) dosimetry to the 1% level. We correlated the dosimetric deficiencies with recurrences, using deformable registration to determine the dose delivered to the site of recurrence.

RESULTS: With a median follow-up of 2 years, the local control at 1 year was 90%, declining to 70% at 2 years. The total number of local recurrences was 10, and 8 of these patients died with progressive disease. Two recurrences occurred synchronously with metastases, and two recurrences were in palliative cases treated to lower doses with tight margins, and disease was never cleared locally. MC calculations showed that the mean dose delivered to the planning target volume (PTV), averaged over all cases, was 7% lower than planned. Most cases were planned with an expansion on the PTV (PTVmicro = GTV + 8-mm expansion in lung + 3 mm), representing a region at risk for microscopic extension, and were intended to receive a minimum dose of 80% of the prescription dose. MC calculations showed that the minimum dose to this structure, averaged over all cases, was 47% lower than the intended dose. For cases that recurred, the mean dose to the PTVmicro was 8% lower than intended, while it was only 2% lower for those whose disease was controlled. There were no other significant differences in target coverage between patients with local control and local recurrence. The PB algorithm and MC estimates for pulmonary exposure were assessed, recording the V5, V10, and V20 for the ipsilateral and total lung volumes. These estimates roughly agreed for the two algorithms, with the MC results almost universally lower than PB by an absolute difference of 1% to 3% on average.

CONCLUSIONS: Without the use of MC planning, target structures were substantially underdosed. Local failures were associated with PTVmicro undercoverage, which suggests that delivering a therapeutic dose to this expanded microscopic disease target volume is beneficial. MC dosimetry is preferable for lung SBRT, while the PB algorithm is adequate for predicting pulmonary toxicity.

Proceedings of the 97th Annual Meeting of the American Radium Society - americanradiumsociety.org