CBCT-based dosimetric verification and alternate planning techniques to reduce the normal tissue dose in SBRT of lung patients

Ganesh Narayanasamy Ganesh Narayanasamy,Jonathan Feddock Jonathan Feddock,John Gleason John Gleason

doi:10.14319/ijcto.32.18

Ganesh Narayanasamy Ganesh Narayanasamy, Jonathan Feddock Jonathan Feddock + Show 1 more

Open Access

https://doi.org/10.14319/ijcto.32.18

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Abstract

Purpose: Confirmation of treatment delivery accuracy in stereotactic body radiotherapy (SBRT) of lung tumors suggests the possibility of treatment margin, or aperture reduction. In this investigation, the dose delivery to lung tumors using SBRT techniques was verified, and the feasibility of normal tissue sparing via aperture reduction or altered prescription isodose line was assessed. Methods: Planned and delivered doses to the gross tumor volume (GTV) and planning target volume (PTV) were compared for 10 patients using planning CT and conebeam CT image. Potential for reduction in normal tissue dose were assessed using 2 alternate treatment plans – reduced PTVs and alternate prescription techniques. Plans were assessed using conformity index, homogeneity index and the ratio of 50% / 100% isodose volumes (R 50% ). Results: The planned and delivered mean doses were consistent to within 4%. However, the mean dose delivered to the GTV exceeded the prescription dose (Rx) by 19% and is consistent with our planning technique of prescribing to the 80% isodose line. When reducing treatment margins and retaining a constant dose-volume constraint, block margins had to be increased which produced a constant effective field aperture outside of the GTV. Prescription to a lower isodose line using stereotactic-like planning techniques yielded the only method by which the volume of the prescription isodose could be affected, although this yielded increases in normal tissue dose due to the increased monitor units required. Conversely, conventional prescription techniques using wider field apertures were effective in reducing absolute values of normal tissue dose. Although dose conformity was similar across different prescription isodose lines, homogeneity index and R 50% values were significantly different in the 60%-70% prescription isodose line plans than the 80%, 90% prescription plans. Conclusion: Traditional margin reduction techniques did not affect a reduction in the volume of normal tissue irradiated to the prescribed dose. Prescribing to low isodose lines yields reduced volumes of the prescribed dose, but at the expense of normal tissue dose.

Highlights

Stereotactic body radiation therapy (SBRT) is a widely used treatment modality for early-stage non-small cell lung cancer (NSCLC).[1]
Non-coplanar, non-opposing, 3D conformal 6MV photon beams were used with 80% isodose line covering at least 95% of planning target volume (PTV)
The planning, immobilization and set-up techniques used in our clinic appear to result in set-up reproducibility that results in agreement between planned and delivered mean dose to the gross tumor volume (GTV) to within 2%

Summary

Introduction

Stereotactic body radiation therapy (SBRT) is a widely used treatment modality for early-stage non-small cell lung cancer (NSCLC).[1]. There is a mature literature on the efficacy of SBRT, and studies vary in terms of the specifics used for treatment planning, immobilization and daily setup.[2, 3] We began the present investigation to study the degree to which our planning and treatment techniques result in dosimetric accuracy. The results supported the assumption of dose delivery accuracy and prompted an investigation into the feasibility of reducing the planning target volumes (PTVs) for this class of tumors. This latter investigation revealed certain potentially counter-intuitive aspects to the prospects of margin reduction for these very small tumors.

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