A Planning Comparison of IMRT vs. Pencil Beam Scanning for Deep Inspiration Breath Hold Lung Cancers
Introduction
Lung cancer is among the leading causes of cancer-related deaths.1,2 Radiation plays an important role in early stage, locally-advanced and advanced stage cancers. For locally-advanced tumors, definitive radiation concurrent with chemotherapy remains an established approach for treatment, however concurrent chemotherapy and radiation leads to higher toxicity.3,4 Radiation toxicity can be mitigated by reduction of dose to the organs at risk (OAR). The lung shows a strong volume effect5 hence, mechanisms to facilitate target margin reduction using tumor motion management approaches have been implemented.6,7.
One technique for motion management is deep inspiration breath hold (DIBH).8,9,10,11 DIBH has two primary advantages. First, deep inspiration maximally inflates the lung moving the greatest volume of healthy lung out of the high dose volume. Second, breath hold minimizes respiratory motion, reducing the target motion margin.12 The breath hold is achieved through a breathing maneuver and the patient's breath is monitored by a spirometer,8 a surface imaging system13, external14 or internal markers15. Unfortunately, while many patients have been treated with DIBH, 16,17 others cannot perform DIBH due to other factors (e.g., COPD, co-morbidities), limiting its widespread application.18
Proton beams can spare lung due to the finite range of the beam. Recent studies suggest that clinical outcomes with proton therapy are comparable or better than photon therapy.19,20 Pencil beam scanning (PBS) has been used, in which a beam is raster-scanned layer by layer over the treatment volume. Beam scanning may result in interplay effects between respiratory motion and beam delivery resulting in dose inhomogeneities. 21,22,23 While there are multiple different factors that affect the robustness of the proton plan to motion, breath hold alone is sufficient to mitigate interplay effects. Specifically, for proton plans, motions <5 mm do not require motion management22 and intra-breath hold target motion is approximately 4 mm for DIBH.9
The purpose of this study was to evaluate the dosimetric impact of DIBH with PBS proton therapy and to quantify the relative gains of DIBH and PBS protons separately. We perform a planning comparison between free breathing (FB) and DIBH scans with both intensity modulated radiation therapy (IMRT) and PBS plans.
Section snippets
Methods and materials
Patients were treated on a prospective protocol evaluating the use of Calypso-based24 DIBH photon treatments in lung cancer patients. From the 42 accrued patients to date, 8 were treated with conventional fractionation. Of these, six anonymized patients had both DIBH and FB data sets and were included in this study under an IRB exempted protocol. At simulation, each patient had a DIBH scan monitored using a commercial reflective marker system as well as a FB scan and a 4DCT. Patients were
Results
Figure 1 shows a typical plan comparison between FBx, DIBHx, FBp, and DIBHp plans. The high dose volume among all the plans is comparable while the low dose bath in the photon plans is reduced in the proton plans.
Table 2 summarizes the mean and range of dosimetric parameters to the lung, heart and esophagus for DIBH and FB scans for PBS and IMRT plans rescaled to a prescription dose of 60 Gy(RBE). For all cases, the spinal cord dose was <45 Gy(RBE).
Table 3 compares the mean population
Discussion
The data presented demonstrate an advantage of DIBH over FB in sparing lung, regardless of treatment modality. This is due to the complementary effects of a smaller target volume from the breath hold and lung sparing from deep inspiration. The individual contributions of these two effects are patient specific.30 DIBH plans for both protons and photons resulted in increased lung volume outside of the high dose region which reduced the mean lung dose, lung V20 and lung V5. Proton plans
Conclusions
DIBH for lung cancer treatment offers lung dosimetric gains (V20, MLD) and heart dosimetric gains (V5, V45) compared to free breathing, irrespective of the treatment modality. Clinical implementation of DIBH for proton centers may be warranted for a subpopulation of patients. Free breathing proton plans also offer dosimetric gains (lung V5, MLD, heart V5, heart V45 and MHD) compared to DIBH IMRT plans, but similar or better lung sparing, suggesting that free breathing treatments with proton
Conflict of interest
The authors have no conflict of interest to report.
Acknowledgments
The authors would like to acknowledge an MSK institutional core grant, NCI P30 CA008748.
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A version of this manuscript was presented at the 2019 ASTRO Annual Meeting