A Planning Comparison Of Imrt Versus Pencil Beam Scanning For Deep Inspiration Breath Hold Lung Cancers

Deep inspiration breath hold (DIBH) has dosimetric advantages for lung cancer patients treated with external beam therapy, but is difficult for many lung cancer patients to perform. Proton therapy for lung cancer permits sparing of the contralateral lung and other organs at risk (OAR) as the finite range spares downstream OARs. We compared conventionally fractionated proton (p) and photon(x) plans on both free breathing (FB) and DIBH planning CTs to determine how much DIBH would affect proton therapy. We also compared DIBH IMRT to FB proton plans We evaluated 7 patients treated with photon DIBH on a prospective protocol from 5/2014-10/2018. Six patients had locally advanced disease and were treated with conventional fractionation. One patient had early stage disease and was treated with moderate hypofractionation (60 Gy in 15 fractions). All patients were re-planned using pencil beam scanning proton (PBS) therapy. Additionally, new plans were generated for FB datasets with both modalities for a total of 4 plans/patient. Prescription dose ranged from 50-70 Gy or Gy(RBE) with the goal of having the 95% isodose line cover 95% of the PTV. To compare the effect of DIBH vs FB, we calculated the difference between dosimetric parameters for heart and lung for x-ray and proton plans. We also compared FBp to DIBHx parameters to quantify how FB proton plans compare to DIBH IMRT plans. A 5 mm PTV margin was used for all plans, which was sufficient to ensure ITV coverage within a ±3.5 % range uncertainty. Table 1 (below) summarizes the population differences between the FB and DIBH plans for the two different modalities. For V20, the differences were comparable, suggesting that DIBH has comparable advantages for both PBS and IMRT at the V20 level. For all other metrics, the proton gains of DIBH over FB were smaller than IMRT. Since PBS produces a smaller low-isodose volume than IMRT, the relative advantage that DIBH offers is reduced. The population mean lung for DIBHx V20 (29.6%) and MLD (16.6 Gy) were comparable to FBp V20 (28.4%) and 16.7 Gy (RBE) while other metrics favored FB PBS. For instance, for DIBHx vs FBp the population mean lung V5 and MHD were 57.7% vs. 40.7%, and 15.8 Gy vs. 6.8 Gy (RBE), respectively. DIBH may offer similar relative advantages for PBS as it does for IMRT as measured by V20. The gains from DIBH for V5, MHD and MLD are smaller for PBS than IMRT. On average, FB proton plans offer similar or better lung and heart sparing compared to DIBH IMRT plans. For IMRT patients who have difficulty performing DIBH, FB protons may offer an alternative.Abstract 3660; Table 1Differences between FB and DIBH metrics averaged over population of patients.IMRT (FB-DIBH) [range]PBS (FB-DIBH) [range]V20 (lung)6.6% [1.3%-20.1%]6.2% [0.7%-14.8%]V5 (lung)12.5% [2.7%-23.3%]4.3% [-2.5%-14.6%]Mean Lung Dose (MLD) [Gy/Gy(RBE)]16.6 Gy [2.33-29.8]12.1 Gy (RBE) [1.6-24.2]Mean Heart Dose (MHD) [Gy/Gy(RBE)]3.05 Gy [-3.9-9.2]1.28 Gy(RBE) [-0.3-2.83] Open table in a new tab