A Planning Comparison of IMRT vs. 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 patients to perform. Proton therapy permits sparing of the downstream organs at risk (OAR). We compared conventionally fractionated proton (p) and photon(x) plans on both free breathing (FB) and DIBH planning CTs to determine the effect of DIBH with proton therapy. We evaluated 24 plans from 6 lung cancer patients treated with photon DIBH on a prospective protocol. All patients were re-planned using pencil beam scanning (PBS) proton therapy. New plans were generated for FB datasets with both modalities. All plans were renormalized to 60 Gy. We evaluated dosimetric parameters for heart, lung and esophagus. We also compared FB_p to DIBH_x parameters to quantify how FB_p plans compare to DIBH_x plans. Significant differences were found for lung metrics V20 and mean lung dose between FB and DIBH plans regardless of treatment modality. Furthermore, lung metrics for FB_p were comparable or superior to DIBH_x, suggesting that FB protons may be a viable alternative for those patients that cannot perform DIBH with IMRT. The heart dose metrics were significantly different for the 5 out of 6 patients where the PTV overlapped the heart as DIBH moved heart out of the high dose volume. Heart dose metrics were further reduced by proton therapy. DIBH offers similar relative advantages for lung sparing for PBS as it does for IMRT but the magnitude of the DIBH related gains in OAR sparing were smaller for PBS than IMRT. FB_p plans offer similar or better lung and heart sparing compared to DIBH_x plans. For IMRT patients who have difficulty performing DIBH, FB protons may offer an alternative.