A treatment planning study of proton arc therapy for para-aortic lymph node tumors: dosimetric evaluation of conventional proton therapy, proton arc therapy, and intensity modulated radiotherapy

Rah et al. Radiation Oncology (2016) 11:140 DOI 10.1186/s13014-016-0717-4 RESEARCH Open Access A treatment planning study of proton arc therapy for para-aortic lymph node tumors: dosimetric evaluation of conventional proton therapy, proton arc therapy, and intensity modulated radiotherapy Jeong-Eun Rah1, Gwe-Ya Kim2, Do Hoon Oh1, Tae Hyun Kim3, Jong Won Kim4, Dae Yong Kim3, Sung Yong Park5 and Dongho Shin3* Abstract Background: The purpose of this study is to evaluate the dosimetric benefits of a proton arc technique for treating tumors of the para-aortic lymph nodes (PALN). Method: In nine patients, a proton arc therapy (PAT) technique was compared with intensity modulated radiation therapy (IMRT) and proton beam therapy (PBT) techniques with respect to the planning target volume (PTV) and organs at risk (OAR). PTV coverage, conformity index (CI), homogeneity index (HI) and OAR doses were compared. Organ-specific radiation induced cancer risks were estimated by applying organ equivalent dose (OED) and normal tissue complication probability (NTCP). Results: The PAT techniques showed better PTV coverage than IMRT and PBT plans. The CI obtained with PAT was 1.19 ± 0.02, which was significantly better than that for the IMRT techniques. The HI was lowest for the PAT plan and highest for IMRT. The dose to the OARs was always below the acceptable limits and comparable for all three techniques. OED results calculated based on a plateau dose–response model showed that the risk of secondary cancers in organs was much higher when IMRT or PBT were employed than when PAT was used. NTCPs of PAT to the stomach (0.29 %), small bowel (0.69 %) and liver (0.38 %) were substantially lower than those of IMRT and PBT. Conclusion: This study demonstrates that there is a potential role for PAT as a commercialized instrument in the future to proton therapy. Keywords: Proton arc therapy (PAT), Proton beam therapy (RBT), Intensity modulated radiation therapy (IMRT), Organ equivalent doses (OED), Normal tissue complication probability (NTCP) Introduction New technologies in the delivery of radiation therapy have included the use of intensity-modulated radiation therapy (IMRT) or volumetric modulated arc therapy (VMAT) with linear accelerators, as well as the development of proton beam therapy (PBT), which has increased the ability to maximize the dose to the tumor while sparing normal structures. The VMAT approach has a number of potential * Correspondence: 3 Proton Therapy Center, National Cancer Center, Goyang, Korea Full list of author information is available at the end of the article advantages compared to IMRT, such as significantly reducing the treatment time and the number of MUs, as well as improving normal tissue sparing while keeping adequate coverage. Also, proton beams, unlike X-ray beams, have a low entrance dose, followed by a region of uniform high dose (the spread out Bragg peak) at the target, and then a steep fall-off to zero dose. These characteristics minimize the dose delivered to normal tissues while maximizing the dose delivered to the tumor. Better or comparable dose conformity with decreased low dose volume can be achieved with proton beams than with advanced photon techniques © The Author(s). 2016 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The Creative Commons Public Domain Dedication waiver (http://creativecommons.org/publicdomain/zero/1.0/) applies to the data made available in this article, unless otherwise stated. Rah et al. Radiation Oncology (2016) 11:140 [1–3]. This is because of the advantageous physical properties of protons, including a near zero exit or distal dose just beyond the target volume, resulting in reduced proton doses to normal tissue, with better conformation of the dose to the target volume. These unique dose characteristics of protons may reduce the risk of acute as well as late side effects [4]. In recent years, several studies on treatment planning or dosimetric validation with those obtained by proton arc therapy (PAT) can be found in the literature [5, 6]. Rechner et al. [5] assessed the predicted risk of a second cancer following proton arc therapy and VMAT technique for prostate cancer. This study used proton arc plans with 16 equally spaced, static, passively-scattered proton beams. They reported that the predicted risk of cancer for an outof-field organ such as the bladder or rectum following proton arc therapy is either less than or approximately equal to the risk with VMAT. Seco et al. [6] compared a proton arc technique using passively scattered beams and IMPT for early-stage non-small cell lung cancer. Their study used 8– 14 beams with a maximum arc of 150°. They observed that passive-arc therapy produced comparable tumor conformity to VMAT and significantly reduced the low dose to the lungs. Although the proton arc technique has been reported in the literature, extensive studies on treatment planning or dosimetric validation of PAT plans have not yet been conducted. Therefore, the purpose of the present study was to compare dosimetric properties of PAT, PBT, and IMRT techniques for tumors of the para-aortic lymph nodes (PALN). We also compared the dose distribution, organ equivalent doses (OED) and normal tissue complication probability (NTCP) resulting from IMRT, PBT, and PAT techniques in the nine patients based on analysis of dose-volume histograms (DVHs). Methods and materials Patient data and planning techniques We randomly selected nine patients who were to be treated with IMRT or PBT for PALN tumors at the National Cancer Center (NCC) in Korea. The proton system consists of a 230 MeV proton cyclotron, two gantries with rotating beamlines and one gantry with a stationary horizontal beamline. The two gantries with rotating beamlines utilize passive scattering and uniform scanning delivery techniques. The horizontal beam is usually used to treat tumors of the eyes and prostate cases. The maximum extracted beam current of the cyclotron is 300 nA at a 106 Hz radio frequency. Minimum and maximum ranges of the proton beam in water are 5 and 28 cm, respectively, with 0.1 cm accuracy. When data from all nine patients were analyzed, proton therapy was simulated to prescribed dose with the beam range of 7.59 to 17.28 mm and a modulation width of 5.48 to 11.78 mm. Page 2 of 10 For all patients, plans were designed on a CT scan (RT 16 PRO CT Simulator, General Electric Medical Systems, Waukesha, WI) acquired with 2.5 mm slice thickness extending the scan from the 11th thoracic vertebral level to include the proximal third of the femur’s diathesis. Patients were simul (...truncated)