Radiation Physics

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TU_13_3244 - A Novel Jawless Linac with Fast MLC Collimation Achieved Comparable Plan Quality and Improved Delivery Efficiency Compared to Conventional Linacs for Challenging Head and Neck Cases

Tuesday, October 23
1:00 PM - 2:30 PM
Location: Innovation Hub, Exhibit Hall 3

A Novel Jawless Linac with Fast MLC Collimation Achieved Comparable Plan Quality and Improved Delivery Efficiency Compared to Conventional Linacs for Challenging Head and Neck Cases
Z. L. Shen1, P. Kosterin1, P. T. Teo1, R. J. Lalonde1, K. Plakan2, D. Peskorski2, D. E. Heron3, and M. S. Huq1; 1Department of Radiation Oncology, UPMC Hillman Cancer Center, University of Pittsburgh School of Medicine, Pittsburgh, PA, 2D3 Oncology Solutions, Pittsburgh, PA, 3UPMC Hillman Cancer Center, Pittsburgh, PA

Purpose/Objective(s): We investigated the dosimetric quality and delivery efficiency of head and neck (HN) plans achieved by a novel jawless 6 MV flattening-filter free (6FFF) linac with fast MLC collimation.

Materials/Methods: Five challenging HN cases treated with simultaneous integrated boost (SIB) were retrospectively selected. There were two to three prescription dose levels with the high dose of 60-70 Gy and the low dose of 54-59.4 Gy. The treatment goal was that ³95% of both high dose PTV (HD_PTV) and low-dose PTV (LD_PTV) will receive the prescription doses. The OARs included brainstem, spinal cord, parotids, esophagus, larynx, oral cavity, and mandible. The original 9-field IMRT clinical plans were done using a treatment planning system for conventional linacs. Each plan was replanned with a 9-field IMRT technique using an unreleased version of the treatment planning system for the novel jawless linac with 1-cm-wide dual-layer MLCs. The lower layer MLCs were used for beam modulation while the upper layer MLCs were used to block inter-leaf leakage. The original clinical plans and the newly generated replans were compared using dosimetric endpoints (such as PTV coverages and OAR max/mean doses), homogeneity index (HI), conformity index (CI) and total MU. Paired t-tests were performed and the results were considered significant if the p-values were ≤ 0.05.

Results: The population averaged dosimetric endpoints, HI, CI, and total MU are summarized in the table below. All newly generated replans for the jawless linac met the clinical dose limit requirements. The PTV coverage, OAR doses, HI, and CI in the replans were all comparable to those in the clinical plans. The total MUs in the replans (2584 ± 407) were significantly greater than those in the clinical plans (1495 ± 207). However, the delivery times for the replans were still shorter than that for the clinical plans because the jawless linac has a much higher dose rate (800 MU/min) compared to the conventional linac (300 MU/min).

Conclusion: For challenging HN cases, the novel jawless linac with fast MLC collimation achieved similar plan quality and improved delivery efficiency compared to the conventional linacs. Table: Plan comparison (n=5)
Clinical Replan Clinical vs. Replan (p-value)
HD_PTV VHD (%) 96.6 ± 1.9 96.9 ± 1.6 0.639
LD_PTV VLD (%) 96.0 ± 1.7 97.3 ± 1.8 0.166
Brainstem Dmax (Gy) 31.1 ± 15.6 29.2 ± 13.7 0.290
Spinal_cord Dmax (Gy) 39.6 ± 3.9 38.4 ± 1.1 0.541
Parotid_L Dmean (Gy) 29.0 ± 6.1 27.5 ± 6.6 0.099
Parotid_R Dmean (Gy) 26.4 ± 3.8 25.5 ± 4.1 0.484
Esophagus Dmean (Gy) 52.9 ± 18.5 52.5 ± 19.4 0.553
Larynx Dmean (Gy) 52.3 ± 22.2 53.5 ± 22.3 0.438
Oral Cavity Dmean (Gy) 31.2 ± 11.0 30.4 ± 9.4 0.497
Mandible Dmean (Gy) 30.5 ± 13.6 29.4 ± 11.6 0.530
HI 1.09 ± 0.01 1.09 ± 0.02 0.541
CI 0.96 ± 0.02 0.97 ± 0.02 0.109
MU 1495 ± 207 2584 ± 407 0.002

Author Disclosure: Z.L. Shen: None. P. Kosterin: None. P.T. Teo: None. R.J. Lalonde: None. D.E. Heron: No personal compensation; Accuray Exchange in Radiation Oncology. Partnership; Cancer Treatment Services International. Vice Chairman of Clinical Affairs; University of Pittsburgh School of Medicine. Director of Radiation Services; UPMC CancerCenter. M. Huq: Honoraria; Varian Medical Systems. Chair, Therapy Physics Committee (TPC); AAPM. Vice chair, Science Council; American Association of Physicists in Medicine.

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