The effect of arm position on the dosimetry of thoracic stereotactic ablative radiation therapy using volumetric modulated arc therapy.
OBJECTIVE
Patient comfort and positioning stability may be improved in the arms down (AD) compared with the typical arms up (AU) position in thoracic stereotactic ablative radiation therapy (SABR). We compared plan quality for AD vs AU when using volumetric modulated arc therapy (VMAT), and evaluated the sensitivity of AD plans to arm positioning variability.


METHODS
We took plans of 14 patients with 17 lung tumors treated with thoracic SABR using VMAT in the AD position and simulated the same treatments in the AU position by re-optimizing after digitally removing the ipsilateral arm. To evaluate the sensitivity of AD plans to arm positioning variability, all plans were recalculated without re-optimization after assigning water density to the ipsilateral arm (AD-W) and then digitally shifting the arm 2.5 cm anterolaterally (AD-WS).


RESULTS
Between AD and AU plans, statistically significant but clinically insignificant (all original planning constraints met) differences were found for the following parameters: mean planning target volume maximum dose, difference of 2.3% of prescription dose (P = .049); mean intermediate dose conformity index, difference of 0.27 (P = .012); median percent lung volume receiving a minimum of 10, 20, and 30 Gy (V10, V20, and V30), differences of 0.5%, 0.2%, and 0.1%, respectively (P = .040, .007, and .001); and median spinal cord maximum dose, difference of 33.5 cGy (P = .017). Similarly, between AD-W and AD-WS plans, statistically significant but clinically insignificant differences were found for median lung V20 and V30, difference of 0.0% for both (P = .034 and .016, by matched pair analysis).


CONCLUSIONS
Our exploratory planning study suggests that when using VMAT for lung tumor SABR, AD and AU positioning achieve clinically equivalent plan quality, and AD plans are insensitive to relatively large variability in arm position.
Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California.
David B
DB
Shultz
Department of Radiation Oncology, College of Medicine, The Catholic University of Korea, Seoul, Korea.
Seong Sun
SS
Jang
Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California; School of Nursing, University of Pennsylvania, Pennsylvania.
Alexandra L
AL
Hanlon
Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California; Stanford Cancer Institute, Stanford University School of Medicine, Stanford, California; Institute for Stem Cell Biology and Regenerative Medicine, Stanford University School of Medicine, Stanford, California.
Maximilian
M
Diehn
Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California; Stanford Cancer Institute, Stanford University School of Medicine, Stanford, California. Electronic address: BWLoo@Stanford.edu.
Billy W
BW
Loo
Department of Radiation Oncology, Stanford University School of Medicine, Stanford, California; Stanford Cancer Institute, Stanford University School of Medicine, Stanford, California. Electronic address: pmaxim@stanford.edu.
Peter G
PG
Maxim
D001132:Arm / Q000033:anatomy & histology*; D054893:Cone-Beam Computed Tomography / Q000379:methods; D006801:Humans; D008175:Lung Neoplasms / Q000601:surgery*; D016634:Radiosurgery / Q000379:methods*; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated / Q000379:methods; D012189:Retrospective Studies