Author Disclosure: A.I. Saito, ViewRay, Inc., A. Employment; J.G. Li, None; C. Liu, None; K.R. Olivier, None; J.F. Dempsey, ViewRay, Inc., A. Employment; ViewRay, Inc., E. Ownership Interest. 2832 The Trade-Off Between Neutron Dose and Skin Dose for 6MV Versus 18MV for Prostate IMRT: Does the 20cm Rule Still Apply? R. M. Howell, W. Koontz-Raisig, P. A. S. Johnstone Emory University Hospital, Atlanta, GA Background: A general rule in radiotherapy is to prescribe high beam energies when treating patients with AP/PA separations greater than 20cm. Beams with energies greater than 10MV can generate unwanted secondary neutrons. This author previously reported results of organ equivalent doses (x-ray and neutron doses). Peripheral organ dose was much greater for high energy IMRT compared to high energy conventional radiotherapy; using low beam energies for IMRT signiﬁcantly decreased unwanted dose to peripheral organs. Yet, many clinicians remain reluctant to prescribe low energies when using IMRT for deep seated targets due to concerns about possible skin reactions. Purpose/Objective(s): This research compared measured skin dose for 6MV and 18MV IMRT and conventional prostate radiation therapy. Materials/Methods: In an earlier investigation, 6MV and 18MV IMRT and conventional treatment plans were created using a CT scan of the ART male dosimetry phantom. Since the average male patient is larger than the ART phantom, the phantom was rescanned with 3cm of bolus added to the anterior surface and 2 cm added to each side (ARTB), increasing the AP/PA sep from 20cm to 23cm and the Rt/Lt sep from 32cm to 36 cm. Both the static MLC shaping for the conventional and the dynamic MLC ﬁeld for the IMRT treatment plans were identical for the ART and ARTB treatment plans, comparable DVHs achieved. MU given in table below. TLD measurements were performed at 9 locations on the phantom surface; at central axis, and at 2 and 4 cm superior, inferior, right and left of beam cax. Standard deviation was 0.085 and batch error was 4%. Results: Skin dose results are given as a ratio of measured dose at each TLD location to the prescribed dose. While 6MV IMRT results in higher skin doses compared to 18MV IMRT, it still results in substantially lower skin doses than 18MV conventional radiotherapy. It reasonable to enter a new era where 6MV IMRT is used to treat even large patients with deep seated tumors. Author Disclosure: R.M. Howell, None; W. Koontz-Raisig, None; P.A.S. Johnstone, None. cax 2cm Avg 4cm Avg Total MU (1.8Gy/Fx) 6Conv ART 27.15% 27.01% 8.98% 291 6IMRT ART 10.24% 9.92% 1.68% 571 18Conv ART 18.91% 17.97% 9.50% 225 18IMRT ART 5.24% 4.87% 1.61% 460 6Conv ARTB 28.05% 28.87% 8.51% 315 6IMRT ARTB 11.60% 11.94% 1.66% 614 18Conv ARTB 19.29% 18.66% 9.46% 234 18IMRT ARTB 5.81% 5.29% 1.68% 488 S678 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006

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Author Disclosure: A.I. Saito, ViewRay, Inc., A. Employment; J.G. Li, None; C. Liu, None; K.R. Olivier, None; J.F. Dempsey,ViewRay, Inc., A. Employment; ViewRay, Inc., E. Ownership Interest.

2832 The Trade-Off Between Neutron Dose and Skin Dose for 6MV Versus 18MV for Prostate IMRT: Doesthe 20cm Rule Still Apply?

R. M. Howell, W. Koontz-Raisig, P. A. S. Johnstone

Emory University Hospital, Atlanta, GA

Background: A general rule in radiotherapy is to prescribe high beam energies when treating patients with AP/PA separationsgreater than 20cm. Beams with energies greater than 10MV can generate unwanted secondary neutrons. This author previouslyreported results of organ equivalent doses (x-ray and neutron doses). Peripheral organ dose was much greater for high energyIMRT compared to high energy conventional radiotherapy; using low beam energies for IMRT significantly decreasedunwanted dose to peripheral organs. Yet, many clinicians remain reluctant to prescribe low energies when using IMRT for deepseated targets due to concerns about possible skin reactions.

Purpose/Objective(s): This research compared measured skin dose for 6MV and 18MV IMRT and conventional prostateradiation therapy.

Materials/Methods: In an earlier investigation, 6MV and 18MV IMRT and conventional treatment plans were created usinga CT scan of the ART male dosimetry phantom. Since the average male patient is larger than the ART phantom, the phantomwas rescanned with 3cm of bolus added to the anterior surface and 2 cm added to each side (ART�B), increasing the AP/PAsep from 20cm to 23cm and the Rt/Lt sep from 32cm to 36 cm.

Both the static MLC shaping for the conventional and the dynamic MLC field for the IMRT treatment plans were identicalfor the ART and ART�B treatment plans, comparable DVHs achieved. MU given in table below. TLD measurements wereperformed at 9 locations on the phantom surface; at central axis, and at 2 and 4 cm superior, inferior, right and left of beamcax. Standard deviation was 0.085 and batch error was �4%.

Results: Skin dose results are given as a ratio of measured dose at each TLD location to the prescribed dose. While 6MV IMRTresults in higher skin doses compared to 18MV IMRT, it still results in substantially lower skin doses than 18MV conventionalradiotherapy. It reasonable to enter a new era where 6MV IMRT is used to treat even large patients with deep seated tumors.

Author Disclosure: R.M. Howell, None; W. Koontz-Raisig, None; P.A.S. Johnstone, None.

cax 2cm Avg 4cm Avg Total MU (1.8Gy/Fx)

6Conv ART 27.15% 27.01% 8.98% 2916IMRT ART 10.24% 9.92% 1.68% 57118Conv ART 18.91% 17.97% 9.50% 22518IMRT ART 5.24% 4.87% 1.61% 4606Conv ART�B 28.05% 28.87% 8.51% 3156IMRT ART�B 11.60% 11.94% 1.66% 61418Conv ART�B 19.29% 18.66% 9.46% 23418IMRT ART�B 5.81% 5.29% 1.68% 488

S678 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006