Proceedings of the 51st Annual ASTRO Meeting S705

(G4NeutronHPThermalScatteringData available in library G4NDL3.1) was plugged for computation when neutron energyis lower than 4 eV. The ambient dose equivalent, H*(10), was calculated as a product of particle fluence, F, and the flu-ence to dose equivalent conversion factors which were taken from ICRP 74 report (1996). The absorbed dose per protonwas 5x1011 proton/Gy used from available data.

Results: The ambient dose equivalent per therapeutic absorbed dose (H*(10)/D) is 3.5 mSv/Gy for neutron and 0.25 mSv/Gy forphoton. The average fluence to dose equivalent conversion factor (H*(10)/ F) is 377.7 pSv$cm2 for neutron and 12.2 pSv$cm2 forphoton.

Conclusions: Monte Carlo simulations with GEANT4 thus provide an improved understanding of the ambient dose equivalentsrequired for radiation shielding calculations.

Author Disclosure: Y. Chen, None; S. Ahmad, None.

3164 Dosimetric Comparison of Proton Radiotherapy and Photon Stereotactic Body Radiotherapy for

Treatment of Liver Metastases

A. W. Jensen, O. K. Macdonald, J. J. Kruse, J. M. Miller, R. C. Miller, M. G. Haddock, M. G. Haddock, R. L. Foote

Mayo Clinic, Rochester, MN

Purpose/Objective(s): We analyzed the dosimetric differences between proton radiotherapy (PT) and photon stereotactic bodyradiotherapy (SBRT) for treatment of liver oligometastases to clarify the potential advantages and disadvantages of the two mo-dalities.

Materials/Methods: Eight patients with metastases to the liver were simulated, planned and treated with SBRT according to ac-cepted techniques. PT treatment plans were retrospectively generated using CMS XiO 4.34.0 planning software. For each patient,three separate two- or three-field PT plans were designed with actively scanned beams without intensity modulation (ASPT), ac-tively scanned beams with intensity modulation (IMPT), and passively scattered beams (PSPT), respectively. Calculated dose char-acteristics were compared using a 2-sided t-test with significance defined as p # 0.05.

Results: Compared to SBRT, target dose homogeneity was improved with PT. Dose conformality (the ratio of the volume ofthe prescription isodose line to the PTV volume) with ASPT and IMPT (median 0.98 for both) were not significantly differentthan SBRT (median 1.1); dose conformality for PSPT was significantly worse (median 1.7, p \ 0.01). Doses to the liver for alltested parameters (maximum, minimum, and mean doses, and minimum dose to 50%, 30%, and total liver volume-700cc) weresignificantly reduced with all PT techniques except for PSPT, where the maximum dose and the minimum dose to 30% of liverwere not significantly different from SBRT. Skin maximum doses for ASPT (median 26.4 Gy) and PSPT (median 27.1 Gy)were significantly higher compared to SBRT (median 23.5 Gy); skin maximum doses with IMPT (median 23.0 Gy) were notsignificantly different from SBRT. The median skin mean dose with all PT modalities (range 5.2-8.7 Gy) was significantlylower than SBRT (13.2 Gy). The calculated point and integral doses to most other normal tissue structures (e.g. kidney, spine,esophagus, heart, stomach, and lung) were significantly less with PT, but the absolute dose differences compared to SBRTwere small.

Conclusions: In these hypothetical treatments, PT achieved comparable target dose coverage with better sparing of normaltissues than SBRT. Active scanning PT was more conformal and delivered fewer doses to normal liver than PSPT. The useof IMPT can avoid the relatively higher skin doses of other PT modalities. The clinical significance of these findings remainsto be determined.

Author Disclosure: A.W. Jensen, None; O.K. Macdonald, None; J.J. Kruse, Industry agreement for research/development of protonplanning systems, Varian Medical Systems, B. Research Grant; J.M. Miller, None; R.C. Miller, None; M.G. Haddock, None; M.G.Haddock, None; R.L. Foote, None.

3165 Dosimetric Comparison of Helical Tomotherapy against Protons in Treating the Prostate Alone

B. E. Lewis1, N. J. Yue1, R. Millevoi1, S. Kim1, D. Louis2, Z. Li2, N. P. Mendenhall2

1UMDNJ Robert Wood Johnson Medical School, New Brunswick, NJ, 2University of Florida Proton Therapy Institute,Jacksonville, FL

Purpose/Objective(s): Great strides have been made in reducing acute and late bladder and rectal toxicities experienced bymen undergoing definitive radiation therapy for early prostate cancer, and likewise, new techniques have gained popularitydue to the potential for further improvement. In this study, we perform a dosimetric comparison of proton therapy againsthelical tomotherapy with 6 MV photons for the delivery of 78 Gy to the prostate alone.

Materials/Methods: CT images for ten patients who had been treated with helical tomotherapy to the prostate and seminalvesicles were de-identified and replanned in two fashions with the original PTV/OAR contours. Prostate contours had beendesigned based upon noncontrast CT simulation, and prostate expansion to PTV was 0.8 cm except 0.6 cm posteriorly.Proton calculations with two fields assumed one field per day, alternating sides, with 40 CGE and 38 CGE deliveredfrom right and left laterals or slight anterior-obliques, respectively. Expansion to collimator edge from PTV was 1.0 cmexcept 0.7 cm posteriorly. PTV constraints were (100%, 74.1 CGE) and (95%, 78 CGE). OAR constraints were total femoralheads (1 cc, 55 CGE), rectal wall (50%, 50 CGE), (30%, 70 CGE) and bladder wall (35 cc, 30 CGE), (8 cc, 80 CGE), (7 cc,82 CGE). The 98% isodose line was at least 3 mm shy of PTV proximally and at least 5 mm distal to PTV on each slice.Helical tomotherapy plans utilized PTV constraint (95-96%, 78 Gy) and OAR constraints for rectum (35%, 40 Gy) and blad-der (50%, 40 Gy). In some helical tomotherapy plans, artificial rectal shield or bladder shield OAR were drawn to assistoptimization, but importantly, these were utilized only in order to meet institutional standards without specific knowledgeof proton DVH results.

Results: Target coverage criteria were met for all plans. For rectum and bladder, differences in percentage volume receiving $ 65Gy/CGE were insignificant between plans. However, proton plans delivered $ 30 Gy/CGE to 25% less bladder and 25% less

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rectum than corresponding photon plans (p\0.0001), and at the $ 50 Gy/CGE level, proton plans may treat as much as 6% less ofeach OAR.

Conclusions: In blinded comparison, helical tomotherapy photon plans appear equivalent to protons in meeting upper dose-vol-ume constraints for OAR at the cost of considerably more integral dose. Clinical importance of this difference remains to be in-vestigated.

Author Disclosure: B.E. Lewis, None; N.J. Yue, None; R. Millevoi, None; S. Kim, None; D. Louis, None; Z. Li, None; N.P. Men-denhall, None.

3166 Could We use Helical Tomotherapy for Total Skin Irradiation? A Study of the Dose Distribution in the

Rando Alderson Phantom

T. Piotrowski12, J. Kazmierska1

1Wielkopolskie Centrum Onkologii, Poznan, Poland, 2University of Medical Sciences, Poznan, Poland

Purpose/Objective(s): The current method of treatment of diseases like Mycosis Fungoides, Sezary Syndrome, Kaposi’sSarcoma, Leukemia Cutis or Scleromyxoedema is Total Skin Electron Irradiation realized on a conventional linac. In this studythe possibilities of the realization of total skin irradiation on Helical Tomotherapy was verified.

Materials/Methods: A rando Alderson female Phantom was used to create a treatment plan of total skin irradiation on HelicalTomotherapy. The total dose of 36 Gy was delivered in 36 fractions. 5 mm scans were performed for a phantom on a CT. Scanswere outlined on the Eclipse workstation and then sent to the Tomotherapy Planning Station. A skin contour was delineated withthe width of 1,5cm. Additionally the contours of whole body, eyes, lenses and four additional, ‘‘onion’’ structures were establish.‘‘Onion’’ structures were defined as a body area with an inner margin from the body outline. The value of the inner margin wasrespectively: 1.5 cm for structure A, 2 cm for structure B, 2.5 cm for structure C and 3 cm for structure D. Structures C and D wereused for dose minimalization inside the body and structures A, B for controlling of dose gradient between skin and structures C andD. Lenses and structure D were establish as directional blocked. A field width of 2.5 cm, pitch equal to 0.86 and 3.5 value of theplanned modulation factor were used for dose optimalization.

Results: An excellent, homogenous dose in the skin area was obtained for tomotherapy total skin irradiation. The mean dose was36.2 Gy with the range from 32.7 Gy (90.8%) to 39.7 Gy (110.2%). Moreover there was acceptable dose sparing in lenses (max-imum doses 6.2 Gy for the left lens and 6.7 Gy for the right lens) and eyes (mean dose 12.4 Gy for the right eye and 12.2 Gy for theleft eye) was observed. On the other hand doses received by the structure D (body with 3 cm inner margin) was higher than ina classical total skin electron irradiation realized on a conventional linac. For example for the rotational TSEI technique routinelyused in our hospital the contamination of the photons is 2.7 %. In tomotherapy the total skin irradiation mean dose on a wholestructure D was 4.6 Gy (13 % of the total dose) with a range from 0 Gy to 10 Gy. While splitting structure D to four anatomicalregions such as head and neck, thorax, abdomen and pelvis the mean dose was respectively: for head and neck 2.2 Gy (6.1%), forthorax 5.4 Gy (15%), for abdomen 1.7 Gy (4.7%) and for pelvis 3.2 Gy (8.9%). The beam-on time for planned total skin irradiationwas 21 min. The time was three times longer than for a rotational TSEI (7 min).

Conclusions: The longer beam-on time and the higher doses in the body on the Tomotherapy machine in comparison to classicalTSEI techniques reduce the possible clinical benefits of the treatment of patients with total skin diseases.

Author Disclosure: T. Piotrowski, None; J. Kazmierska, None.

3167 VMAT Plans Solution with Single and Double Gantry Rotation Compared with IMRT Step & Shoot for

Different Treatment Sites

L. Masi1, F. Casamassima1,2, R. Doro1, A. Colantuoni1, I. Bonucci1, C. Menichelli1

1Casa di Cura S. Chiara, Firenze, Italy, 2Istituto di radiobiologia clinica Universita’ di Firenze, Firenze, Italy

Purpose/Objective(s): Volumetric Modulated Arc Therapy (VMAT) can ensure dose conformality and sparing of OARs reduc-ing MUs and treatment times with respect to standard IMRT techniques. We compared IMRT step& shoot with VMAT plansfor treatments of prostate and hypo-fractionated treatments of spinal and abdominal tumors, analyzing advantages and draw-backs of a single arc solution (VMAT1) with respect to a 2 rotations approach (VMAT2).

Materials/Methods: IMRT step & shoot plans (Masterplan, Nucletron), single and two rotations VMAT solutions (ERGO 1.7.2Elekta) were created for an Elekta Synergy linac with 1cm leaf MLC. Plans were optimized for 5 prostate (76 Gy, 38 fx),3 spinal and 2 abdominal (30 Gy, 3 fx) cases and evaluated in terms of number of MUs, dose covering 95% (D95) and 90%(D90) of the PTV, conformity and homogeneity index (CI, HI). Due to the different fractionation schemes, MUs were normalizedto 2Gy and D90 and D95 were normalized to the prescription doses. For Prostate plans rectum V60Gy and V50Gy were also usedfor comparison among techniques (bladder and femoral heads were held within RTOG constraints). For spinal cases maximumdose to cord PRV was strictly maintained at 18 Gy (1%) for all plans. In the 2 abdominal cases most critical OARs (tangentialto the PTV) were duodenum (max 27 Gy at 2%) and aorta (max 30 Gy); liver, kidney and stomach were also considered in theoptimization.

Results: Normalized and averaged MUs for VMAT1 (409) and VMAT2 (432) are significantly lower (p \ 0.01) than IMRTMUs (600). Averaged IMRT treatment times (515 s) were longer (p = 0.017) even than VMAT2 times (254 s). Normalizedand averaged D90 values for IMRT and VMAT2 plans show no significant difference (101 and 100.5 respectively). While 1gantry rotation yielded a slightly but significantly lower coverage (98.5 p = 0.03). Double rotation was necessary in 4 out of 10plans to ensure the same PTV coverage obtained by IMRT. Averaged CI is better for the 2 VMAT solutions (respectively 1.54and 1.55) than for IMRT plans (1.69) but statistical significance is not observed. IMRT plans yielded a more homogeneousdose distribution inside the PTV (averaged HI 0.14) than VMAT(1 and 2) (0.17 and 0.18 respectively), but statistical signif-icance was not reached (p = 0.053). Constraints for all OARs were satisfied in all plans. For prostate plans averaged values forrectum V60Gy yielded 20.5% (VMAT 1), 21% (VMAT 2) and 21% (IMRT), while V50Gy values were respectively 36.4,40.8 and 39%.