Proceedings of the 50th Annual ASTRO Meeting S659

HT with its good dose homogeneity and conformity is quite suitable for small animal research.

References:1. DesRosiers et al., 2003, TCRT 2, p4492. Deng. et al., 2007, PMB 52, p27293. Stojadinovic et al., 2006, MP 33, p3834

This work is supported in part by Tomotherapy Inc.Author Disclosure: S.M. Goddu, Tomotherapy Inc., B. Research Grant; S. Mutic, None; D. Chen, None; P. Parikh, None; E. Iza-guirre, None; D. Low, Tomotherapy Inc., B. Research Grant.

3088 The Impact of Patient Immobilization Devices on Skin Dose during IMRT: A Radiochromic EBT Film

Dosimetry Study in Phantom

S. Chiu-Tsao1, S. Sim2, M. F. Chan2

1Quality MediPhys, LLC, Denville, NJ, 2Memorial Sloan Kettering Cancer Center, Basking Ridge, NJ

Purpose/Objective(s): For delivery of dynamic IMRT fields, the MLC leaves move in one direction with variable speeds while thebeam is on. The enhancement effect on the skin dose due to the presence of patient immobilization device could vary with the dy-namic MLC gap width, location and fluence. Radiochromic EBT film (0.24 mm thick, active emulsion in the central layer) is poly-mer-based and tissue equivalent. The useful dose range is wide and coincides with the clinical doses in radiation therapy. The EBTfilm response is weakly dependent on energy. In this study, the two-dimensional (2D) perspective of skin dose increase in the pres-ence of patient alpha cradle immobilization devices during IMRT treatment was quantified using EBT film dosimetry in phantom.

Materials/Methods: An IMRT field (6MV or 15MV photon from a Varian Cl-iX Linac) was delivered to expose a stack of 2 EBTfilms placed on the surface of a solid phantom, with or without an alpha cradle immobilization device above the film stack. Thealpha cradle was not of even thickness, simulating the patient treatment situation. Both films simulated the patient’s skin layers. Allthe films were scanned using an Epson 4870 flatbed scanner with 48 bit color (RGB) and 150 dpi (0.0169 cm per pixel). The pixelvalues were converted to doses based on an established calibration curve. Cross sectional profiles and 2D isodose curves were gen-erated. The enhancement factor, defined as the ratio of doses with and without the presence of alpha cradle, was also determined in2D film plane.

Results: The 2D isodose curve plots extracted from the EBT films illustrated non-uniform increase of the surface doses due to thepresence of the alpha cradle. For both 6MV and 15MV beams, the dose enhancement factors in the top film (in direct contact withthe alpha cradle) and the lower film can be as high as 400% and 250%, respectively, demonstrating the loss of skin sparing effectattributed to the presence of the alpha cradle.

Conclusions: Significant increase in skin doses in the presence of an alpha cradle was confirmed and quantified with radiochromicEBT film in 2D for an IMRT field. In-vivo EBT film dosimetry would be an important and effective method in measuring the 2D dosesduring IMRT treatments across the skin surface beyond point dose measurements. Although the use of immobilization device improvesthe setup reproducibility and comfort in patients receiving IMRT treatment, the impact of the immobilization devices on skin dose canbe clinically significant. The 2D skin dose measurement would allow us to correlate skin reaction in any immobilization device.

Author Disclosure: S. Chiu-Tsao, None; S. Sim, None; M.F. Chan, None.

3089 Use of Music-based Breathing Training to Stabilize Breathing Motion in Respiration Correlated Imaging

and Radiation Delivery

T. J. Waldron, J. E. Bayouth, S. Bhatia, J. M. Buatti

Department of Radiation Oncology, University of Iowa College of Medicine, Iowa City, IA

Purpose/Objective(s): Assess the stability of measured breathing patterns of patients undergoing respiratory-correlated radiationtherapy (RCRT), and impact of a music-based breathing training device.

Materials/Methods: 46 patients identified as potentially benefiting from RCRT underwent respiratory training prior to 4DCTstudy. Training consisted of 10 minutes of monitored use of a music-based hypertension treatment device (RESPeRATE, IntercureLtd., Lod Israel). RESPeRATE plays music that increases in pitch to cue inhalation and decreases to cue exhalation. A strain sensormeasures initial breathing rate and subsequent compliance. Voluntary synchronized respiration results in a decrease in prompt rateat 30 second intervals. Rate change ceases when the programmed session time or a steady state of breathing rate is reached. En-trainment success was assessed by the change in respiratory rate during the training session and used to identify the fixed musicalpacing rate needed for 4DCT acquisition and gated treatment delivery. Patient compliance with music-paced breathing was mea-sured by power spectrum analysis (n = 18): 10 lung lesions, 6 hepatobiliary, 1 adrenal and 1 breast. Patient treatment fractionsranged from 1 to 28, with 336 samples spanning 181 fractions. Data were analyzed in samples of approximately 3.5 minutes du-ration. Dominant frequency (DF) was examined in comparison to pacing rate, and DF spectral power band +/� 0.5 breaths perminute (bpm) was examined as an indication of stability.

Results: 38 of 46 patients successfully completed respiratory training prior to imaging; 18 patients went on to receive RCRT due toclinically sufficient motion (.1 cm). 15 of the 18 patients were successful in training and treated with the fixed-rate musical pacingidentified during training. 10 patients performed within 0.5 bpm of the target rate (on average) and were able to do so for approx-imately 67% of the time (range, 55 - 78%). 2 patients exhibited treatment fractions of both excellent (72% and 81%) and poor (28%and 24%) compliance with the target breathing rate. The final 3 of 15 patients showed poor compliance (27% average), similar tothe 3 patients who did not entrain well to music-paced breathing (30%).

Conclusions: Respiratory training was successful for the majority of patients (83%) using the RESPeRATE device. As indicatedby power spectrum analysis, most successfully trained patients were effective in maintaining their respiratory pace established at4DCT during respiration-gated treatment delivery (67%). Power spectrum analysis was a powerful and sufficiently accurate tool toassess rate compliance over time periods that encompassed full course of treatment.

Author Disclosure: T.J. Waldron, Shareholder, Varian Medical Systems, E. Ownership Interest; J.E. Bayouth, None; S. Bhatia,None; J.M. Buatti, None.