M Heard

COMPARISON OF MONTE CARLO CALCULATIONS AROUND A FLETCHER SUIT DELCLOS OVOID WITH RADIOCHROMIC FILM AND NORMOXIC POLYMER GEL DOSIMETRY

The Fletcher Suit Delclos (FSD) ovoids employed in intracavitary brachytherapy (ICB) for cervical cancer contain shields to reduce dose to the bladder and rectum. Many treatment planning systems (TPS) do not include the shields and other ovoid structures in the dose calculation. Instead, TPSs calculate dose by summing the dose contributions from the individual sources and ignoring ovoid structures such as the shields. The goal of this work was to calculate the dose distribution with Monte Carlo around a Selectron FSD ovoid and compare these calculations with radiochromic film (RCF) and normoxic polymer gel dosimetry. Monte Carlo calculations were performed with MCNPX 2.5.c for a single Selectron FSD ovoid with and without shields. RCF measurements were performed in a plane parallel to and displaced laterally 1.25 cm from the long axis of the ovoid. MAGIC gel measurements were performed in a polymethylmethacrylate phantom. RCF and MAGIC gel were irradiated with four 33μGym2h-1 Cs-137 pellets for a period of 24 h. Results indicated that MCNPX calculated dose to within ±2% or 2 mm for 98% of points compared with RCF measurements and to within ±3% or 3 mm for 98% of points compared with MAGIC gel measurements. It is concluded that MCNPX 2.5.c can calculate dose accurately in the presence of the ovoid shields, that RCF and MAGIC gel can demonstrate the effect of ovoid shields on the dose distribution and the ovoid shields reduce the dose by as much as 50%.

Measurement of a 200 MeV proton beam using a polyurethane dosimeter

PRESAGETM (Heuris Pharma LLC, Skillman, NJ) is a three-dimensional polyurethane dosimeter containing a leuco dye that generates a color change when irradiated. The dosimeter is solid and does not require a container to maintain its shape. The dosimeter is transparent before irradiation and the maximum absorbance of the leuco dye occurs at 633 nm which is compatible with the OCT-OPUSTM laser CT scanner (MGS Research, Inc., Madison, CT). The purpose of this study was to investigate the response of PRESAGETM to proton beam radiotherapy.

Verification of Monte Carlo calculations around a Fletcher Suit Delclos ovoid with normoxic polymer gel dosimetry

The goal of this work is to calculate the effect of including the anterior and posterior ovoid shields on the dose distribution around a Fletcher Suit Delclos (FSD) ovoid (Nucletron Trading BV, Leersum, Netherlands) and verify these calculations with normoxic polymer gel dosimetry. To date, no Monte Carlo results verified with dosimetry have been published for this ovoid.

SU-FF-T-263: Evaluation of a PAGAT Gel for Use in An Anthropomorphic Phantom

Purpose: To evaluate the use of PAGAT, a normoxic polymergel formulation, in the Radiological Physics Centers (RPC) Head and Neck Phantom. Methods and Materials: Conventional and IMRT treatment plans were developed for the RPCs head and neck phantom, which contained simulated planning target volumes and an organ at risk. A PAGAT dosimeter was placed in the phantom and the conventional treatment plan was delivered. A different PAGAT dosimeter was used to measure the IMRT treatment. An additional group of PAGAT dosimeters were irradiated to doses between 1 and 5 Gy through parallel‐opposed beams to develop a calibration curve. The dosimeters were imaged using an OCT‐OPUS™ laser CT scanner 24 hours after irradiation. Profile, isodose, and gamma index comparisons were performed between the treatment plan and the distribution measured with PAGAT. Results: Good agreement was observed between the conventional treatment plan and the gel distribution with the exception of one target region where the dose was most fractionated. The gel overestimated the dose in this region. The gel used to measure the IMRT treatment overestimated the dose by 35%. Relative comparisons of the IMRT treatment showed agreement consistent with that seen in the conventional treatment plan. Conclusions: The over‐response of the gel is attributed to the fractionation dependence of the gel, which was investigated. Calibrationgels should be irradiated with a fractionation scheme similar to the experimental gels, particularly for IMRT treatments, so that absolute dose comparisons can be performed. The investigation was supported in part by PHS grant CA 10953 awarded by the NCI, DHHS.

SU‐GG‐T‐308: Polymer Cross‐Linking and Sensitivity Enhancement in Normoxic Polymer Gels Used for Three‐Dimensional Dosimetry

Purpose: To discuss the relation between cross‐linking and sensitivity enhancement in normoxic polymergels in a dose range suitable for intensity modulated radiotherapy, based on considerations related to actual sensor preparation methods and a theoretical description of the optical behavior of irradiated polymergel when dose maps are obtained through optical CT.Method and Materials: Two set of samples of MAGIC gel (9% in weight of methacrylic acid) were prepared at 37°C and 45°C and irradiated with a 60 Co and 6 MV photon beams to doses in the range from 0.1 to 5 Gy. The samples were scanned in an optical CT. Non‐irradiated samples were also scan. A theory based on the number of sites for water solvation, which takes into account polymer cross‐linking, is introduced in order to explain the observed changes in the optical density. The slope of the polymergel response to dose is a measurement of sensitivity and it is analyzed for the actual experimental conditions as well as those for the optimum in connection to cross‐linking phenomena. Conclusion: The analysis showed a proper monotonic behavior for the polymergel response and maximum sensitivity when cross‐linking occurs in an important fraction in the preparation process.

SU‐FF‐T‐19: A Method for Extracting the Relevant MRI Information From Normoxic Polymer Gels Exposed to Low Doses

Purpose: To extract the contribution due to the formation of polymer at low dose from the MRI multi‐echo signal using a numerical method for Laplace transform inversion and to analyze the transverse relaxation rate spectrum Materials and Methods: Several samples of MAGIC gel (9% by weight of methacrylic acid) were prepared and irradiated with 60 Co and 6 MV photon beams to doses in the range of 0.1 to 0.5 Gy. The samples were scanned in a 7 Tesla MRI Bruker BioSpec using a Carr‐Purcell‐Meiboom‐Gill sequence. The signals were analyzed with the inversion algorithm. Samples of pure gelatin and a combination of gelatin and methacrylic acid were scanned in an MR spectrometer from Bruker. The samples were also scanned in an optical CT.Conclusions: Data analysis showed that polymer formation can be used as a better parameter than the average transverse relaxation rate for polymergel dose calibration at low dose. The chemical shift spectrum showed that gelatin was not affected by radiation. The optical CT scan correlated well with the other results.

SU-FF-T-150: Development of a Modified 3D Radiochromic Dosimeter for Clinical Proton Beams

Purpose: To develop a formulation of PRESAGE™, a three‐dimensional radiochromic dosimeter, that does not under‐respond in the high LET region of a protonbeam. Previous investigations with PRESAGE™ report a 38% under‐response when irradiated in the SOBP of a protonbeam compared to 6 MV photons.Method and Materials: A new PRESAGE™ formulation was developed for this study by replacing the reporter molecule in the original formulation, leucomalachite green (LMG), with a new LMG derivative. Dosimeters were manufactured using LMG and the LMG derivative and irradiated in the SOBP region of a 250 MeV protonbeam to doses from 1 Gy to 5 Gy to develop a dose response curve for each formulation. An additional dose response curve was developed for the LMG derivative by irradiating dosimeters to doses from 1 Gy to 5 Gy using 6 MV photons.Results: The response of the LMG derivative was 52% more than that of the original formulation when irradiated using a protonbeam. The response of the LMG derivative to protons and photons was within 3% over the entire dose range. The LMG derivative was also found to be more stable post‐irradiation than the original formulation. Conclusion: The under‐response of PRESAGE™ to protons has been corrected by replacing the LMG reporter molecule with a new LMG derivative. The molecular structure of the new derivative was based on the presumption that the under‐response in the original formulation was due to shifting of the equilibrium of the colored malachite green form back to the non‐colored leuco form when irradiated in high LET beams. The data for the new LMG derivative supports this mechanism. Additional studies will be presented that further characterize the response of the new PRESAGE™ formulation. The investigation was supported by PHS grant CA 10953 awarded by the NCI, DHHS.

SU‐FF‐T‐284: Linear Energy Transfer (LET) Dependence of BANG® Polymer Gel Dosimeters in Proton Beams

Purpose: Technological advances of radiation therapydelivery systems, like proton therapy, have prompted research towards the development of accurate and reliable 3D dose verification systems, such as polymergeldosimetry. Furthermore, preliminary research has shown a decrease in geldosimeter response in high LET regions of a dose distribution. The aim of this study is to assess the response of BANG® polymergeldosimeters in proton beams. Method and Materials:Dose response curves were first obtained for a range of doses of 6 MV x‐rays and 250 MeV protons by irradiating BANG® geldosimeters immersed in a water phantom. The plateau region of the proton beam was used. The gels were then scanned using Optical CT (OCT) and ion chamber measurements for the proton and photon beams were collected. Next, several BANG® gels were irradiated in the peak of a 140 MeV pristine proton beam to acquire the depth dose distribution. OCT scans of the gels were compared with ion chamber percent depth dose data to determine and correct for the relative sensitivity of the geldosimeter at various depths. Results: BANG® geldosimeters showed a linear response with dose of optical attenuation acquired with the OCT scanner for both proton and photon beams in the range 0 Gy to 6 Gy. The relative sensitivity of the geldosimeters at various depths in a 140 MeV proton beam showed an 8% underdose in the Bragg peak region. Additional energies will be investigated. Conclusion: BANG® polymergeldosimeters can be utilized to assess the three‐dimensional character of the resultant dose distributions in proton beams. Further research includes irradiating the RPC pelvis phantom to evaluate the radiation treatment plan calculations. Conflict of Interest: Work supported by PHS grant CA10953 and ATC grant CA081647 awarded by NCI, DHHS.

SU‐FF‐T‐231: Evaluation of the Spatial and Dose Resolution of a New 3D Polyurethane Dosimeter

Purpose: To determine the dose resolution and spatial stability of PRESAGE™, a new three‐dimensional (3D) polyurethane dosimeter. Methods and Materials: PRESAGE™ dosimeters were irradiated to doses between 0.5 Gy and 10 Gy using stereotactic beams to develop a dose response curve and determine the dose resolution. A PRESAGE™ dosimeter was also placed in a water tank with the top surface coincident with the water surface and irradiated using a half‐blocked field delivered by a linear accelerator to investigate the spatial integrity of the dose distribution. An additional PRESAGE™ was irradiated in a similar fashion; however the total dose was delivered in 3 fractions given over 3 days in order to investigate the affect of fractionation on spatial stability. All dosimeters were scanned using an OCT‐OPUS™ laser CT scanner. Results: The PRESAGE™ dosimeter showed a monotonic and easily characterized response with dose. The dose resolution, determined at the 95% confidence level, was found to be comparable to polymergel formulations. The width of the measured penumbra was 3.98 mm when irradiated in a single fraction and 4.23 mm when irradiated in multiple fractions. Neither dosimeter demonstrated a dose overshoot near the steep dose gradient. Conclusions: This work demonstrated the potential for PRESAGE™ to be used for 3D dosimetry. The dose distributions were found to be spatially stable in high‐dose gradient regions. Also, the dosimeter did not exhibit the dose overshoot often observed with polymergel dosimeters. Further work is required to optimize the dose resolution of the dosimeter. The investigation was supported by PHS grants CA 10953 awarded by the NCI, DHHS.