Proceedings of the 49th Annual ASTRO Meeting S729

2941 Evaluation of a 2D Array Planar Dose Verification System for IMRT Dose Delivery

H. K. Malhotra, J. Liu, T. Tran, M. B. Podgorsak

Roswell Park Cancer Institute, Buffalo, NY

Purpose/Objective(s): 2D array based multi detector planar dose verification systems play an important role in the pre-treat-ment quality assurance of an IMRT plan. There are a number of devices available in the market which differ from eachother in the type of detectors employed, detector size, spatial distribution of the detectors, maximum field-size availableetc. In the present study, we have evaluated the usefulness of PTW729 2D array system in a clinical environment forthe pretreatment QA of IMRT dose delivery technique. The results were also compared with those from SunNuclear Map-check system.

Materials/Methods: 5 patients having IMRT plans generated with Eclipse Treatment Planning system for different targets wereused in this study. The plans were designed for sliding-window IMRT dose delivery technique on a Varian 6 MV linear acceleratorfitted with millennium MLC. An individual verification plan per field was generated from the clinically approved plans on a stan-dard solid water phantom after resetting the gantry, collimator and couch angles to 0. The SSD of the fields was set to 95 cm and thedose computation was carried out using a grid-size of 2.5 mm. The computed planar dose matrix in the coronal section was thenexported to the PTW729 system as well as Sun Nuclear Mapcheck system.

PTW729 incorporates 729 5 mm * 5 mm * 5 mm sized cube vented ion-chambers uniformly spaced with center to center distanceof 10 mm and useful field size of 27 * 27 cm2. The effective point of measurement of the chambers is assumed to be the back of thefront surface of the ion-chambers. Mapcheck systems uses 445 diode detectors [0.8 mm * 0.8 mm] designed for use up to 20� 20cm2 field-size. More over the system has differential spacing of detectors with more detectors located near its central region. Prior touse, both the systems were self-calibrated for absolute dosimetry using a 10 * 10 cm2 field at 95 cm SSD and 5 cm depth in thephantom and 10 cm back-scatter medium.

Results: The two systems were compared using the following criterion: the percentage of detectors passing 3%/3 mm criterionas well as the CAX dose measured by the system and the computed plan in absolute dose values. Analysis of our results showedthat the mean, median & standard deviation at CAX for the 30 fields were 0.46, 0, 2.32 for PTW729 while the correspondingvalues for Mapcheck were �2.29, �1.8, �2.59, respectively. Analysis of detectors satisfying 3%/3 mm criterion for PTW729showed the mean, median & standard deviation of 90.2, 92.16, 6.3 while the respective values for Mapcheck were 90.44, 91.65,6.68.

It is important to know that Mapcheck by default uses a dose threshold of 10% which excludes the detectors having\10% of themaximum dose value from 3%/3 mm computation. These detectors usually lie at the field edges. In PTW729, this threshold is set to5% and can’t be changed by user. It has been observed that the pass-rate of PTW729 is slightly poorer than Mapcheck as thesefailures occurs at the detectors which are outside the useful fluence pattern. Once these outliers are removed, PTW729 gives resultswhich are consistent with the treatment planning system computations.

Conclusions: Our preliminary measurements demonstrate the usefulness of PTW729 as a pretreatment IMRT QA device for planardose verification.

Author Disclosure: H.K. Malhotra, The 2D array system was provided by PTW-New York Corporation for evaluation, C. OtherResearch Support; J. Liu, None; T. Tran, None; M.B. Podgorsak, None.

2942 Scalp and Ear Sparing Whole Brain Radiation Therapy Using Helical Tomotherapy

J. P. Limmer1, J. Buskerud1, D. Henrich1, R. Adkins2, V. Mullins2, J. S. Welsh2,3

1UW Cancer Center-Aspirus, Wausau, WI, 2UW Cancer Center-Riverview, Wisconsin Rapids, WI, 3University of WisconsinSchool of Medicine, Madison, WI

Purpose/Objective(s): Whole brain radiation therapy is routinely used for palliation of brain metastases and prophylactic cranialirradiation to prevent brain metastases in small cell lung cancer patients. Common acute adverse effects include scalp irritation,otitis externa, otitis media, and alopecia. We explored the ability of helical tomotherapy to minimize such adverse effects througha scalp and ear sparing approach to whole brain radiotherapy.

Materials/Methods: Three patients received whole brain radiation therapy that was planned and administered using the HI-ARTTomoTherapy treatment planning software and treatment unit. Dose-volume histograms were generated to evaluate the potential ofthe scalp-sparing technique in minimizing dose to the scalp and ears. Patient treatment consisted of 30 Gy in 10 fractions for 2patients. One patient received 30 Gy in 10 fraction and a simultaneous in-field boost to 35 Gy for 4 large symptomatic lesions.Thermoluminescent diodes were used to verify actual dose to the scalp.

Results: The DVH’s illustrated effective sparing of the scalp and ears in every patient. The average maximum dose to the scalp was25 Gy with no more than 50% of the scalp receiving more than 15 Gy. TLD’s confirmed that the expected doses to the scalp basedon the isodose curves and DVH’s were what was received. Treatments were efficient with an average beam on time of 5 minutesand pretreatment limited field-of-view MVCT imaging taking about 90 seconds. Overall treatment time including set up, MVCTimaging, readjustment and treatment was typically under 20 minutes even with simultaneous in-field boost. Treatment planningtook on average 2 hours. Before, during and after pictures were obtained in all 3 patients and showed no early alopecia but onepatient died before the next set of follow-up photographs, another patient shaved his head in anticipation of planned stereotacticradiosurgery and the third patient went on to receive chemotherapy and developed alopecia. No patient complained of any otic/auditory symptoms during or after the course of treatment.

Conclusions: Scalp and ear sparing whole brain radiation therapy is feasible and relatively easily implemented using helical to-motherapy. TLD’s confirmed the low doses predicted from the planning software. Treatment planning and daily treatment requiresmore input from physicians, radiation therapists, physicists and dosimetrists but can be accomplished efficiently without affectingthroughput in a busy clinic. The full value of this approach in terms of reducing alopecia and otitis awaits further clinical experi-ence.

Author Disclosure: J.P. Limmer, None; J. Buskerud, None; D. Henrich, None; R. Adkins, None; V. Mullins, None; J.S. Welsh,None.