Author Disclosure: J.P. Kirkpatrick, None; F.R. Irani, None; S.E. Johnston, None; A.M. Stalnecker, None; T.M. Cooney, None;D.L. Georgas, None; M. Oldham, None.

2876 A New Method for Real-Time Confirmation of Irradiated Area of Therapeutic Photon Beams

K. Maruyama1, F. Matsubayashi2, T. Magara1, H. Kojima1, M. Ishigami1, K. Hayakawa1,3, M. Hashimoto4

1Graduate School of Medical Science, Kitasato University, Sagamihara, Japan, 2Department of Radiology, KitasatoUniversity Hospital, Sagamihara, Japan, 3Kitasato University School of Medicine, Sagamihara, Japan, 4Department ofRadiology, Chiba University Hospital, Chiba, Japan

Background: External irradiation of energetic photons is used in cancer therapy. There are established methods to estimate theabsorbed dose in the target area, determined by therapeutic planning. It is possible to confirm the irradiated area in advance andin the follow-up, however, no established method is available to confirm it during irradiation therapy.

Purpose/Objective(s): We propose a new method utilizing positron production by incident photons in order to measure theirradiated area directly. This study is to evaluate the method theoretically and to estimate its feasibility by measurements.

Materials/Methods: When photons with higher energies than 1.022MeV are incident on matter, electrons and positrons areproduced in the reaction of electron-positron pair production. Therefore, one can expect that the therapeutic photon beamirradiation on matter will produce positrons. The positron traverses through matter to combine with an electron, and thepositron-electron system annihilates into two photons. We theoretically estimated the number of positrons generated by thephoton-beam irradiation on a water phantom by using the Monte Carlo simulation program (Geant 4). We counted theannihilation photons from the region of interest (ROI) to estimate the positrons yield, and found that a large number ofannihilation photons were produced. The position distribution of generated positrons and that of annihilation photons are foundto conform to the irradiation area shown in Fig. 1. If we could detect the photons from outside of the phantom, it might bepossible to confirm the irradiation area determined by therapeutic planning. This measurement can be performed duringtherapeutic irradiation.

Results: A photon detection system has been designed and fabricated by us to confirm the calculation. Measurements ofgenerated photon yield from the ROI have successfully performed during irradiation. It proved that the yield is proportional tothe intensity of the irradiated photon beam.

Conclusions: In conclusion, we calculated the position distributions of produced positrons by a therapeutic photon beam in theGeant 4 Monte Carlo simulation. The distributions are useful to confirm the irradiated area in a water phantom. The calculationsare confirmed by photon yield measurements.

S705Proceedings of the 48th Annual ASTRO Meeting

Author Disclosure: K. Maruyama, None; F. Matsubayashi, None; T. Magara, None; H. Kojima, None; M. Ishigami, None; K.Hayakawa, None; M. Hashimoto, None.

2877 Image Guided In Vivo Dose Verification for Quality Assurance in Intensity Modulated Prostate CancerRadiotherapy

H. Wertz, J. Boda-Heggemann, C. Walter, B. Dobler, S. Mai, F. Wenz, F. Lohr

Mannheim Medical Center, Mannheim, Germany

Purpose/Objective(s): In external beam radiotherapy (EBRT) and particularly in intensity modulated radiotherapy (IMRT) theaccuracy of the dose distribution in the patient is essential. It was investigated whether image guided dose measurements in therectum are a reliable direct method for online dose verification during prostate radiotherapy.

Materials/Methods: In seven patients undergoing IMRT for prostate cancer an ionization chamber was placed in the rectumat the height of the symphysis and a total of twenty-one measurements were performed. With cone beam CT (CBCT) imagingthe position of the probe was determined before and after treatment. The point of measurement was determined relative to theisocenter and relative to an anatomical reference point (infundibulum of the seminal vesicles). The dose deviations relative tothe corresponding doses in the treatment plan were calculated and analyzed. With a manual offline CT soft tissue match patientpositioning after ultrasound was verified and the differences were calculated.

Results: The mean magnitude � SD of patient positioning errors was (3.0 � 2.5) mm, (5.1 � 4.9) mm and (4.3 � 2.4) mmin the left-right, anterior-posterior and cranio-caudal direction. The dose deviations in points at corresponding positions relativeto the isocenter were (-1.4 � 4.9) % (mean � SD). 86 % of the deviations were smaller or equal 5 % and 95 % were smalleror equal 10 %. The maximal deviation was -15 % (in a steep dose gradient). The mean dose deviation at correspondinganatomical positions was (6.5 � 21.6) %. 48 % of the deviations were smaller or equal 5 % and 71 % were smaller or equal10 %. In the rare event of insufficient patient positioning dose deviations could be more than 30 % because of the closeproximity of the probe and the posterior dose gradient.

Conclusions: Image guided dosimetry in the rectum during intensity modulated prostate cancer radiotherapy is a feasible andreliable direct method for online dose verification when probe position and patient setup are effectively controlled.

Author Disclosure: H. Wertz, Nomos, Elekta, B. Research Grant; J. Boda-Heggemann, Nomos, Elekta, B. Research Grant; C.Walter, Nomos, Elekta, B. Research Grant; B. Dobler, Nomos, Elekta, B. Research Grant; S. Mai, Nomos, Elekta, B. ResearchGrant; F. Wenz, Nomos, Elekta, B. Research Grant; F. Lohr, Nomols, Elekta, B. Research Grant.

2878 An Evaluation of Carbon Beam Attenuation Using Inhomogeneous Layered Phantoms: Comparison Withthe Present Method Using a Water Phantom

T. Magara, T. Ikeda, K. Maruyama

Graduate School of Medical Science, Kitasato University, Sagamihara 228–8555, Japan

Background: Since 1994, cancer therapy has been successfully accomplished at HIMAC, Japan using a high-energy carbonbeam. This method offers two advantages over the traditional radiation therapy: dose concentration due to the Bragg peak andthe high RBE. However, there is still an unsolved problem of beam attenuation due to fragmentation reactions of the carbonbeam. In the planning of treatment with carbon beam therapy, the dose distribution in the patient’s body is calculated based onthe data of the depth-dose distribution in water, and it does not take into account the beam attenuation in tissue other than water.There are few experimental methods for verifying the accuracy of the treatment plan.

Purpose/Objective(s): The purpose of this study is to evaluate beam attenuation in inhomogeneous layered phantoms as a newmethod of verifying beam attenuation in body.

Materials/Methods: The evaluation of carbon beam attenuation due to fragmentation reactions was performed by applying themethod which the Kitasato University group established1). Measuring devices used in this method consist of a �E counter, aphantom and six thin plastic scintillation counters surrounding the phantom. This method identified the occurrence offragmentation reactions by recording fragment-particle hits on plastic scintillation counters. For this study, we made three typesof phantoms using CT images: a hepatocyte-cancer phantom and two lung-cancer phantoms (one with bone and one withoutbone). The phantoms consist of plates of tough-lung (lung area), tough-bone (bone tissue), acrylic (soft tissue and tumor) andmuscular-fat equivalent material (hepatic tissue), and the plates were stacked along the beam direction. Then beam attenuationin the inhomogeneous layered phantoms was compared to attenuation in a water phantom, which is currently used to verify thetreatment plan.

Results: The beam attenuation in the hepatocyte-cancer phantom was about 33%, lower than the result in the water phantomby 3% (relatively 8%). We speculated that the presence of the tough-bone plate accounted for this difference. However, whilebeam attenuation in the lung-cancer phantom (with bone) was about 23%, similar to the result in the water phantom, attenuationin the lung-cancer phantom (without bone) was about 25%, which is higher than the result in the water phantom by 2%(relatively 9%). We speculated that the presence of the tough-lung plate, which differs greatly from water in terms of the atomicnumber and density accounts for this difference.

Conclusions: We succeeded in evaluating the carbon-beam attenuation using three types of inhomogeneous layered phantomsby applying the method which the Kitasato University group established. The results show that it is more effective to verifythe carbon-beam attenuation in the body by using our phantoms rather than the water phantom, which is currently used inplanning of patient treatment.

References1) T. Magara et al.: Jpn. J. Med. Phys. 25 Sup. 2: 148–149, 2005.

Author Disclosure: T. Magara, None; T. Ikeda, None; K. Maruyama, None.

S706 I. J. Radiation Oncology ● Biology ● Physics Volume 66, Number 3, Supplement, 2006