- Email: [email protected]
Evaluation of a tissue expander in external radiation therapy authors
Proceedings of the 34thAnnual ASTRO Meeting collimators fields.
as beam-edge
modifiers
is described
and applied
to the problem
285
of abutting
The problem is investigated in two parts - matching field edges Materials and Methods: along a common ray (the penumbra problem) and matching field edges with different divergences A computerat a specific plane of interest (the penumbra plus divergence problem). is employed. with dynamic collimator capability controlled Philips SL25 accelerator Generated-beam and stored-beam treatment planninq systems are tested for their capability of Dynamic beam-edge modification is simulated handling matching fields with sharp penumbra. by treatment planning and realized by actual dynamic collimation with the SL25 accelerator. Dosimetry measurements in phantom are performed for verification. Results: Dose inhomogeneities in the match-line region of abutting fields are substantially reduced by beam-edge modification using dynamic collimators. Dynamic beam-edge modification can be simulated by treatment planning and Conclusions: realized by computer-controlled dynamic collimation. This method can be used to improve dose homogeneity in clinical applications requiring abutment of adjoining fields.
1077 ROTATIONAL.AND MULTIFIELDDOSIMETRY BACKPROJECTION AND MEASURED TRANSMISSION VALUES Robert G. Waggener. Ph.D.. E. Bumell Hranitsky. M. S.. John J Feldmeier. D. 0.. and MelJssa Miller Division of Radiation Oncology. Department of Radiology. University of Texas Health Science Center. San Antonio. Texas 78284 Purpose: To compare therapy beam values in an anthropoid phantom using conventional computer dosimetry techniques and a new method based upon modified backprojection techniques from Computed Tomography (m. Materiels and Methods: Transmission values of rotational and multiple Melds were measured using an anthropomorphic phantom and fflm dosimetry for 60 Co and 15 MV X-rays. Dose values in the anthropomorhpic phantom for selected points were calculated using a modified form of backprojection as defined in Computed Tomography (CT) reconstruction from the orlglnal Radon transform. The backprojection was modlfled: (1) for the divergent beam geometry of the therapy beams (as contrasted to parallel beam geometry for the original Radon algorithm); (2) for inverse square fall-off from the point of interest to the point of measurement: and 13) for scatter using both conventional techniques and an approximation method derived from the transmlsslon measurements. Results: The backprojection method when compared to conventional techniques differs by up to 8% for small fields with a relatively small amount of scatter. The discrepency is 8 to 12% for larger fields with more scatter. The use of conventional scatter estimation methods greatly increases the accuracy of the new method. Conclusions: The use ofthenew method for estfmating primary dose components coupled with conventional scatter estimation methods provides a relatively quick and easy way to implement method for dosimetry for use in cases where the beam transmission is known.
1078 EVALUATION
OF A TISSUE EXPANDER
IN EXTERNAL
RADIATION THERAPY
Authors- J.K.Bareng,J.T.Nugent, S.G. Chenery, H.B. Wolkov, M.D., W.I. Yamaha& M.D. Radiological Associates of Sacramento Medical Group, Inc., Radiation Oncology Center The dosimetric effect on treatment of a breast with a tissue expander in place has been studied. The metal component found in a type of expander which has been seen in a number of patients sent for radiation therapy has raised questions as to possible underdosing of tissue located in the shadow of the component as well as to possible increase in dose to the tissue in proximity to it. Owing to an absence of information in the literature, patients have been hitherto referred for removal of the expander prior to radiation therapy. Our evaluation is based on planned 4 MV treatment of a patient with an expander in place and upon phantom measurements. The means of analysis used included computed tomography (CT) scan and thermoluminescence dosimetry (TLD). The chief components of the expander’include an all silicone textured surface expansion envelope and an injection site composed of a stainless steel or titanium needle guard encapsulating a permanent magnet interfaced with a self sealing silicone membrane. The CT scan was used to determine the patient contour and location of the magnet. Attenuation produced by the magnet was
as beam-edge
modifiers
is described
and applied
to the problem
285
of abutting
The problem is investigated in two parts - matching field edges Materials and Methods: along a common ray (the penumbra problem) and matching field edges with different divergences A computerat a specific plane of interest (the penumbra plus divergence problem). is employed. with dynamic collimator capability controlled Philips SL25 accelerator Generated-beam and stored-beam treatment planninq systems are tested for their capability of Dynamic beam-edge modification is simulated handling matching fields with sharp penumbra. by treatment planning and realized by actual dynamic collimation with the SL25 accelerator. Dosimetry measurements in phantom are performed for verification. Results: Dose inhomogeneities in the match-line region of abutting fields are substantially reduced by beam-edge modification using dynamic collimators. Dynamic beam-edge modification can be simulated by treatment planning and Conclusions: realized by computer-controlled dynamic collimation. This method can be used to improve dose homogeneity in clinical applications requiring abutment of adjoining fields.
1077 ROTATIONAL.AND MULTIFIELDDOSIMETRY BACKPROJECTION AND MEASURED TRANSMISSION VALUES Robert G. Waggener. Ph.D.. E. Bumell Hranitsky. M. S.. John J Feldmeier. D. 0.. and MelJssa Miller Division of Radiation Oncology. Department of Radiology. University of Texas Health Science Center. San Antonio. Texas 78284 Purpose: To compare therapy beam values in an anthropoid phantom using conventional computer dosimetry techniques and a new method based upon modified backprojection techniques from Computed Tomography (m. Materiels and Methods: Transmission values of rotational and multiple Melds were measured using an anthropomorphic phantom and fflm dosimetry for 60 Co and 15 MV X-rays. Dose values in the anthropomorhpic phantom for selected points were calculated using a modified form of backprojection as defined in Computed Tomography (CT) reconstruction from the orlglnal Radon transform. The backprojection was modlfled: (1) for the divergent beam geometry of the therapy beams (as contrasted to parallel beam geometry for the original Radon algorithm); (2) for inverse square fall-off from the point of interest to the point of measurement: and 13) for scatter using both conventional techniques and an approximation method derived from the transmlsslon measurements. Results: The backprojection method when compared to conventional techniques differs by up to 8% for small fields with a relatively small amount of scatter. The discrepency is 8 to 12% for larger fields with more scatter. The use of conventional scatter estimation methods greatly increases the accuracy of the new method. Conclusions: The use ofthenew method for estfmating primary dose components coupled with conventional scatter estimation methods provides a relatively quick and easy way to implement method for dosimetry for use in cases where the beam transmission is known.
1078 EVALUATION
OF A TISSUE EXPANDER
IN EXTERNAL
RADIATION THERAPY
Authors- J.K.Bareng,J.T.Nugent, S.G. Chenery, H.B. Wolkov, M.D., W.I. Yamaha& M.D. Radiological Associates of Sacramento Medical Group, Inc., Radiation Oncology Center The dosimetric effect on treatment of a breast with a tissue expander in place has been studied. The metal component found in a type of expander which has been seen in a number of patients sent for radiation therapy has raised questions as to possible underdosing of tissue located in the shadow of the component as well as to possible increase in dose to the tissue in proximity to it. Owing to an absence of information in the literature, patients have been hitherto referred for removal of the expander prior to radiation therapy. Our evaluation is based on planned 4 MV treatment of a patient with an expander in place and upon phantom measurements. The means of analysis used included computed tomography (CT) scan and thermoluminescence dosimetry (TLD). The chief components of the expander’include an all silicone textured surface expansion envelope and an injection site composed of a stainless steel or titanium needle guard encapsulating a permanent magnet interfaced with a self sealing silicone membrane. The CT scan was used to determine the patient contour and location of the magnet. Attenuation produced by the magnet was