Medical D~tsimclr.r. Vol. 17. pp. 217-220
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ADVANTAGES OF USING HIGH ACTIVITY 12sI SEEDS IN TEMPORARY INTERSTITIAL BREAST IMPLANTS SURENDRA N . RUSTGI, P H . D . a n d SEUNG S. H A H N , M . D . Department of Radiology, MetroHealth Medical Center/Case Western Reserve University, 2500 MetroHealth Drive, Cleveland, OH 44109, U.S.A. Abstract--There has been considerable interest in the use of high activity 12si sources as a substitute for 1921r seeds for removable implants of the breast and prostate. 12Sl seeds with an initial activity of approximately 5 mCi per seed, loaded in special afterloading nylon catheters, are used to improve dose distribution in the tumor volume and minimize dose to the adjacent critical organs and normal tissues. Seed spacing in strands is adjusted to maintain a dose rate of 40-60 cGy per hour at a distance of 5 mm from the plane of the implant. Implants custom loaded with ~ZSlsources achieve superior isodose distribution compared to implants loaded with standard zgZlr seed strands. High activity 'ZSl seeds also offer the advantage of reduced exposure to radiation oncology staff, nurses, and visitors leading to better patient care. Due to reduced exposure to personnel, the accuracy of the actual implant geometry can be verified by taking a localization film with actual 'zsI sources placed in the tumor bed. Key Words: iodine-125seed, Interstitial brachytherapy implants, Temporary implants.
INTRODUCTION
while the lumpectomy incision is still open. To standardize the implant procedure and to achieve excellent geometry between afterloading nylon catheters, a template shown in Fig. 1 is used to guide the needles parallel to each other in the tumor volume. With the aid of the lucite template, an inter-plane and intercatheter distance of 1.2 cm is achieved. This idealized
For most temporary interstitial brachytherapy implants, 192It has been used as the radioisotope of choice. ~ )92Ir implants require radiation isolation of the patient by placing portable bedside shields around the patient to reduce exposure to personnel and visitors. If an implant requires differentially loaded 192Ir seeds catheters, it would be difficult to prepare them in-house due to safety reasons. Such sources have to be ordered in advance from the manufacturer. Low activity ~251seed sources have been primarily used for permanent implants of the prostate 2 and brain. 3 Because of the low energy (27.4-35.5 keV) of the photons emitted by ~251radioisotope, the photons are easily attenuated in tissue (HVL = 2 cm) and require minimum external shielding. Due to the radiation safety advantage, a few radiation oncology centers have initiated the use of high activity '-'~I sources as a substitute for '92Ir in removable implants. 4'5 In addition to reducing radiation exposure to personnel and visitors, ~25Iimplant has many other advantages over a typical '921r implant. '251 sources can be differentially loaded in catheters in-house to optimize dose distribution in the tumor volume. Whereas the dose distribution in the t u m o r volume is similar for ~92Irand ~25Iimplants, the dose to normal tissue and surrounding critical structures is much reduced in '2sI implants. Even though the high activity ~25I sources are expensive, the cost per procedure can be reduced by reusing the seeds for several weeks.
Fig. 1. A template made of lucite to guide needles in the breast tissue. An interneedle distance in a plane and the interplane distance of 1.2 cm is achieved using this template.
M E T H O D S AND M A T E R I A L S For breast implants, 16-gauge needles are inserted in the tumor volume at the time of surgery 217
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Volume 17, Number 4, 1992
Fig. 2, A stainless steel jig used to check the loading of t251seeds in ribbons. It is covered with a leaded glass shield (5 HVL) for safe viewing.
source distribution is determined from computerized treatment plans. The afterloading nylon catheters used for t25I implants have a narrow leader on one end and a halfmoon and a flat button at the other closed end. The leader is inserted in the stainless steel needle and the needle along with the leader is withdrawn from the breast tissue leaving the nylon catheter in the volume to be implanted. The catheters contain an inner support tube to prevent damage to the catheter during this pulling process. After surgery the patient is brought to the simulation room to localize the seeds in the t u m o r volume. The radiation oncologist places a radio-opaque marker at the center of the implant volume during surgery. Ribbons containing d u m m y 125I seeds with interseed spacing identical to ribbons containing active seeds are inserted in the nylon catheters after the
inner support tube is removed by cutting the outer catheter. A film perpendicular to the plane of implant is obtained to assess the location of the ribbons in the afterloading nylon catheters. Using this film as a guide, the exact location of ~25I ribbon with its active length centered over the t u m o r volume, is determined. After adjustments are made, another film is obtained to verify the accuracy of this procedure. The d u m m y seed ribbons are then marked where they protrude from the afterloading catheters. An orthogohal pair of films is obtained for calculating isodose distributions in the implant volume. Approximately 130 t25I seeds are purchased every 12 weeks with an initial activity of 5 mCi per seed. They are reused till the activity decays below 2 mCi when they are returned to the manufacturer. Initially the seeds have a spacing of 1.2 cm with a resulting dose rate of 65 cGy/hr at a distance of 0.5 cm from
Advantages of using high activity ~251seeds • S. N. RUSTGI and S. S. HAHN
the plane of the implant. When the dose rate falls below 45 c G y / h r after about 3-4 weeks, spacing between ~25Iseeds is adjusted to maintain an initial dose rate of 65 cGy/hr. Loose ~zsI seeds are loaded in hollow tubes in the brachytherapy room in a shielded seed loader with a funnel attached to the open end of the tube. The desired interseed spacing is achieved by placing plastic spacers of proper lengths between active seeds. When the desired active length has been achieved, loose seeds and spacers are held in place by a pusher plastic filament. The end of the active ribbon is sealed securely with glue. The accuracy of loading can be checked with the help of a shielded jig shown in Fig. 2 which has grooves to hold the ribbons and a scale to measure distances. The jig is covered with a 5 HVL-thick leaded glass shield for viewing seeds without any significant exposure to the viewer. After an acceptable treatment plan displaying isodose distribution is obtained, the d u m m y seed ribbons are placed adjacent to customized active seed ribbons, and the marks made during the simulation process are transferred accurately. The jig containing the active ribbons is brought to the simulation room for loading. To minimize exposure during loading, the breast is covered with a leaded rubber shield. Use is also made of leaded gloves when handling ribbons with active seeds. A film representing the perpendicular view of the actual implant is obtained and compared against the final simulation film with d u m m y seeds to verify the accuracy of implant loading. Then active ribbons are fixed in outer catheters by crimping the metallic buttons. 1
(a) Fig. 3a. Isodose distribution of a breast implant using 5 mCi t251seeds with an interseed and intercatheter spacing of 1.2 cm. The two planes of this implant are separated by 1.2 cm.
219 L_
(b) Fig. 3b. lsodose distribution of the breast implant in Fig. 3a using 1.04 mCi ~921rseeds.
R E S U L T S AND D I S C U S S I O N One of the several advantages of using ~25Iseeds as a substitute for 192Irseeds for temporary implants is the reduction in radiation exposure to radiation oncology personnel, patient care nurses, and visitors. Low energy 125I photons (27-35 keV) have an HVL of 0.025 mm in lead and a 0.2 mm thick lead shield reduces the exposure by approximately 99%. Flexible lead rubber sheets can be incorporated in the dressing or placed over the implanted area to provide localized shielding obviating the need for shielded rooms and bedside shields. Nursing staff can deliver better patient care without any time constraints and visitation by adult family members is unrestricted. The exposure rate at 1 meter from a typical t25I breast implant containing approximately 200 mCi activity and shielded locally by lead rubber shield is less than 1 mR per hour. The surgical procedure for implanting afterloading catheters in breast for removable ~25Iseeds is very similar to the widely used 192Ir implants. The procedures for loading active seed ribbons in the nylon catheters and the technique of fixing them in the tumor volume is also very similar in the two approaches. Figure 3 displays transverse views of isodose distribution of two identical implants loaded with ~25I and ~92Ir seeds of activity 5 mCi and 1.04 mCi per seed, respectively. The two implants have the same interseed and intercatheter spacing within a plane and the same interplane distance. Within the tumor volume, which is usually enclosed by the 60 to 40 cGy per hr dose rate line, the two isodose distribu-
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tions are indistinguishable, because at short distances the inverse square law predominates. Outside the tum o r volume, J92Ir implant imparts a higher dose to the healthy tissue resulting in higher integral dose. This higher dose outside the t u m o r v o l u m e in t92Ir implants is produced by reduced tissue attenuation o f ~9-'Ir photons. E m p l o y m e n t o f a template to guide afterloading catheters through the t u m o r v o l u m e produces fairly idealized implant configuration with ~zsI ribbons running parallel to each other in each plane o f the implant. A distance o f about 1.2 c m between needles in a plane and between planes is achieved, which minimizes the use o f differentially loaded ribbons to reduce high and low dose regions in the t u m o r volume. In some cases, where c u s t o m loaded ribbons are required due to uneven spacing between catheters, they can be easily and safely assembled in the brachytherapy room. Such c u s t o m loading o f tgZlr seed is not possible on-site due to radiation safety constraints. Customized 192Ir catheters can be ordered only in advance from the m a n u f a c t u r e r and c a n n o t be modified in-house. Implants requiring differentially loaded 1251 ribbons are preplanned for optimization, easily assembled in the brachytherapy r o o m , and loaded in the patient room. Although high activity '25I seeds are expensive, the cost per procedure can be c o m p a r a b l e to t92Ir im-
Volume 17, Number 4, 1992 plants if a sufficient n u m b e r o f implants are carried out in a given time period (approximately 10 per m o n t h or more) as the ~25I seeds are used over a 12week period. Use o f t25I seeds for removable implants offers safer and m o r e optimal t r e a t m e n t c o m p a r e d to the standard 192Ir implants.
Acknowledgements--The authors wish to thank Heidi Ewing for
her help in the preparation of this manuscript.
REFERENCES I. Zwicher, R.D.; Schmidt-Ulrich, R.; Schiller, B. Planning of Ir-192 seed implants for boost irradiation to the breast. Int. J. Radiat. Oncol. Biol. Ph)'s, 11:2163-2170: 1985. 2. Hilaris, B.S.:Whitmore, W.F.; Batata, M.A.; Barzell, W. Behavioral patterns of prostate adenocarcinoma following an I- 125 implant and pelvic node dissection. Int. J. Radiat. Oncol. Biol. Phys. 2:631-637: 1977. 3. Gutin, P.H.: Phillips, T.L.; Hosobuchi, Y.; Wara, W.M.; Mackay, A.R.: Weaver, K.A.; Lamb, S.: Hurst, S. Permanent and removable implants for the brachytherapy of brain tumors. Int. J. Radiat. Oncol. Biol. Ph)w. 7:1371-1381 : 1981. 4. Clarke, D.H.: Edmundson, G.K.; Martinez, A.; Matter, R.C.; Warmelink, M.S. The utilization of !-I 25 seeds as a substitute for Ir-192 seeds in temporary interstitial implants: An overview and a description of the William Beaumont Hospital Technique. Intl..I. Radiat. Oncol. Biol. Plow. 15:1027-1033; 1988. 5. Gotfinet, D.R.; Ling, C.; Mariscal, M.: Phillips, T.L. Removable iodine-125 breast implants: Preliminary report. Endocurietherapy Hyperthermia Oncolog)' 3:121- 125; 1987.