Dose Variation Due to Differences in Applicator Placement Used for Intracavitary Brachytherapy of Cervical Cancer

Medical Dosimetry, Vol. 23, No. 1, pp. 57– 63, 1998 Copyright © 1998 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/98 $19.00 1 .00

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DOSE VARIATION DUE TO DIFFERENCES IN APPLICATOR PLACEMENT USED FOR INTRACAVITARY BRACHYTHERAPY OF CERVICAL CANCER RICHARD R. RUTTEN, B.S.R.T.,(T), ANN A. LAWYER, M.S., and PAULA BERNER, B.S. The University of Texas M.D. Anderson Cancer Center, Department of Radiation Physics, 1515 Holcombe Boulevard, Houston, TX 77030 Abstract—The prognosis for women with early stage cancers of the cervix is excellent. The cervix and proximal vagina are relatively radioresistant structures; dose limiting critical structures for radiation treatment in this area are tissues such as the bladder and rectum. The high dose rate gradient of brachytherapy allows high doses to the tumor volume, while doses to the neighboring structures are lower. Intracavitary treatment of cervical cancer is performed by insertion of tandem and ovoid applicators. Correct tandem and ovoid placement is verified and documented with orthogonal radiographs. If placement of the tandem and ovoid is not acceptable, the packing is removed, and the tandem or ovoid is repositioned or exchanged as necessary. To examine the difference in dose to specific reference points when tandem or ovoid placement was changed by the physician, three patients’ initial insertion radiographs were compared to those that were accepted to treat. Points of comparison selected for the patients on each of the insertions were: point A (L and R), external iliac nodes (L and R), bladder, and rectum. Each of the placement changes for the three patients resulted in differences in distance, and therefore, differences in dose. Changes as high as 823 cGy resulted from the adjustment of only one component. In the cases examined, repositioning improved dose distribution to the cervix, and lowered the dose to adjacent critical structures. © 1998 American Association of Medical Dosimetrists. Key Words: Intracavitary brachytherapy, Cervical cancer, Orthogonal film, Tandem, Ovoid.

specific reference points when a tandem or ovoid is repositioned in intracavitary brachytherapy for cervical cancer. Repositioning may correct doses that would otherwise compromise tumor control or dose-limiting critical structures and result in an unacceptable treatment outcome. Specific methods for locating dose points are in accordance with International Commission on Radiation Units and Measurements Report 38 (ICRU 38).7

INTRODUCTION The prognosis for women with early-stage cancers of the cervix is excellent. The Papanicolaou smear has made early detection of neoplastic activity possible. Most of these patients can be treated with curative intent.1 The mean age of diagnosis is 52 years, but cervical cancers may occur at a wide range of ages.2 The cervix and proximal vagina are not radiosensitive structures. Total doses of more than 120 Gy are tolerated without significant morbidity, and radiation therapy can be used effectively in managing cervical cancers; however, dose limits for cancer treatment in this area are imposed by the sensitivity to ionizing radiation of nearby structures such as the bladder and rectum.3,4 Doses of 80 Gy or higher are required for control of cervical cancer, whereas the rectum and bladder will tolerate only 60 – 80 Gy.5,6 The rapid dose-rate gradients achievable with brachytherapy allow effective placement of high-dose areas on the tumor volume, while doses to the neighboring critical structures are lower; however, if the sources are not carefully placed, this significant dose gradient will also increase the possibility of underdosing the tumor or tumor margins. Effective use of brachytherapy requires the accurate positioning of radioactive sources and precise calculation of dose. This paper will examine the difference in dose to

METHODS AND MATERIALS Insertion procedure Intracavitary treatment of cervical cancer is performed with insertion of a tandem and ovoids system. Low-dose rate insertions (40 –200 cGy/h)7 are typically prescribed for either 48 or 72 mg radium hours. Actual time of treatment is adjusted for the isotope being used, i.e., 137Cs. To begin the procedure for intracavitary insertion of a tandem and ovoids the patient may be placed under general anesthesia or receive a spinal block. Following a rectovaginal bimanual examination, the curvature of the tandem and ovoid size are selected by the physician. After the external os is identified, two radio-opaque seeds are inserted in the cervical os, one anteriorly and one posteriorly. Superior extension of the tandem into the uterus is determined by a uterine sound. Once that distance is determined, a lockable flange is placed on the tandem at the sounded distance. The tandem is inserted through the cervical os and into the uterus. The flange

Reprint requests to: Richard R. Rutten, The University of Texas M.D. Anderson Cancer Center, Department of Radiation Physics, 1515 Holcombe Boulevard, Box 701, Houston, TX 77030. 57

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will abut the external os, avoiding further superior movement of the tandem and preventing uterine perforation. On radiograph, the flange will indicate whether the tandem has been displaced inferiorly after placement. Ovoids are positioned within the vaginal vault in the lateral cervical fornices. An optimum fit may be achieved by using ovoid angulation of 15 or 30 degrees. Caps of different sizes may be placed on the ovoid to increase length and diameter. The largest ovoid allowed by the anatomy should be used to increase distance and minimize dose to the vaginal mucosa. Incorrectly sized ovoids will not fit properly within the fornix. If the vault is too narrow for ovoids, a tandem and cylinder may be used. Once placed, the tandem and ovoids are immobilized by the physician with gauze packing, which prevents the system from changing position during the insertion. Packing also aids in anterior and posterior distention of the vaginal mucosa and keeps the tandem and ovoids in contact with the cervix. Correct tandem and ovoid placement is verified and documented with orthogonal radiographs. On the anterior radiograph, the tandem should be approximately at the midpoint of the pelvic brim. This will be determined largely by the patient’s anatomy. On the lateral radiograph, the tandem is seen passing through the midpoint of the ovoids. If visible, the flange is at the external os, as indicated by the anterior and posterior marker seeds.8,9 If placement of the tandem and ovoid is not acceptable, the packing is removed, the tandem or ovoid is repositioned or modified as necessary, repacked, and orthogonal radiographs are taken again. The process is repeated until satisfactory positioning is achieved. Source loading Loading of radioactive sources in the tandem and ovoids is expressed as milligrams radium equivalent (mgRaeq). Sources in the tandem are listed first followed by loading for each ovoid. In this study, for example, patient 1 was loaded 10 1 15 1 10/15, 15. This indicates that the tandem was loaded superiorly to inferiorly with mgRaeq activities of 10, 15, 10, and that each of the ovoids received a 15 mgRaeq source. For the cases examined these activities were selected by the physician to deliver 3150 cGy to a specific volume. An example of this volume is shown in Fig. 1. Comparison Radiographs for three patients were selected for study. Radiographs of initial tandem and ovoid insertions chosen for repositioning by the physician were compared to those that were accepted for treatment. Dosimetry was performed in the same manner for the initial placement films as for treatment films. Points of comparison selected for the patients on each of the insertions were: point A left and right, external iliac nodes left and right, bladder, and rectum. Location of the bladder, rectal, and

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Fig. 1. Anterior view of the 3150 cGy dose volume represented by the 68 cGy/h line. Reference points are: point A right (a rt), point A left (a lt), bladder, rectum, external iliac node right (rt ext), and external iliac node left (lt ext). Other points located are the symphis pubis (sp) and the junction of S1/S2 (sacrum). The internal os and the mid-point of the pelvic brim plane are nearly co-located in the center of the volume.

external iliac points was in accordance with the definitions of ICRU 38,7 as follows: Bladder. On the lateral radiograph, the reference point is obtained on an anterioposterior line drawn through the center of the (bladder) balloon. The reference point is taken on this line at the posterior surface of the balloon. On the frontal radiograph, the reference point is taken at the center of the balloon. Rectum. On the lateral radiograph, an anteroposterior line is drawn from the lower end of the intrauterine source (or from the middle of the intravaginal sources). The point is located on this line 5 mm behind the posterior vaginal wall. On the AP radiograph, this reference point is at the lower end of the intrauterine source or at the middle of the vaginal source(s). External iliac nodes. A line is drawn from the junction of S1-S2 to the top of the (pubic) symphysis. A point 6 cm lateral to midline is used to give an estimate of the dose rate to mid-external iliac lymph nodes (Fig. 2). Point A is defined as 2 cm lateral of the internal os on a plane perpendicular to a line along the longitudinal axis of the endocervical canal.6 Point A (L and R), external iliac nodes (L and R), and bladder are selected to represent points of reference that will not move as the positions of the tandem and ovoids change. The rectum is selected because of the critical nature of dose to this point when using intracavitary brachytherapy. RESULTS Patient 1 demonstrated an inferior movement of the tandem (Figs. 3– 6). This is seen in the position of the flange on the tandem relative to the seeds inserted

Intracavitary brachytherapy for cervical cancer ● R. R. RUTTEN et al.

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Fig. 2. Anterior view for location of right and left external iliac points.

in the external os. As a result, the sources in the tandem were closer to the sources in the ovoids. This movement reduces the length of tandem available for loading without having a protruding source between the ovoids. Dose changes from repositioning of the system can

Fig. 4. Patient 1, lateral view of initial insertion.

Fig. 3. Patient 1, anterior view of initial insertion. A 5 point A, N 5 external iliac nodes, B 5 bladder, R 5 rectum.

Fig. 5. Patient 1, anterior view of repositioned placement.

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Fig. 6. Patient 1, lateral view of repositioned placement.

be seen in Table 1. Significantly, the rectal dose was decreased 823 cGy by superior movement of the tandem sources from the ovoids. Points A right and left also decreased by 428 cGy and 253 cGy, respectively. This resulted from movement of the 15 mgRaeq source in the tandem. The bladder point showed a minor change, as did both nodal points. In patient 2, the ovoids moved from the fornices (Figs. 7–10). Movement occurred as a result of constriction of the fornices, or during tandem and ovoid placement and packing. The gauze packing, which contains radio-opaque material, is seen on the radiographs as the irregular line around the ovoids. As the ovoids were displaced, their distance relative to the bladder decreased. When repositioned, the bladder

Table 1. Dose changes (cGy) from repositioning tandem and ovoids in patient 1 Point

Initial placement

Reposition

Change

% of Change

A right A left Bladder Rectum External Iliac right External Iliac left

2817 2196 791 1674 454 383

2389 1943 840 851 478 408

2428 2253 49 2823 24 25

218 213 6 297 5 6

Loading: 10 1 15 1 10/15, 15.

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Fig. 7. Patient 2, anterior view of initial insertion.

dose decreased 475 cGy (Table 2). Ovoid movement has also caused the tandem to move inferiorly. The rectal dose increased by 434 cGy when the tandem was repositioned because of a closer location to the tandem sources. Both points A demonstrated a decrease of 389 cGy and 670 cGy when repositioned. This is because the most superior source in the tandem had the highest activity (15 vs. 10 mgRaeq). Repositioning of the tandem to the sounded depth increased the distance of the 15 mgRaeq source from points A. The repositioning also caused the left nodal point to decrease by 124 cGy, while the right nodal point remained unchanged. In patient 3, both tandem and ovoid had moved (Figs. 11–14). In addition, the tandem did not bisect the ovoids on the lateral radiograph. Much of the movement was caused by the ovoids’ being too large for the fornices. During repositioning, the ovoids were changed to a smaller size (medium to short-small), which allowed a proper fit. The tandem moved superiorly and better bisected the ovoids. As a result, the dose to the bladder decreased by 770 cGy (Table 3). The right external iliac point decreased 96 cGy because of better tandem alignment. The smaller changes in points A were caused by replacement of the 15 mgRaeq source at the superior aspect of the tandem with the contribution of other sources in the tandem, and by the closer proximity of the ovoids.

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Fig. 10. Patient 2, lateral view of repositioned placement. Fig. 8. Patient 2, lateral view of initial insertion.

CONCLUSION Each placement change resulted in differences in distance, and therefore, differences in dose. Changes as high as 823 cGy resulted from the adjustment of only one component. Less than optimum positioning may be caused by patient anatomy, selection of improper components for the insertion, or by movement of the tandem and ovoids during packing. Repositioning allows better dose distribution to the cervix and for better dose control to critical structures. Table 2. Dose changes (cGy) from repositioning tandem and ovoids in patient 2

Fig. 9. Patient 2, anterior view of repositioned placement.

Point

Initial placement

Reposition

Change

% of Change

A right A left Bladder Rectum External Iliac right External Iliac left

2016 2162 2260 1174 264 382

1627 1492 1785 1608 265 258

2389 2670 2475 434 1 2124

224 245 227 27 0 248

Loading: 15 1 10 1 10/10, 10.

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Fig. 11. Patient 3, anterior view of initial insertion. Fig. 13. Patient 3, anterior view of repositioned placement.

Fig. 12. Patient 3, lateral view of initial insertion.

Fig. 14. Patient 3, lateral view of repositioned placement.

Intracavitary brachytherapy for cervical cancer ● R. R. RUTTEN et al.

Table 3. Dose changes (cGy) from repositioning tandem and ovoids in patient 3 (cGy) Point

Initial placement

Reposition

Change

% of Change

A right A left Bladder Rectum External Iliac right External Iliac left

2510 2404 1743 1049 569 393

2639 2707 973 1125 473 467

129 303 2770 76 296 74

5 11 279 7 220 16

Loading: 15 1 10 1 10/10, 10.

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3. Eifel, P.J. Carcinoma of the cervix. Lecture to American Society of Therapeutic Radiation Oncologists. Los Angeles, CA; October 1996. 4. Crook, J.; Esche, B.A. The uterine cervix. In: Cox, J.D., editor. Moss’ radiation oncology. 7th ed. St. Louis, MO: Mosby-Year Book Inc; 1994:617,644 – 646. 5. Emani, B.; Lymann, J.; Brown, A.; et al. Tolerance of normal tissue to therapeutic irradiation. Int. J. Radiat. Oncol. Biol. Phys. 21:109 –122; 1991. 6. Crook, J.M.; Esche, B.A.; Chaplain, G.; et al. Dose volume analysis and the prevention of radiation sequelae in cervical cancer. Radiother. Oncol. 8:321–332; 1987. 7. International Commission on Radiation Units and Measurements. Report 38. Dose and volume specification for reporting intracavitary therapy in gynecology. Bethesda, MD: ICRU; 1985. 8. Delclos, L.; Fletcher, G.H. Gynecologic cancers. In: Levitt, S.H.; Kahn, F.M.; Potish, R.A., editors. Technological basis of radiation therapy, practical applications. 2nd ed. Malvern, PA: Lea & Febiger; 1992:272–273. 9. Fletcher, G.H. Squamous cell carcinoma of the uterine cervix. Textbook of radiotherapy. 3rd ed. London: Lea & Febiger; 1980: 742–746.