British Journal of Radiology 75 (2002),234-237 © 2002 The British Institute of Radiology
The influence of applicator angle on dosimetry in vaginal vault brachytherapy
P J Hoskin, MD, FRCP, FRCR
P Bownes, MSc
and
A Summers, BSc
Mount Vernon Centre for Cancer Treatment, Mount Vernon Hospital, Rickmansworth Road, Northwood, Middlesex HA6 2RN, UK
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Abstract
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In vaginal vault brachytherapy, the critical normal tissues are bladder and rectum; doses to these tissues may be affected by the position of a single line applicator placed in the vagina. Dosimetry with the applicator lying at its "natural" angle in the vagina with the patient in the lithotomy position has been compared with the applicator held horizontal as defined by a spirit level in 30 consecutive patients. A mean change in angle of 19.7° was found. This resulted in a mean decrease in ICRU (International Commission of Radiation Units and Measurements) rectal point dose when the applicator is horizontal of 12.9%, equivalent to a mean absolute dose reduction of 1.3 Gy for a prescription dose of 5.5 Gy at 5 mm depth. An increase in mean dose to the ICRU bladder point when the applicator is horizontal of 13.3%, equivalent to an absolute mean dose increase of 0.5 Gy per fraction for the same prescription dose, was also found. On the basis of these findings, it is recommended that vaginal vault brachytherapy is performed with a single line source held in the "corrected" horizontal position to reduce bowel dose as this is the most sensitive critical normal tissue.
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Introduction
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Vaginal vault brachytherapy remains an important component of post-operative treatment in patients presenting with early stage endometrial cancer. It is generally well tolerated and major complications are few, but moderate to severe late bowel and bladder problems are reported in approximately 5% of patients following this treatment [1–6]. There are a number of variations in technique, and little attention has been paid to the influence of these variables on treatment outcome. Although some centres may prefer to use vaginal ovoids or individualized vaginal moulds, the most common technique uses a single line source vaginal cylinder applicator. Once placed in the vagina, the position of the cylinder in relation to the couch and pelvic structures can vary enormously between individual treatments. We have sought to document this variability, to assess its impact on dosimetry and to develop standardized criteria for the delivery of vaginal vault brachytherapy.
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Patients and method
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The routine technique at Mount Vernon Hospital uses high dose rate afterloading with a vaginal cylinder (Gammamed; MDS Nordion, Haan, Germany) placed in position within the vagina with the patient in the lithotomy position; the cylinder is then secured with a clamp attached to the side rail of the couch. This is effectively a single line source for brachytherapy dosimetry using 5 mm stepping dwell positions. Until this study, it was standard practice to secure the applicator in the natural position that it adopted within the vagina.
Standard prescriptions are delivered at 5 mm depth from the surface of the vaginal applicator using 5 mm dwell positions with a 250% weighting of the first dwell position to achieve adequate dose to the surface at the vault. Depending upon the external beam component, doses of 4 Gy or 5.5 Gy at 5 mm depth are delivered. Treatment length is designed to treat the upper third of the vagina, the typical length being 5 cm. For the first treatment of any course, standard anteroposterior and lateral radiographs are taken with an indwelling urinary catheter (with the balloon containing 7 ml of contrast material) and rectal barium.
In this study the patient was set up for treatment in the lithotomy position with the vaginal applicator lying at its natural angle and standard verification radiographs, as described above, were taken. The vaginal applicator was then adjusted to lie horizontal to the couch in all patients. This was defined using a spirit level along the line of the rigid applicator source tube. Repeat lateral radiographs were then taken.
The two lateral radiographs from each patient were then analysed to determine the effect of the change in applicator angle on dosimetry. Isodose distributions are plotted using the Abacus software program and they are superimposed on the film after correction for magnification. The angle between the line applicator and the femur was measured on each film, as shown in Figure 1
, together with the doses at the ICRU (International Commission on Radiation Units and Measurements) bladder and rectal points as well as the dose at the nearest barium-containing bowel to the source, as shown in Figure 1.
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Figure 1. Measurement and dosimetry points with the vaginal cylinder (a) at the natural angle and (b) at the corrected horizontal position. A, angle of applicator; B, bladder dosimetry point; C, rectal dosimetry point; D, additional bowel dosimetry point at closest measured barium-containing bowel position to source.
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Differences between the natural dwell angle and the corrected horizontal position were tabulated and compared using a paired t-test and two-tailed p-value.
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Results
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A series of 30 consecutive patients was studied, from which 28 pairs of films could be analysed, the remaining 2 patients having poorly defined rectal images preventing accurate dosimetry. The details of the treatment are given in Table 1.
The results of the differences in angle and dosimetry are shown in Figure 2 and Table 2.
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Figure 2. Effect of vaginal applicator angle on isodose distribution (a) at the natural angle and (b) at the corrected horizontal position.
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Table 2. Differences in angle and dosimetry, subtracting corrected horizontal position measurements from "natural" applicator position measurements
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Statistically significant differences between the two applicator positions were seen with respect to the angle of the source tube, ICRU bladder and rectal doses, and dose to nearest region of bowel opacified with barium.
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Discussion
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The results of this study confirmed that there is considerable variation in the natural angle at which a single line source will be positioned. This was defined relative to the line of the femur with the patient in the lithotomy position in an attempt to relate the position to a reproducable anatomical structure. When the angle between the source tube and a line estimated to represent the couch top was measured, the difference between the two angles was found to be the same, ±3°, as when the angle was measured to the femoral shaft. In general, the "natural" angle points posteriorly, with the effect that the first (distal) dwell position is moved closer to the rectum and rectosigmoid bowel. This is partially balanced by the source position moving away from the bladder as defined by the ICRU bladder point. The magnitude of this effect is demonstrated by the change in mean dose when the source position is corrected to the horizontal. The average reduction in rectal dose is 1.3 Gy for a prescribed dose of 5.5 Gy at 5 mm from the applicator surface, compared with an increase in bladder dose of 0.5 Gy.
The ICRU bladder dose is often a poor reflection of absolute bladder dose [7], and CT or ultrasound imaging, which were not carried out in this study, would be required to define this dose more closely. However, this limitation should not detract from the overall conclusions of this study. It would be expected that volume estimates would follow the trend shown here but would give greater accuracy of absolute dose effects.
As has been noted previously, the results of gynaecological brachytherapy using vaginal vault treatment demonstrate considerably greater bowel toxicity than bladder toxicity, reflecting the lower radiation tolerance of bowel compared with bladder [8]. The reduction in rectal dose by this simple adjustment to applicator position may therefore be quite significant in patients who are at risk of bowel damage. On average, a patient receiving two fractions of vaginal vault brachytherapy would be spared a maximum dose of 2.6 Gy. Delivered at high dose rate this would be equivalent to at least one extra external beam fraction to that region of bowel, a particularly important consideration in patients receiving combined external beam and intravaginal brachytherapy. One study has reported that the incidence of late bowel damage increased from 2.5% with external beam treatment alone to 18% when vaginal vault brachytherapy was added to the treatment schedule [5]. This may be expected, since the addition of vault brachytherapy to an external beam course of treatment delivering 40–50 Gy will take the adjacent bowel very close, and clearly at times well beyond, normal tissue tolerance. A difference of 2.6 Gy at this level of tolerance on a steep dose–response curve could therefore be critical in reducing late morbidity. This is illustrated if the effect of delivering 4.2 Gy instead of the prescription isodose of 5.5 Gy with each brachytherapy fraction after 40 Gy is calculated in terms of biologically effective dose (BED) using an 
/ß ratio of 3.5 for rectal tolerance. This results in a reduction in dose from 91.1 Gy3.5 to 81.3 Gy3.5. In contrast, hyperfractionation schedules have demonstrated that bladder tolerance is greater than 80 Gy when delivering 1 Gy fractions three times daily [9] and therefore an addition of 0.5 Gy to bladder dose at this level is likely to be inconsequential for bladder morbidity.
In conclusion this study has demonstrated that when using a single line source vaginal cylinder for high dose rate afterloading vaginal vault brachytherapy, correction of the position of the vaginal cylinder to the horizontal position defined by a spirit level will result in a significant reduction in both maximum bowel dose and ICRU rectal dose, with only a modest change in bladder dose. Given the limiting tolerances of these tissues, it is recommended that a corrected horizontal position is adopted as standard practice for this treatment.
Received for publication April 24, 2001.
Revision received August 14, 2001.
Accepted for publication August 28, 2001.
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References
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Abstract
Introduction
