Brachytherapy 6 (2007) 49e52

Dosimetry of anal radiation in high-dose-rate brachytherapyfor prostate cancer

Karl Mikael K€alkner1,*, Emil Bengtsson2, Simone Eriksson2, Carina Holmberg1,Sten Nilsson1, Seymour Levitt1,3, Marie Lundell2

1Department of Oncology, Radiumhemmet, Karolinska University Hospital, Stockholm, Sweden2Department of Medical Physics, Radiumhemmet, Karolinska University Hospital, Stockholm, Sweden

3Department of Therapeutic Radiology, University of Minnesota, Minneapolis, MN

ABSTRACT PURPOSE: The objective of this study is to determine the radiation dose to the anus during bra-chytherapy using high-dose-rate Ir-192 sources.METHODS AND MATERIALS: Thermoluminescence dosimeters were used for measuring thedose to the distal part of the anus in 10 patients, and in a prostate phantom to measure the radiationdose during the transport of the radiation source.RESULTS: The measured dose to the anus in vivo was on average 0.85 Gy (range, 0.48e1.37 Gy) per treat-ment. The transport dose using 15 and 19 needles in the prostate phantom was 0.07 and 0.08 Gy, respectively.CONCLUSIONS: The dose delivered to the anus using high-dose-rate brachytherapy with Ir-192sources is quite low. There is a contribution to the anal radiation dose during the transport of the Ir-192 source into the needles. However, in clinical practice when using 15e20 needles, the dose fromtransporting the Ir-192 source can be ignored. � 2007 American Brachytherapy Society. All rightsreserved.

Keywords: Prostate cancer; Dosimetry; High-dose-rate brachytherapy; Thermoluminescence dosimeters

Introduction

The incidence of prostate cancer is increasing in Sweden(1) and was 178 per 100,000 males in 2002. Radiotherapy andsurgery are the two options for curative treatment and thetechniques differ in their side effects. The development ofproctitis is of particular concern after radiotherapy. A strongrisk factor for this development is the rectal volume irradiated(2, 3). Recently there has been a focus on the symptoms thatmight be due to disturbed function of the anus, i.e., fecal leak-age and incontinence. The absorbed dose to the anal regionhas been considered a prognostic factor (4). Brachytherapyper se has the possibility of minimizing the radiation doseto normal tissue outside the target. The use of a perineal ap-proach with transrectal ultrasound guided implants into theprostate has made it possible to treat with either low-dose-rate or high-dose-rate (HDR) brachytherapy. Our study

Received 30 June 2006; received in revised form 21 August 2006;

accepted 24 August 2006.

* Corresponding author. Department of Oncology, Radiumhemmet,

Karolinska University Hospital, SE-171 76 Stockholm, Sweden. Tel.:

þ46-8-517-79927.

E-mail address: karl.kalkner@karolinska.se (K.M. K€alkner).

1538-4721/07/$ e see front matter � 2007 American Brachytherapy Society. A

doi:10.1016/j.brachy.2006.08.008

addresses the question of absorbed dose to the anus and thedose contribution from source transportation into the needlesat HDR brachytherapy.

Methods and materials

Radiotherapy

At the Radiumhemmet, we use HDR brachytherapy asa boost after external beam radiotherapy (EBRT). The EBRTconsists of 50 Gy delivered to the planning target volume(PTV), at 2 Gy per fraction, 5 days per week. The techniquehas been described by Borghede et al. (5). The HDR dose isdelivered to the PTV in two 10 Gy fractions with an interven-ing 2-week interval. The HDR brachytherapy dose planningis based on transrectal ultrasound images. The images aretaken at 5-mm intervals covering the whole prostate gland.In every image, the gland is delineated and the PTVis definedas the gland plus a 3-mm margin. The seminal vesicles are notincluded in the target. The dose per fraction to the rectum isnot allowed to exceed 6 Gy. The rectum is defined as the vol-ume containing the anterior part of the probe with a chord setat 15 mm. The brachytherapy is performed under spinal

ll rights reserved.

50 K.M. K€alkner et al. / Brachytherapy 6 (2007) 49e52

anesthesia and usually takes a total of 2 h. The dose planningsystem used is the BrachyVision, version 6.1 (Varian MedicalSystems, Inc., Palo Alto, CA). Usually, 18e20 needles areused to obtain a dose distribution as homogenous as possible.For the treatment, a VariSource HDR afterloader (VarianMedical Systems, Inc.) is used. Measurements with thermo-luminescence dosimeters (TLDs) were performed at the analverge using two TLDs (LiF chips, 3� 3� 1 mm) each time.The TLDs were individually calibrated and correction forbackground irradiation was performed. The TLDs wereplaced at the anal verge, see Figure 1. The distance fromthe anal verge to the base of the prostate was measured usingthe stepping unit used for the ultrasound probe (AccuSeed,CMS, Inc., St. Louis, MO). A comparison was performed be-tween the radiation dose measured with TLDs and the calcu-lated dose in the dose plan.

In vitro measurements using a prostate phantom

Nineteen and 15 needles were inserted in the phantom,and the dose at 12 mm from the lowest center needle wasmeasured with TLDs (Fig. 2). There was only one dwellposition in each needle, as close to the needle tip as possi-ble. The dwell time for this position was 0.1 s. The TLDswere put two by two in small plastic bags. The distance be-tween the centers of the TLDs was approximately 15 mm.The midline between the TLDs was aligned to be in thecenter of the needle setup. The source transportation sched-ule was set to run eight times to obtain better statistics.Since the dwell time was small, it was assumed that thedose delivered to the TLDs was the transportation dose.

Matrix

The Prostate

12 mm

TLDpositioned indistal anus

Needles

Ultrasound probe in rectum

Fig. 1. Principal sketch of the setup during dose measurements in a

patient. The distance between the central needle in the lowest row in the

matrix and the ultrasound probe is 12 mm. TLD 5 thermoluminescence

dosimeter.

The number of needles was chosen according to our clinicalexperiences as representative for an average treatment plan(19 needles) and a treatment plan using few needles (15needles). When reducing the number of needles, peripheralneedles were removed to simulate a clinical situation.

Patients

The average age of the 10 investigated patients was 64years (range, 54e80 years), with a PSA average of13.8 mmol/L (range, 5.6e36 mmol/L). Five patients werestage T2 and 3 patients stage T3, according to the TNMclassification. Two patients did not have their T stagedefined. Seven patients had a Gleason score of 6e7, 2patients had a Gleason score of 8, and 1 patient had aGleason score of 5.

Results

In vivo measurements

In the 10 patients, two TLDs were placed anteriorly atthe anal verge. The measured dose per treatment averaged0.85 Gy (range, 0.48e1.37 Gy). According to the pretreat-ment dose plan the anal dose should have averaged 0.79 Gy(range, 0.54e1.36 Gy). Pearson’s correlation coefficientbetween the measured and the planned dose was r 5 0.83( p!0.01). In three cases, the TLD dose was lower thanthe planned dose and in the remaining cases the TLD dosewas equal or higher than the planned one. On average, thedifference between the measured dose and the dose accord-ing to the dose plan was 14%.

Phantom measurements using several needles

The average dose per transport was 0.081 Gy using 19needles and 0.068 Gy when using 15 needles. The differ-ence in dose is statistically significant using Student’s t test

Matrix

12 mm

TLD

Needles

Ultrasound probe

Water-Phantom

Fig. 2. Principal sketch of the setup using the prostate phantom. The dis-

tance between the central needle in the lowest row in the matrix and the

ultrasound probe is 12 mm. TLD 5 thermoluminescence dosimeter.

51K.M. K€alkner et al. / Brachytherapy 6 (2007) 49e52

for unrelated pair ( p!0.01). The measured results arepresented in Table 1.

Discussion

Radiation proctitis, is characterized with symptoms suchas increased frequency and urgency of defecation, tenesmusand rectal bleeding, and is a common complication of radia-tion therapy in the curative treatment of prostate cancer. Thedevelopment of these symptoms is primarily related tothe dose and volume of rectum irradiated (2, 3, 6). In thecombined modality treatment using conformal external ra-diotherapy and a HDR brachytherapy boost, there is a possi-bility of reducing the rectal volume receiving high doses.There are only a few reports that address the problems of analleakage after radiotherapy. Yeoh et al. concluded that an in-creased sensitivity in the rectum rather than decreased analsphincter pressure (7) caused the anorectic symptoms includ-ing fecal leakage. However, a relation between a high mini-mum absorbed dose to the anal canal and the risk of fecalincontinence was reported by Vordermark et al. (4), but noother dose volume histogram parameter was found to be pre-dictive. al-Abany et al. have hypothesized that the irradiationdose to the anus is crucial to anal function (8).

In this study, it was found that the dose contribution tothe anus during HDR brachytherapy was 0.85 Gy (range,0.48e1.37 Gy) per treatment. Using the linear-quadraticformula and assuming that a/b is 3 Gy, the late reaction ef-fect is predicted to be equivalent to 2.39 Gy in 2 Gy frac-tions provided two fractions of 1.37 Gy HDR. This is lessthen 5% of the maximal dose of 50 Gy that might be deliv-ered by EBRT. Differences in the development of fecalleakage among individual patients can probably be ex-plained by different doses to the anus delivered by theEBRT, rather than explained by differences in the dosedelivered by HDR brachytherapy.

The measured anal dose from HDR brachytherapy wasfound to be quite low. To evaluate the fraction of this dosedue to the source being transported in the needle, we useda prostate phantom. In this phantom, the dose measuredwith TLDs calibrated for low doses revealed that verylow doses of irradiation were delivered during transfer. Atthe Radiumhemmet, we plan a dose distribution that is ashomogenous as possible. As a consequence, we more fre-quently use a maximum of 20 needles than other centers.In our study, we found no clinically significant differences

Table 1

The measured transport dose using TLDs in the prostate phantom

with different number of needles

Configuration

Dose to TLD/transport

[Gy]

Standard deviation

[Gy]

19 needles 0.081 0.0048

15 needles 0.068 0.0045

TLD 5 thermoluminescence dosimeter.

in the dose emitted during source transfer when 15e19 nee-dles were used in the prostate phantom. The dose deliveredduring transfer is in the range of 12% of the dose measuredat the anal verge.

The correlation between the actually measured dose inthe anus and the dose according to the dose plan was high(r 5 0.83). The average of the absolute difference betweenthe measured dose in the anus and that observed in the doseplan was 14%, this is actually higher than that recently re-ported where the difference was reported to be 6%, using aninvasive method (9). The difference in results can possiblybe explained by the uncertainty in measuring the distancefrom the base of prostate to the anal verge. The use ofa stepping device creates an error when measuring the dis-tance in range of 2.5 mm (a half step). A 2.5-mm increasein distance from the clinical target volume (CTV) is equiv-alent to a dose decrease of 10%. The small contributionfrom transferring the Ir-192 source might also interfere tosome extent. However, using TLD on the ultrasound probeprovides advantage as a simple noninvasive method.

Conclusion

HDR brachytherapy treatment of the prostate results inlow radiation dose to the anus. Even after adding the dosefrom transporting the radioactive source into and out of theneedles the anus dose is still very low and contributes onlywith 12% of the total dose to the anal verge from combinedHDR and EBRT. Therefore, to preserve anal function, by re-ducing the radiation dose delivered, it would seem to be moreeffective to decrease the dose to the anus from the EBRT.

Acknowledgments

This project was supported by grants from Cancerfore-ningen i Stockholm. We are indebted to Ulla Lundbladfor handling the TLDs.

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