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The effect of constipation on rectal dosimetry following prostate brachytherapy
Medical Dosimetry, Vol. 25, No. 4, pp. 237–241, 2000 Copyright © 2000 American Association of Medical Dosimetrists Printed in the USA. All rights reserved 0958-3947/00/$–see front matter
PII: S0958-3947(00)00047-9
THE EFFECT OF CONSTIPATION ON RECTAL DOSIMETRY FOLLOWING PROSTATE BRACHYTHERAPY GREGORY S. MERRICK, M.D., WAYNE M. BUTLER, PH.D., ANTHONY T. DORSEY, M.S., and J. THOMAS DORSEY III, M.D. Schiffler Oncology Center, Wheeling Hospital, Wheeling, WV and The George Washington University Medical Center, Division of Radiation Oncology & Biophysics, Washington, D.C. (Received 13 April 2000; Accepted 8 August 2000)
Abstract—The purpose of this study is to report the effect of dilatation of the anorectum on rectal dosimetry following an 125I prostate implant. Three months following prostate brachytherapy, 2 computed tomography (CT) scans of the prostate gland were obtained within 90 minutes of each other. The first CT scan revealed a dilated anorectum secondary to constipation. The second CT was obtained following the administration of an enema with a successfully evacuated rectum. Differences in radiation doses to the distended and empty rectum were computed via the mean dose, the maximum dose per slice, the distance from the base, and in terms of the surface of the anterior quadrant of the rectum receiving 100%, 125%, 150%, 175%, 200%, and 250% of the prescribed dose. The dose to the rectal wall was substantially increased in the distended state for all evaluated parameters. In general, the mean dose to the rectal wall was increased by a factor of 1.5 in the distended state. In both scenarios, the dose to the rectal wall peaked near midgland. In terms of 10° rectal wall sectors receiving a given percentage of the prescribed minimal peripheral dose, S%mPD, the S100, S125, S150, S175, S200, and S250 were substantially greater for the distended versus the empty rectum. The magnitude of the percentage difference in dose between the distended and evacuated rectum increased with dose level while the difference in the number of sectors receiving a given dose level was greatest at 125% and 150% of the prescribed dose. We recommend detailed postimplant attention to bowel habits for at least 2 half-lives of the implanted isotope to minimize rectal distention, decrease radiation dose to the anterior rectal wall, and subsequently minimize potential constipation related rectal toxicity. © 2000 American Association of Medical Dosimetrists. Key Words: Prostate, Brachytherapy, Dosimetry, Rectum, Constipation.
INTRODUCTION
iopathic constipation requires at least 2 of the following criteria to be present: less than 3 bowel movements per week, straining with defecation at least 25% of the time, lumpy and/or hard stools at least 25% the time, or the sensation of incomplete evacuation at least 25% of the time.13 Herein, we present detailed rectal dosimetry in a constipated patient with dilatation of the anorectum.
Over the past decade, transperineal ultrasoundguided permanent prostate brachytherapy with either iodine 125 (125I) or palladium 103 (103Pd) has been increasingly utilized as definitive management for earlystage carcinoma of the prostate gland. Multiple recent studies evaluating postoperative dosimetry have supported the ability of brachytherapists to adequately encompass the target volume.1–9 However, only 2 of these studies have evaluated the relationship between the radiation dose to the rectum and toxicity.4,9 Fortunately, significant rectal injury following prostate brachytherapy is relatively rare. An estimated 55 million American adults complain of constipation at least once every 3 months, and 4 million adults report chronic constipation. The prevalence of constipation increases with age and is particularly prevalent in patients 65 years of age or older.10 In most patients, constipation is a result of insufficient dietary fiber.11 One study12 reported that 70% of patients with idiopathic chronic constipation have dilatation of anorectum via barium enema. The establishment of id-
METHODS AND MATERIALS The patient is a 65-year-old white male who, in December 1998, was diagnosed with a Gleason score 6 (3 ⫹ 3) adenocarcinoma of the prostate gland, clinical stage T1c. The patient was diagnosed via a transperineal ultrasound-guided needle biopsy. Gleason score-6 histology was noted to involve 10% of the right-sided specimen without evidence of perineural invasion. Biopsies of the left lobe revealed no evidence of malignancy. A prostate-specific antigen (PSA) in November 1998 was 5.1. On April 13, 1999, the patient underwent prostate brachytherapy utilizing 125I radionuclide (145 Gy calculated by TG 43 dosimetry delivered via 133 125I seeds of 0.376-mCi activity [NIST 85 standard, 0.478 U]). Day 0 dosimetry was significant for the following: V100 (volume of the gland receiving 100% of the prescribed dose) 96.8%, V150 (volume of the gland receiving 150% of the
Reprint requests to: Dr. G. S. Merrick, Schiffler Oncology Center, Wheeling Hospital, 1 Medical Park, Wheeling, WV 26003-6300. E-mail: [email protected] 237
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Medical Dosimetry
prescribed dose) 58.5% and V200 (volume of the gland receiving 200% of the prescribed dose) 24.8%. The average urethral and rectal maximum point doses were 123.5% and 81.7% of prescribed dose, respectively. On July 15, 1999, a repeat computed tomography (CT) scan was obtained as part of an ongoing dosimetric study. The July 15 CT scan revealed significant dilatation of the anorectum. The patient reported 2–3 bowel movements per week (prior to implantation, the patient noted 4 –5 bowel movements per week). Approximately 90 minutes after the CT study and the administration of an enema, a repeat CT scan was obtained with a successfully evacuated rectum. As of November 22, 1999, the patient remains free of any rectal complications, including proctitis, ulceration, or fistula formation. Both July 15 spiral CT scans were acquired at 5-mm thickness and 5-mm spacing extending 2 cm above and below the superior and inferior implanted seeds. A set of CT images were printed at a magnification of 1.5 using soft tissue windows to identify the prostate and surrounding structures and a second set of images using bone windows to better distinguish seeds from calcifications and eliminate redundant seed images. Subsequently, all seeds were identified, accounted for, and appropriately assigned. The actual dose distribution to the prostate/ periprostatic region and rectal wall/mucosa was generated via a dedicated treatment planning computer (Prowess-3000, SSGI, Chico, CA). The rectal dose was defined in terms of the number of 10° sectors of the anterior quadrant of the rectal wall receiving a given dose level. Assuming the circumferential stretch of the rectal wall upon distention is isotropic, the sector approach allows the same tissue to be compared when analyzing similar sectors of the anterior quadrant of the rectal wall in an empty and distended rectum. Figure 1 illustrates the placement of dose points on the rectal wall at 10° increments from the vertical midline of the rectum. Each dose point samples a surface ⫾ 5° and ⫾ 5 mm superior and inferior.
Volume 25, Number 4, 2000
Fig. 1. Schematic of the method used to place rectal dose points. An origin for drawing the 10° sectors was placed at the lateral midpoint of the rectum and one half of the maximum width of the rectum posterior to the anterior rectal wall. Sectors were drawn at 10°, 20°, 30°, and 40° clockwise and counterclockwise from the vertical midline. Dose points were placed wherever the sector lines crossed the anterior rectal wall. Each point, therefore, represents the dose to a 10° sector of the anterior quadrant of the rectal wall.
included in the table are values of DR10 and DR25, the minimal dose covering the hottest 10% and 25%, respectively, of the sectors. Figures 2 and 3 illustrate the mean and maximal rectal doses of each CT slice in the distended and evacuated states as a function of position relative to the base of the prostate. At all offsets from the base except for the maximum dose at an offset of 2 cm, the dose to the rectal wall is significantly greater in the distended state. In both scenarios, the dose to the rectal wall peaks near midgland. Slice by slice, the magnitude of the difference in doses is up to 2 times greater for the
RESULTS The lateral dimensions of the evacuated rectum ranged from 2.44 to 3.35 cm and the distended rectum dimensions ranged from 3.15 to 5.53 cm. In the distended rectum, the maximum dimension occurred at the base of the prostate and the smallest dimension near the apex. The posterior border of the prostate lying against the empty rectum is nearly flat over most of the length of the gland and becoming convex at the base and apex. The posterior prostate border against the distended rectum, however, is indented over the length of the prostate by as much as 5 mm. Table 1 details the mean value and range of the 81 sector points evaluated for each state of the rectum. The mean dose to the distended rectal wall was 50% greater than to the empty rectum, 276 Gy vs. 184 Gy. Also
Fig. 2. The mean dose of the 9 sectors on each CT slice as a function of offset from the base. Filled circles and solid line ⫽ distended rectum, open circles and dotted line ⫽ empty rectum.
Effect of constipation on rectal dosimetry ● G. S. MERRICK et al.
239
Fig. 3. The maximum point dose on each CT slice as a function of offset from the base. Filled circles and solid line ⫽ distended rectum, open circles and dotted line ⫽ empty rectum.
mean dose (490 Gy vs. 252 Gy for distended and empty, respectively) and 4 times greater for the maximal dose (1,237 Gy vs. 295 Gy). The mean of the slice maxima differed by a factor of 2.4 (523 Gy vs. 215 Gy). As shown in Figs. 4a and 4b, the number of sectors receiving greater than 150% and 250% of prescribed dose are significantly greater for the distended rectum. At 250% of the prescription dose, a total of 16 sectors in the distended rectum are covered, while no sector in the empty rectum received doses of such magnitude. Once again, the number of sectors involved peaked near midgland for both evaluations. Figure 5 displays histograms of the number of sectors receiving 100 –250% of the prescribed dose for the 2 states of the rectum. At all evaluated doses, the number of sectors receiving the given dose level were greater for the distended rectum, and the maximum difference was 29 sectors at 125% and 150% of mPD. The magnitude of the percentage difference between the distended and empty rectum increases approximately exponentially with dose, while at lower dose levels (not shown on the graph), both the distended and empty rectum cover all 81 measured sector dose points. DISCUSSION Over the past decade, transperineal ultrasoundguided permanent prostate brachytherapy with either 125I
Fig. 4. The number of sector dose points on each CT slice with dose greater than or equal to 150% of the prescribed minimal peripheral dose (a) or 250% of the mPD (b). Filled circles and solid line ⫽ distended rectum, open circles and dotted line ⫽ empty rectum.
or 103Pd has been increasingly utilized as definitive management for early-stage carcinoma of the prostate gland. Rectal complications have been relatively rare. Complications have primarily consisted of mild self-limited proctitis.4,9,14,15 Rectal ulceration and fistula formation have occasionally been reported.4,14 –17 Merrick et al.9 reported minimal self-limited proctitis with doses to the anterior rectal wall averaging 85% of the prescription dose (regardless of the choice of isotope or whether moderate doses of external beam radiation therapy were employed). Additional recommendations regarding rectal dosimetry included limiting the average maximum dose to 120% of prescribed and limiting the length of the anterior rectal mucosa receiving 100% and 120% of
Table 1. Sector doses to the empty and distended rectum Rectum State
No. of Sectors
Mean ⫾ SD (Gy)
Minimum (Gy)
Maximum (Gy)
DR10* (Gy)
DR25* (Gy)
Empty Distended
81 81
184 ⫾ 52 276 ⫾ 158
95 80
351 1,237
257 405
217 306
* DR10 and DR25 are the minimal doses to the hottest 10% and 25% of the rectum, respectively.
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Medical Dosimetry
Fig. 5. Histograms of the number of sector dose points in the distended or empty rectum at a given percentage of the prescribed minimal peripheral dose. Solid bars ⫽ distended rectum, shaded bars ⫽ empty rectum.
the prescribed dose to approximately 10 mm and 5 mm, respectively. These results and recommendations were based on day 0 dosimetric evaluation. No cases of rectal ulceration and/or fistula formation were reported.9 Idiopathic chronic constipation affects up to 20% of the population at any time13 and is usually a result of a deficiency in dietary fiber.11 In one study, 70% of patients with idiopathic chronic constipation were diagnosed with dilatation of the anorectum via barium enema.12 Our case study suggests a potentially deleterious effect of chronic rectal distention on rectal morbidity. The slight indentation or invagination of the seeded prostate by a distended rectum shapes and focuses the implant radiation onto the anterior quadrant of the rectal wall. Our results indicate the average rectal wall dose is increased by a factor of 2 in the distended state. In addition, the maximum dose per slice is increased on average by a factor of 2.4 for the distended rectum. Figure 5 illustrates that the number of sectors receiving doses greater than or equal to the prescription dose is substantially greater for the distended vs. the empty rectum. These differences are also more pronounced as the magnitude of the rectal dose increases. Our dosimetric evaluation does not take into account decay of the isotope. This, however, does not alter the magnitude of the differences between a distended and an empty rectum. The doses reported for the empty rectum are also significantly greater than the day 0 evaluation (average dose 118 Gy). The CT scan was obtained 3 months following implantation. Recently, Waterman and Dicker18 reported the dose to the rectal wall increases significantly with resolution of postimplant edema. They reported the D10 (the minimal dose which encompassed 10% of the surface area of the rectum) increased on
Volume 25, Number 4, 2000
average by 68% from day 0 through day 30 following implantation. Rectal morbidity data, however, was not presented. Our corresponding values of DR10 from Table 1 were 257 Gy for the empty rectum and 405 Gy for the distended, an increase of 58% without including the effects of day 0 edema resolution. In most patients, idiopathic chronic constipation can be successfully managed by an increase in dietary fiber via fruit and vegetables or the addition of psyllium hydrophilic colloids (Metamucil, 1 teaspoon 3 times daily).11 Patients with chronic or new onset constipation who undergo prostate brachytherapy should be carefully counseled regarding the importance of routine daily bowel movements. We recommend detailed postimplant attention to bowel habits for at least 2 half-lives of the implanted isotope to minimize rectal distention and subsequently decrease radiation dose to the anterior rectal wall. CONCLUSIONS For acutely or chronically constipated patients undergoing prostate brachytherapy, diets rich in high fiber foods and/or the supplementation with psyllium hydrophilic colloids (Metamucil) should be encouraged for at least 2 half-lives of the implanted isotope to minimize rectal distention, decrease the radiation dose to the anterior rectal wall, and to minimize potential constipationrelated rectal toxicity. REFERENCES 1. Merrick, G.S.; Butler, W.M.; Dorsey, A.T.; et al. Prostatic conformal brachytherapy: I-125/Pd-103 post-operative dosimetric analysis. Radiat. Oncol. Invest. 5:305–13; 1997. 2. Merrick, G.S.; Butler, W.M.; Dorsey, A.T.; et al. The influence of timing on the dosimetric analysis of transperineal ultrasound guided prostatic conformal brachytherapy. Radiat. Oncol. Invest. 6:182–90; 1998. 3. Merrick, G.S.; Butler, W.M.; Dorsey, A.T.; et al. Potential role of various dosimetric quality indicators in prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 44:717–24; 1999. 4. Wallner, K.; Roy, J.; Harrison, L. Dosimetry guidelines to minimize urethral and rectal morbidity following transperineal I-125 prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 32:465– 71; 1995. 5. Willins, J.; Wallner, K. CT based dosimetry for transperineal I-125 prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 39:347– 53; 1997. 6. Prestidge, B.R.; Bice, W.S.; Kiefer, E.T., et al. Timing of computed tomography based post-implant assessment following permanent transperineal prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 40:1111–15; 1998. 7. Stock, R.G.; Stone, N.N.; Tabert, A.; et al. A dose response study for I-125 prostate implants. Int. J. Radiat. Oncol. Biol. Phys. 41:101– 8; 1998. 8. Waterman, F.M.; Yue, N.; Corn, B.W.; et al. Edema associated with I-125 or Pd-103 prostate brachytherapy and its impact on post-implant dosimetry: An analysis based on serial CT acquisition. Int. J. Radiat. Oncol. Biol. Phys. 41:1069 –77; 1998. 9. Merrick, G.S.; Butler, W.M.; Dorsey, A.T.; et al. Rectal dosimetric analysis following prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 43:1021–27; 1999. 10. Johanson, J.F. Geographic distribution of constipation in the United States. Am. J. Gastroenterol. 93:188 –91; 1998.
Effect of constipation on rectal dosimetry ● G. S. MERRICK et al. 11. Camilleri, M.; Thompson, W.G.; Fleshman, J.W.; et al. Clinical management of intractable constipation. Ann. Intern. Med. 121: 520 –28; 1994. 12. Mishalany, H. Seven years’ experience with idiopathic unremitting chronic constipation. J. Pediatric. Surg. 24:360 –2; 1989. 13. Prather, C.M.; Ortiz-Camachio, C.P. Evaluation and treatment of constipation and fecal impaction in adults. Mayo Clin. Proc. 73: 881–7; 1998. 14. Blasko, J.C.; Grimm, P.D.; Ragde, H. Brachytherapy and organ preservation in the management of carcinoma of the prostate. Semin. Radiat. Oncol. 3:240 –9; 1993. 15. Stock, R.G.; Stone, N.N.; DeWyngaert, J.K.; et al. Prostate specific
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antigen findings and biopsy results following interactive ultrasound guided transperineal brachytherapy for early stage prostate cancer. Cancer 77:2386 –92; 1996. 16. Wallner, K.; Roy, J.; Zelefsky, M.; et al. Short term freedom from disease progression after I-125 prostate implantation. Int. J. Radiat. Oncol. Biol. Phys. 30:405–9; 1994. 17. Wallner, K.; Roy, J.; Harrison, L. Tumor control and morbidity following transperineal I-125 implantation for stage T1/T2 prostatic carcinoma. J. Clin. Oncol. 14:449 –53; 1996. 18. Waterman, F.M.; Dicker, A.P. Effect of post-implant edema on the rectal dose in prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 45:571– 6; 1999.
PII: S0958-3947(00)00047-9
THE EFFECT OF CONSTIPATION ON RECTAL DOSIMETRY FOLLOWING PROSTATE BRACHYTHERAPY GREGORY S. MERRICK, M.D., WAYNE M. BUTLER, PH.D., ANTHONY T. DORSEY, M.S., and J. THOMAS DORSEY III, M.D. Schiffler Oncology Center, Wheeling Hospital, Wheeling, WV and The George Washington University Medical Center, Division of Radiation Oncology & Biophysics, Washington, D.C. (Received 13 April 2000; Accepted 8 August 2000)
Abstract—The purpose of this study is to report the effect of dilatation of the anorectum on rectal dosimetry following an 125I prostate implant. Three months following prostate brachytherapy, 2 computed tomography (CT) scans of the prostate gland were obtained within 90 minutes of each other. The first CT scan revealed a dilated anorectum secondary to constipation. The second CT was obtained following the administration of an enema with a successfully evacuated rectum. Differences in radiation doses to the distended and empty rectum were computed via the mean dose, the maximum dose per slice, the distance from the base, and in terms of the surface of the anterior quadrant of the rectum receiving 100%, 125%, 150%, 175%, 200%, and 250% of the prescribed dose. The dose to the rectal wall was substantially increased in the distended state for all evaluated parameters. In general, the mean dose to the rectal wall was increased by a factor of 1.5 in the distended state. In both scenarios, the dose to the rectal wall peaked near midgland. In terms of 10° rectal wall sectors receiving a given percentage of the prescribed minimal peripheral dose, S%mPD, the S100, S125, S150, S175, S200, and S250 were substantially greater for the distended versus the empty rectum. The magnitude of the percentage difference in dose between the distended and evacuated rectum increased with dose level while the difference in the number of sectors receiving a given dose level was greatest at 125% and 150% of the prescribed dose. We recommend detailed postimplant attention to bowel habits for at least 2 half-lives of the implanted isotope to minimize rectal distention, decrease radiation dose to the anterior rectal wall, and subsequently minimize potential constipation related rectal toxicity. © 2000 American Association of Medical Dosimetrists. Key Words: Prostate, Brachytherapy, Dosimetry, Rectum, Constipation.
INTRODUCTION
iopathic constipation requires at least 2 of the following criteria to be present: less than 3 bowel movements per week, straining with defecation at least 25% of the time, lumpy and/or hard stools at least 25% the time, or the sensation of incomplete evacuation at least 25% of the time.13 Herein, we present detailed rectal dosimetry in a constipated patient with dilatation of the anorectum.
Over the past decade, transperineal ultrasoundguided permanent prostate brachytherapy with either iodine 125 (125I) or palladium 103 (103Pd) has been increasingly utilized as definitive management for earlystage carcinoma of the prostate gland. Multiple recent studies evaluating postoperative dosimetry have supported the ability of brachytherapists to adequately encompass the target volume.1–9 However, only 2 of these studies have evaluated the relationship between the radiation dose to the rectum and toxicity.4,9 Fortunately, significant rectal injury following prostate brachytherapy is relatively rare. An estimated 55 million American adults complain of constipation at least once every 3 months, and 4 million adults report chronic constipation. The prevalence of constipation increases with age and is particularly prevalent in patients 65 years of age or older.10 In most patients, constipation is a result of insufficient dietary fiber.11 One study12 reported that 70% of patients with idiopathic chronic constipation have dilatation of anorectum via barium enema. The establishment of id-
METHODS AND MATERIALS The patient is a 65-year-old white male who, in December 1998, was diagnosed with a Gleason score 6 (3 ⫹ 3) adenocarcinoma of the prostate gland, clinical stage T1c. The patient was diagnosed via a transperineal ultrasound-guided needle biopsy. Gleason score-6 histology was noted to involve 10% of the right-sided specimen without evidence of perineural invasion. Biopsies of the left lobe revealed no evidence of malignancy. A prostate-specific antigen (PSA) in November 1998 was 5.1. On April 13, 1999, the patient underwent prostate brachytherapy utilizing 125I radionuclide (145 Gy calculated by TG 43 dosimetry delivered via 133 125I seeds of 0.376-mCi activity [NIST 85 standard, 0.478 U]). Day 0 dosimetry was significant for the following: V100 (volume of the gland receiving 100% of the prescribed dose) 96.8%, V150 (volume of the gland receiving 150% of the
Reprint requests to: Dr. G. S. Merrick, Schiffler Oncology Center, Wheeling Hospital, 1 Medical Park, Wheeling, WV 26003-6300. E-mail: [email protected] 237
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prescribed dose) 58.5% and V200 (volume of the gland receiving 200% of the prescribed dose) 24.8%. The average urethral and rectal maximum point doses were 123.5% and 81.7% of prescribed dose, respectively. On July 15, 1999, a repeat computed tomography (CT) scan was obtained as part of an ongoing dosimetric study. The July 15 CT scan revealed significant dilatation of the anorectum. The patient reported 2–3 bowel movements per week (prior to implantation, the patient noted 4 –5 bowel movements per week). Approximately 90 minutes after the CT study and the administration of an enema, a repeat CT scan was obtained with a successfully evacuated rectum. As of November 22, 1999, the patient remains free of any rectal complications, including proctitis, ulceration, or fistula formation. Both July 15 spiral CT scans were acquired at 5-mm thickness and 5-mm spacing extending 2 cm above and below the superior and inferior implanted seeds. A set of CT images were printed at a magnification of 1.5 using soft tissue windows to identify the prostate and surrounding structures and a second set of images using bone windows to better distinguish seeds from calcifications and eliminate redundant seed images. Subsequently, all seeds were identified, accounted for, and appropriately assigned. The actual dose distribution to the prostate/ periprostatic region and rectal wall/mucosa was generated via a dedicated treatment planning computer (Prowess-3000, SSGI, Chico, CA). The rectal dose was defined in terms of the number of 10° sectors of the anterior quadrant of the rectal wall receiving a given dose level. Assuming the circumferential stretch of the rectal wall upon distention is isotropic, the sector approach allows the same tissue to be compared when analyzing similar sectors of the anterior quadrant of the rectal wall in an empty and distended rectum. Figure 1 illustrates the placement of dose points on the rectal wall at 10° increments from the vertical midline of the rectum. Each dose point samples a surface ⫾ 5° and ⫾ 5 mm superior and inferior.
Volume 25, Number 4, 2000
Fig. 1. Schematic of the method used to place rectal dose points. An origin for drawing the 10° sectors was placed at the lateral midpoint of the rectum and one half of the maximum width of the rectum posterior to the anterior rectal wall. Sectors were drawn at 10°, 20°, 30°, and 40° clockwise and counterclockwise from the vertical midline. Dose points were placed wherever the sector lines crossed the anterior rectal wall. Each point, therefore, represents the dose to a 10° sector of the anterior quadrant of the rectal wall.
included in the table are values of DR10 and DR25, the minimal dose covering the hottest 10% and 25%, respectively, of the sectors. Figures 2 and 3 illustrate the mean and maximal rectal doses of each CT slice in the distended and evacuated states as a function of position relative to the base of the prostate. At all offsets from the base except for the maximum dose at an offset of 2 cm, the dose to the rectal wall is significantly greater in the distended state. In both scenarios, the dose to the rectal wall peaks near midgland. Slice by slice, the magnitude of the difference in doses is up to 2 times greater for the
RESULTS The lateral dimensions of the evacuated rectum ranged from 2.44 to 3.35 cm and the distended rectum dimensions ranged from 3.15 to 5.53 cm. In the distended rectum, the maximum dimension occurred at the base of the prostate and the smallest dimension near the apex. The posterior border of the prostate lying against the empty rectum is nearly flat over most of the length of the gland and becoming convex at the base and apex. The posterior prostate border against the distended rectum, however, is indented over the length of the prostate by as much as 5 mm. Table 1 details the mean value and range of the 81 sector points evaluated for each state of the rectum. The mean dose to the distended rectal wall was 50% greater than to the empty rectum, 276 Gy vs. 184 Gy. Also
Fig. 2. The mean dose of the 9 sectors on each CT slice as a function of offset from the base. Filled circles and solid line ⫽ distended rectum, open circles and dotted line ⫽ empty rectum.
Effect of constipation on rectal dosimetry ● G. S. MERRICK et al.
239
Fig. 3. The maximum point dose on each CT slice as a function of offset from the base. Filled circles and solid line ⫽ distended rectum, open circles and dotted line ⫽ empty rectum.
mean dose (490 Gy vs. 252 Gy for distended and empty, respectively) and 4 times greater for the maximal dose (1,237 Gy vs. 295 Gy). The mean of the slice maxima differed by a factor of 2.4 (523 Gy vs. 215 Gy). As shown in Figs. 4a and 4b, the number of sectors receiving greater than 150% and 250% of prescribed dose are significantly greater for the distended rectum. At 250% of the prescription dose, a total of 16 sectors in the distended rectum are covered, while no sector in the empty rectum received doses of such magnitude. Once again, the number of sectors involved peaked near midgland for both evaluations. Figure 5 displays histograms of the number of sectors receiving 100 –250% of the prescribed dose for the 2 states of the rectum. At all evaluated doses, the number of sectors receiving the given dose level were greater for the distended rectum, and the maximum difference was 29 sectors at 125% and 150% of mPD. The magnitude of the percentage difference between the distended and empty rectum increases approximately exponentially with dose, while at lower dose levels (not shown on the graph), both the distended and empty rectum cover all 81 measured sector dose points. DISCUSSION Over the past decade, transperineal ultrasoundguided permanent prostate brachytherapy with either 125I
Fig. 4. The number of sector dose points on each CT slice with dose greater than or equal to 150% of the prescribed minimal peripheral dose (a) or 250% of the mPD (b). Filled circles and solid line ⫽ distended rectum, open circles and dotted line ⫽ empty rectum.
or 103Pd has been increasingly utilized as definitive management for early-stage carcinoma of the prostate gland. Rectal complications have been relatively rare. Complications have primarily consisted of mild self-limited proctitis.4,9,14,15 Rectal ulceration and fistula formation have occasionally been reported.4,14 –17 Merrick et al.9 reported minimal self-limited proctitis with doses to the anterior rectal wall averaging 85% of the prescription dose (regardless of the choice of isotope or whether moderate doses of external beam radiation therapy were employed). Additional recommendations regarding rectal dosimetry included limiting the average maximum dose to 120% of prescribed and limiting the length of the anterior rectal mucosa receiving 100% and 120% of
Table 1. Sector doses to the empty and distended rectum Rectum State
No. of Sectors
Mean ⫾ SD (Gy)
Minimum (Gy)
Maximum (Gy)
DR10* (Gy)
DR25* (Gy)
Empty Distended
81 81
184 ⫾ 52 276 ⫾ 158
95 80
351 1,237
257 405
217 306
* DR10 and DR25 are the minimal doses to the hottest 10% and 25% of the rectum, respectively.
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Medical Dosimetry
Fig. 5. Histograms of the number of sector dose points in the distended or empty rectum at a given percentage of the prescribed minimal peripheral dose. Solid bars ⫽ distended rectum, shaded bars ⫽ empty rectum.
the prescribed dose to approximately 10 mm and 5 mm, respectively. These results and recommendations were based on day 0 dosimetric evaluation. No cases of rectal ulceration and/or fistula formation were reported.9 Idiopathic chronic constipation affects up to 20% of the population at any time13 and is usually a result of a deficiency in dietary fiber.11 In one study, 70% of patients with idiopathic chronic constipation were diagnosed with dilatation of the anorectum via barium enema.12 Our case study suggests a potentially deleterious effect of chronic rectal distention on rectal morbidity. The slight indentation or invagination of the seeded prostate by a distended rectum shapes and focuses the implant radiation onto the anterior quadrant of the rectal wall. Our results indicate the average rectal wall dose is increased by a factor of 2 in the distended state. In addition, the maximum dose per slice is increased on average by a factor of 2.4 for the distended rectum. Figure 5 illustrates that the number of sectors receiving doses greater than or equal to the prescription dose is substantially greater for the distended vs. the empty rectum. These differences are also more pronounced as the magnitude of the rectal dose increases. Our dosimetric evaluation does not take into account decay of the isotope. This, however, does not alter the magnitude of the differences between a distended and an empty rectum. The doses reported for the empty rectum are also significantly greater than the day 0 evaluation (average dose 118 Gy). The CT scan was obtained 3 months following implantation. Recently, Waterman and Dicker18 reported the dose to the rectal wall increases significantly with resolution of postimplant edema. They reported the D10 (the minimal dose which encompassed 10% of the surface area of the rectum) increased on
Volume 25, Number 4, 2000
average by 68% from day 0 through day 30 following implantation. Rectal morbidity data, however, was not presented. Our corresponding values of DR10 from Table 1 were 257 Gy for the empty rectum and 405 Gy for the distended, an increase of 58% without including the effects of day 0 edema resolution. In most patients, idiopathic chronic constipation can be successfully managed by an increase in dietary fiber via fruit and vegetables or the addition of psyllium hydrophilic colloids (Metamucil, 1 teaspoon 3 times daily).11 Patients with chronic or new onset constipation who undergo prostate brachytherapy should be carefully counseled regarding the importance of routine daily bowel movements. We recommend detailed postimplant attention to bowel habits for at least 2 half-lives of the implanted isotope to minimize rectal distention and subsequently decrease radiation dose to the anterior rectal wall. CONCLUSIONS For acutely or chronically constipated patients undergoing prostate brachytherapy, diets rich in high fiber foods and/or the supplementation with psyllium hydrophilic colloids (Metamucil) should be encouraged for at least 2 half-lives of the implanted isotope to minimize rectal distention, decrease the radiation dose to the anterior rectal wall, and to minimize potential constipationrelated rectal toxicity. REFERENCES 1. Merrick, G.S.; Butler, W.M.; Dorsey, A.T.; et al. Prostatic conformal brachytherapy: I-125/Pd-103 post-operative dosimetric analysis. Radiat. Oncol. Invest. 5:305–13; 1997. 2. Merrick, G.S.; Butler, W.M.; Dorsey, A.T.; et al. The influence of timing on the dosimetric analysis of transperineal ultrasound guided prostatic conformal brachytherapy. Radiat. Oncol. Invest. 6:182–90; 1998. 3. Merrick, G.S.; Butler, W.M.; Dorsey, A.T.; et al. Potential role of various dosimetric quality indicators in prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 44:717–24; 1999. 4. Wallner, K.; Roy, J.; Harrison, L. Dosimetry guidelines to minimize urethral and rectal morbidity following transperineal I-125 prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 32:465– 71; 1995. 5. Willins, J.; Wallner, K. CT based dosimetry for transperineal I-125 prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 39:347– 53; 1997. 6. Prestidge, B.R.; Bice, W.S.; Kiefer, E.T., et al. Timing of computed tomography based post-implant assessment following permanent transperineal prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 40:1111–15; 1998. 7. Stock, R.G.; Stone, N.N.; Tabert, A.; et al. A dose response study for I-125 prostate implants. Int. J. Radiat. Oncol. Biol. Phys. 41:101– 8; 1998. 8. Waterman, F.M.; Yue, N.; Corn, B.W.; et al. Edema associated with I-125 or Pd-103 prostate brachytherapy and its impact on post-implant dosimetry: An analysis based on serial CT acquisition. Int. J. Radiat. Oncol. Biol. Phys. 41:1069 –77; 1998. 9. Merrick, G.S.; Butler, W.M.; Dorsey, A.T.; et al. Rectal dosimetric analysis following prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 43:1021–27; 1999. 10. Johanson, J.F. Geographic distribution of constipation in the United States. Am. J. Gastroenterol. 93:188 –91; 1998.
Effect of constipation on rectal dosimetry ● G. S. MERRICK et al. 11. Camilleri, M.; Thompson, W.G.; Fleshman, J.W.; et al. Clinical management of intractable constipation. Ann. Intern. Med. 121: 520 –28; 1994. 12. Mishalany, H. Seven years’ experience with idiopathic unremitting chronic constipation. J. Pediatric. Surg. 24:360 –2; 1989. 13. Prather, C.M.; Ortiz-Camachio, C.P. Evaluation and treatment of constipation and fecal impaction in adults. Mayo Clin. Proc. 73: 881–7; 1998. 14. Blasko, J.C.; Grimm, P.D.; Ragde, H. Brachytherapy and organ preservation in the management of carcinoma of the prostate. Semin. Radiat. Oncol. 3:240 –9; 1993. 15. Stock, R.G.; Stone, N.N.; DeWyngaert, J.K.; et al. Prostate specific
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antigen findings and biopsy results following interactive ultrasound guided transperineal brachytherapy for early stage prostate cancer. Cancer 77:2386 –92; 1996. 16. Wallner, K.; Roy, J.; Zelefsky, M.; et al. Short term freedom from disease progression after I-125 prostate implantation. Int. J. Radiat. Oncol. Biol. Phys. 30:405–9; 1994. 17. Wallner, K.; Roy, J.; Harrison, L. Tumor control and morbidity following transperineal I-125 implantation for stage T1/T2 prostatic carcinoma. J. Clin. Oncol. 14:449 –53; 1996. 18. Waterman, F.M.; Dicker, A.P. Effect of post-implant edema on the rectal dose in prostate brachytherapy. Int. J. Radiat. Oncol. Biol. Phys. 45:571– 6; 1999.