Factors Influencing the Dose Factors Influencing the Dose to Rectum During the to Rectum During the

Treatment of Prostate Cancer Treatment of Prostate Cancer with IMRTwith IMRT

Nandanuri M.S. Reddy, PhD, Nandanuri M.S. Reddy, PhD, Brij M. Sood, MD, and Brij M. Sood, MD, and

Dattatreyudu Nori, MD, FACR.Dattatreyudu Nori, MD, FACR.Radiation Oncology, NY Radiation Oncology, NY

Hospital Queens, Flushing, NYHospital Queens, Flushing, NY

Purpose/ObjectivePurpose/Objective:: 3D conformal radiotherapy 3D conformal radiotherapy (3D CRT) had shown that smaller the volume of (3D CRT) had shown that smaller the volume of

rectum higher is the dose to rectum and vice rectum higher is the dose to rectum and vice versa (1-5). In addition, patients with small versa (1-5). In addition, patients with small

rectal wall volumes have been shown to be at a rectal wall volumes have been shown to be at a higher risk for rectal bleeding than patients with higher risk for rectal bleeding than patients with

larger rectal wall volumes (1,2). Volumes of larger rectal wall volumes (1,2). Volumes of whole rectum (rectum + fillings) and rectal wall whole rectum (rectum + fillings) and rectal wall

(whole rectum - fillings) vary from patient to (whole rectum - fillings) vary from patient to patient. Whole rectal volumes varied from 50 to patient. Whole rectal volumes varied from 50 to 250 cc (5), and rectal wall volumes varied from 250 cc (5), and rectal wall volumes varied from 12 to 44 cc (5), 18 to 97 cc (1), 30 to 130 cc (2).12 to 44 cc (5), 18 to 97 cc (1), 30 to 130 cc (2).

  

However, with the inverse treatment However, with the inverse treatment planning, the dose-volume constraints planning, the dose-volume constraints placed on rectum are such that the 70, placed on rectum are such that the 70, 50, and 30% volumes of rectum should 50, and 30% volumes of rectum should

not receive more than 40, 60 and 70% of not receive more than 40, 60 and 70% of the prescription dose, respectively. the prescription dose, respectively. However, this type of dose-volume However, this type of dose-volume

constraints do not take into account of the constraints do not take into account of the variation in the rectal volumes among variation in the rectal volumes among

patients.patients.

In this report, we have examined the In this report, we have examined the relationship between dose to 70, 50 and relationship between dose to 70, 50 and 30% volumes of whole rectum or rectal 30% volumes of whole rectum or rectal wall vs. volumes of whole rectum and wall vs. volumes of whole rectum and

rectal wall. We also evaluated the rectal wall. We also evaluated the relationship between dose per cc at 70, 50 relationship between dose per cc at 70, 50 and 30% of the volumes of whole rectum and 30% of the volumes of whole rectum or rectal wall vs. volumes of whole rectum or rectal wall vs. volumes of whole rectum or rectal wall. or rectal wall. This analysis might help This analysis might help to better understand the relationship to better understand the relationship between dose per cc of whole rectum between dose per cc of whole rectum

or rectal wall vs. potential rectal or rectal wall vs. potential rectal toxicity.toxicity.

Materials/Methods:Materials/Methods: Prostate, SV, whole rectum Prostate, SV, whole rectum (with fillings = WR), rectal wall only (WR–(with fillings = WR), rectal wall only (WR–

fillings=RW) and bladder were contoured for 21 fillings=RW) and bladder were contoured for 21 patients patients (Fig. 1).(Fig. 1). The caudal limit of rectum was The caudal limit of rectum was

the first slice above the anal verge and the the first slice above the anal verge and the cranial limit was first slice below the sigmoid cranial limit was first slice below the sigmoid flexure flexure (Fig. 2).(Fig. 2). Variation in the delineation of Variation in the delineation of

rectum at the cranial end could have occurred, rectum at the cranial end could have occurred, up to 10%, for two main reasons: 1) presence of up to 10%, for two main reasons: 1) presence of

air and fecal matter in rectum air and fecal matter in rectum (Fig. 3)(Fig. 3), which , which makes it difficult to discern the sigmoid flexure at makes it difficult to discern the sigmoid flexure at

S3, and the presence of unusually large S3, and the presence of unusually large amounts of bowels in pelvis, amounts of bowels in pelvis, (Fig. 4)(Fig. 4), which , which

makes it difficult to differentiate the rectum from makes it difficult to differentiate the rectum from bowels.bowels.

  

The prescription was 45 Gy to prostate and SV in The prescription was 45 Gy to prostate and SV in 25 fractions (Prostate + SV=CTV, CTV + margin = 25 fractions (Prostate + SV=CTV, CTV + margin =

PTV) with 5F IMRT PTV) with 5F IMRT 66plans. Dose to 30, 50 and plans. Dose to 30, 50 and 70% of whole rectum and rectal wall were 70% of whole rectum and rectal wall were

estimated from the DVH. In addition, dose to 1 cc estimated from the DVH. In addition, dose to 1 cc at 30, 50 and 70% volumes of whole rectum or at 30, 50 and 70% volumes of whole rectum or rectal wall were. At a given % volume level, the rectal wall were. At a given % volume level, the

dose to that % volume was divided by the % dose to that % volume was divided by the % volume in cc of the whole rectum or rectal wall. volume in cc of the whole rectum or rectal wall. The resulting value is expressed as dose per cc The resulting value is expressed as dose per cc at 30, 50 and 70% volumes. These data were at 30, 50 and 70% volumes. These data were

plotted against the volumes of whole rectum or plotted against the volumes of whole rectum or rectal wall. Pearson’s correlation coefficient ‘r’ rectal wall. Pearson’s correlation coefficient ‘r’ and the two-tailed P values were estimatedand the two-tailed P values were estimated..

  ResultsResults:: The volume of whole rectum The volume of whole rectum

varied from 25 to 177 cc between patients varied from 25 to 177 cc between patients (Fig. 5)(Fig. 5). Larger the volume of whole . Larger the volume of whole

rectum, larger was also the volume of rectum, larger was also the volume of rectal wall (rectal wall (Fig. 5Fig. 5, 18-68cc, P<0.01). Dose , 18-68cc, P<0.01). Dose to rectal wall was less in 10 patients by 6-to rectal wall was less in 10 patients by 6-21% at 50% volume, compared to doses 21% at 50% volume, compared to doses to whole rectum. This difference in doses to whole rectum. This difference in doses

seen for large whole rectum volumes seen for large whole rectum volumes decreased beyond 20% volume and decreased beyond 20% volume and

disappeared below 10% volume disappeared below 10% volume (Fig. 6)(Fig. 6)..

Dose to 30%, 50% or 70% volumes of whole Dose to 30%, 50% or 70% volumes of whole rectum rectum (Fig. 7)(Fig. 7) or rectal wall ( or rectal wall (Fig. 8Fig. 8, except at , except at 50% volume, P<0.1), was not correlated to the 50% volume, P<0.1), was not correlated to the volume of whole rectum or rectal wall (p>0.2). volume of whole rectum or rectal wall (p>0.2).

This was because the same dose-volume This was because the same dose-volume constraints were placed on whole rectum constraints were placed on whole rectum independent of variations in whole rectum independent of variations in whole rectum

volumes between patients. However, smaller the volumes between patients. However, smaller the volume of rectum, higher was the dose per cc of volume of rectum, higher was the dose per cc of whole rectum (whole rectum (Fig. 9 and 10Fig. 9 and 10) or rectal wall () or rectal wall (Fig. Fig. 11 and 1211 and 12) and vice versa (p<0.001). This was ) and vice versa (p<0.001). This was

due to the fact that dose per cc takes into due to the fact that dose per cc takes into account of the variations in the volume of whole account of the variations in the volume of whole

rectum or rectal wall between patients.rectum or rectal wall between patients.

  

Dose conformity to PTV and dose to rectum Dose conformity to PTV and dose to rectum were interrelated and the resulting IMRT plan were interrelated and the resulting IMRT plan was a combination a trade of between dose to was a combination a trade of between dose to rectum and dose conformity to PTV. When the rectum and dose conformity to PTV. When the

volume of rectum was small, dose-volume volume of rectum was small, dose-volume constraints (DVC) for rectum could only be constraints (DVC) for rectum could only be

achieved at the expense of dose conformity to achieved at the expense of dose conformity to the PTV. For example, in the case of a rectum the PTV. For example, in the case of a rectum

with a volume of 25 cc, rectal dose to 30 % with a volume of 25 cc, rectal dose to 30 % volume was volume was 35 Gy35 Gy for PTV coverage of for PTV coverage of 87.4-87.4-103.9%103.9% of of prescription dose (heterogeneity prescription dose (heterogeneity

index (HI), max/min, = 1.19). Conversely, for a index (HI), max/min, = 1.19). Conversely, for a rectal dose of rectal dose of 31.831.8 Gy to 30% volume, the dose Gy to 30% volume, the dose

coverage to PTV was coverage to PTV was 72-105.6% 72-105.6% of the of the prescription dose (HI = 1.47).prescription dose (HI = 1.47).

ConclusionsConclusions:: Rectal volumes vary from patient Rectal volumes vary from patient to patient. Larger the volume of whole rectum, to patient. Larger the volume of whole rectum,

larger was also the volume of rectal wall. In larger was also the volume of rectal wall. In IMRT, as in the case of 3D, patients with smaller IMRT, as in the case of 3D, patients with smaller rectal volumes would be at a higher risk for rectal rectal volumes would be at a higher risk for rectal

toxicity because dose delivered to per cc of toxicity because dose delivered to per cc of rectum increases with the decrease in the rectum increases with the decrease in the

volume of rectum. Therefore, it is suggested that: volume of rectum. Therefore, it is suggested that: 1) dose per cc of rectum in addition to the doses 1) dose per cc of rectum in addition to the doses

to 30, 50 and 70% volumes may be used to to 30, 50 and 70% volumes may be used to predict rectal toxicity more reliably and, 2) that predict rectal toxicity more reliably and, 2) that

the use of rectal wall volumes to express dose to the use of rectal wall volumes to express dose to rectum would be more accurate and reliable rectum would be more accurate and reliable

because rectal wall volume changes due to rectal because rectal wall volume changes due to rectal

fillings would be minimal.fillings would be minimal.

  References:References:

  1.   M.W. Skwarchuk et. al. Int. J. 1.   M.W. Skwarchuk et. al. Int. J.

Radiat. Oncol. Biol. Phys. 47, 103-113, Radiat. Oncol. Biol. Phys. 47, 103-113, 2000.2000.

2.   A. Jackson et. al. Int. J. Radiat. 2.   A. Jackson et. al. Int. J. Radiat. Oncol. Biol. Phys. 49, 685-698, 2001.Oncol. Biol. Phys. 49, 685-698, 2001.3.   E.H. Huang et. al. Int. J. Radiat. 3.   E.H. Huang et. al. Int. J. Radiat.

Oncol. Biol. Phys. 54, 1314-1321, 2002.Oncol. Biol. Phys. 54, 1314-1321, 2002.4.   N. Reddy et. al. Med. Phys. 30, 4.   N. Reddy et. al. Med. Phys. 30,

1505, 2003.1505, 2003.5.   P.C.M. Koper et. al. Int. J. Radiat. 5.   P.C.M. Koper et. al. Int. J. Radiat.

Oncol. Biol. Phys. 58, 1072-1082, 2004Oncol. Biol. Phys. 58, 1072-1082, 2004

Whole rectum

Rectal Wall

Rectal Fillings

BladderRectum

Prostate

SV

Sigmoid flexureS3

RectumBowels

Fig 5: Whole rectum vs Rectal wall volumes

y = 0.2308x + 28.701

p<0.01

10

20

30

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50

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70

80

0 50 100 150 200

Whole rectal volume, cc

Rect

al w

all v

olum

e, c

c

Fig. 7: Dose to 30, 50 and 70% volumes of retum vs volumes of whole rectum

30%, P>0.2

50%, P>0.2

70%, P>0.1

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Whole rectal volumes, cc

Dose

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Fig. 8: Dose to 30, 50 and 705 volumes vs. volumes of rectal wall

30%, P>0.2

50%, P=0.01

70%, P>0.2

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Rectal wall volumes, cc

Dose

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Fig. 9: Dose to 1 cc of rectum at 30,50 and 70% volumes of whole rectum

30%, P<0.001

50%, P<0.001

70%, P<0.0010

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Fig. 10: Dose to 1 cc of rectum at 10% volume of whole rectum

Pow er f it, P<0.001

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Fig. 11: Dose to 1 cc of rectal wall at 30, 50 and 70% of rectal wall volumes

30% , P<0.001

50%, P<0.001

70%, P<0.0010

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Fig 12; Dose to 1 cc of rectal wall at 10% rectal wall volume

y = 34012x-0.9543

Power fit, P<0.001

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