Factors Influencing the Dose to Rectum During the Treatment of Prostate Cancer with IMRT Nandanuri M.S. Reddy, PhD, Brij M. Sood, MD, and Dattatreyudu Nori, MD, FACR. Radiation Oncology, NY Hospital Queens, Flushing, NY

Purpose/Objective: 3D conformal radiotherapy (3D CRT) had shown that smaller the volume of rectum higher is the dose to rectum and vice versa (1-5). In addition, patients with small rectal wall volumes have been shown to be at a higher risk for rectal bleeding than patients with larger rectal wall volumes (1,2). Volumes of whole rectum (rectum + fillings) and rectal wall (whole rectum - fillings) vary from patient to patient. Whole rectal volumes varied from 50 to 250 cc (5), and rectal wall volumes varied from 12 to 44 cc (5), 18 to 97 cc (1), 30 to 130 cc (2). Purpose/Objective: 3D conformal radiotherapy (3D CRT) had shown that smaller the volume of rectum higher is the dose to rectum and vice versa (1-5). In addition, patients with small rectal wall volumes have been shown to be at a higher risk for rectal bleeding than patients with larger rectal wall volumes (1,2). Volumes of whole rectum (rectum + fillings) and rectal wall (whole rectum - fillings) vary from patient to patient. Whole rectal volumes varied from 50 to 250 cc (5), and rectal wall volumes varied from 12 to 44 cc (5), 18 to 97 cc (1), 30 to 130 cc (2).

However, with the inverse treatment planning, the dose-volume constraints placed on rectum are such that the 70, 50, and 30% volumes of rectum should not receive more than 40, 60 and 70% of the prescription dose, respectively. However, this type of dose-volume constraints do not take into account of the variation in the rectal volumes among patients.

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

Materials/Methods: Prostate, SV, whole rectum (with fillings = WR), rectal wall only (WR– fillings=RW) and bladder were contoured for 21 patients (Fig. 1). The caudal limit of rectum was the first slice above the anal verge and the cranial limit was first slice below the sigmoid flexure (Fig. 2). Variation in the delineation of rectum at the cranial end could have occurred, up to 10%, for two main reasons: 1) presence of air and fecal matter in rectum (Fig. 3), which makes it difficult to discern the sigmoid flexure at S3, and the presence of unusually large amounts of bowels in pelvis, (Fig. 4), which makes it difficult to differentiate the rectum from bowels. Materials/Methods: Prostate, SV, whole rectum (with fillings = WR), rectal wall only (WR– fillings=RW) and bladder were contoured for 21 patients (Fig. 1). The caudal limit of rectum was the first slice above the anal verge and the cranial limit was first slice below the sigmoid flexure (Fig. 2). Variation in the delineation of rectum at the cranial end could have occurred, up to 10%, for two main reasons: 1) presence of air and fecal matter in rectum (Fig. 3), which makes it difficult to discern the sigmoid flexure at S3, and the presence of unusually large amounts of bowels in pelvis, (Fig. 4), which makes it difficult to differentiate the rectum from bowels.

The prescription was 45 Gy to prostate and SV in 25 fractions (Prostate + SV=CTV, CTV + margin = PTV) with 5F IMRT 6plans. Dose to 30, 50 and 70% of whole rectum and rectal wall were estimated from the DVH. In addition, dose to 1 cc at 30, 50 and 70% volumes of whole rectum or rectal wall were. At a given % volume level, the dose to that % volume was divided by the % volume in cc of the whole rectum or rectal wall. The resulting value is expressed as dose per cc at 30, 50 and 70% volumes. These data were plotted against the volumes of whole rectum or rectal wall. Pearson’s correlation coefficient ‘r’ and the two-tailed P values were estimated.

Results: The volume of whole rectum varied from 25 to 177 cc between patients (Fig. 5). Larger the volume of whole rectum, larger was also the volume of rectal wall (Fig. 5, 18-68cc, P<0.01). Dose to rectal wall was less in 10 patients by 6- 21% at 50% volume, compared to doses to whole rectum. This difference in doses seen for large whole rectum volumes decreased beyond 20% volume and disappeared below 10% volume (Fig. 6). Results: The volume of whole rectum varied from 25 to 177 cc between patients (Fig. 5). Larger the volume of whole rectum, larger was also the volume of rectal wall (Fig. 5, 18-68cc, P<0.01). Dose to rectal wall was less in 10 patients by 6- 21% at 50% volume, compared to doses to whole rectum. This difference in doses seen for large whole rectum volumes decreased beyond 20% volume and disappeared below 10% volume (Fig. 6).

Dose to 30%, 50% or 70% volumes of whole rectum (Fig. 7) or rectal wall (Fig. 8, except at 50% volume, P 0.2). This was because the same dose-volume constraints were placed on whole rectum independent of variations in whole rectum volumes between patients. However, smaller the volume of rectum, higher was the dose per cc of whole rectum (Fig. 9 and 10) or rectal wall (Fig. 11 and 12) and vice versa (p 0.2). This was because the same dose-volume constraints were placed on whole rectum independent of variations in whole rectum volumes between patients. However, smaller the volume of rectum, higher was the dose per cc of whole rectum (Fig. 9 and 10) or rectal wall (Fig. 11 and 12) and vice versa (p<0.001). This was due to the fact that dose per cc takes into account of the variations in the volume of whole rectum or rectal wall between patients.

Dose conformity to PTV and dose to rectum were interrelated and the resulting IMRT plan was a combination a trade of between dose to rectum and dose conformity to PTV. When the volume of rectum was small, dose-volume constraints (DVC) for rectum could only be achieved at the expense of dose conformity to the PTV. For example, in the case of a rectum with a volume of 25 cc, rectal dose to 30 % volume was 35 Gy for PTV coverage of % of prescription dose (heterogeneity index (HI), max/min, = 1.19). Conversely, for a rectal dose of 31.8 Gy to 30% volume, the dose coverage to PTV was % of the prescription dose (HI = 1.47).

Conclusions: Rectal volumes vary from patient to patient. Larger the volume of whole rectum, larger was also the volume of rectal wall. In IMRT, as in the case of 3D, patients with smaller rectal volumes would be at a higher risk for rectal toxicity because dose delivered to per cc of rectum increases with the decrease in the volume of rectum. Therefore, it is suggested that: 1) dose per cc of rectum in addition to the doses to 30, 50 and 70% volumes may be used to predict rectal toxicity more reliably and, 2) that the use of rectal wall volumes to express dose to rectum would be more accurate and reliable because rectal wall volume changes due to rectal fillings would be minimal.

References: 1. M.W. Skwarchuk et. al. Int. J. Radiat. Oncol. Biol. Phys. 47, , A. Jackson et. al. Int. J. Radiat. Oncol. Biol. Phys. 49, , E.H. Huang et. al. Int. J. Radiat. Oncol. Biol. Phys. 54, , N. Reddy et. al. Med. Phys. 30, 1505, P.C.M. Koper et. al. Int. J. Radiat. Oncol. Biol. Phys. 58, , 2004 References: 1. M.W. Skwarchuk et. al. Int. J. Radiat. Oncol. Biol. Phys. 47, , A. Jackson et. al. Int. J. Radiat. Oncol. Biol. Phys. 49, , E.H. Huang et. al. Int. J. Radiat. Oncol. Biol. Phys. 54, , N. Reddy et. al. Med. Phys. 30, 1505, P.C.M. Koper et. al. Int. J. Radiat. Oncol. Biol. Phys. 58, , 2004

Whole rectum Rectal Wall Rectal Fillings

Bladder Rectum Prostate SV Sigmoid flexure S3

Rectum Bowels