Intensity modulated radiation therapy (IMRT): differences in target volumes and improvement in clinically relevant doses to small bowel in rectal carcinoma
Intensity modulated radiation therapy (IMRT): differences in target volumes and improvement in clinically relevant doses to small bowel in rectal carcinoma
Henry Mok 0
Christopher H Crane 0
Matthew B Palmer 2
Tina M Briere 1
Sam Beddar 1
Marc E Delclos 0
Sunil Krishnan 0
Prajnan Das 0
0 Department of Radiation Oncology, The University of Texas, M.D. Anderson Cancer Center , Houston, Texas , USA
1 Department of Radiation Physics, The University of Texas, M.D. Anderson Cancer Center , Houston, Texas , USA
2 Department of Medical Dosimetry, The University of Texas, M.D. Anderson Cancer Center , Houston, Texas , USA
Background: A strong dose-volume relationship exists between the amount of small bowel receiving low- to intermediate-doses of radiation and the rates of acute, severe gastrointestinal toxicity, principally diarrhea. There is considerable interest in the application of highly conformal treatment approaches, such as intensity-modulated radiation therapy (IMRT), to reduce dose to adjacent organs-at-risk in the treatment of carcinoma of the rectum. Therefore, we performed a comprehensive dosimetric evaluation of IMRT compared to 3-dimensional conformal radiation therapy (3DCRT) in standard, preoperative treatment for rectal cancer. Methods: Using RTOG consensus anorectal contouring guidelines, treatment volumes were generated for ten patients treated preoperatively at our institution for rectal carcinoma, with IMRT plans compared to plans derived from classic anatomic landmarks, as well as 3DCRT plans treating the RTOG consensus volume. The patients were all T3, were node-negative (N = 1) or node-positive (N = 9), and were planned to a total dose of 45-Gy. Pairwise comparisons were made between IMRT and 3DCRT plans with respect to dose-volume histogram parameters. Results: IMRT plans had superior PTV coverage, dose homogeneity, and conformality in treatment of the gross disease and at-risk nodal volume, in comparison to 3DCRT. Additionally, in comparison to the 3DCRT plans, IMRT achieved a concomitant reduction in doses to the bowel (small bowel mean dose: 18.6-Gy IMRT versus 25.2-Gy 3DCRT; p = 0.005), bladder (V40Gy: 56.8% IMRT versus 75.4% 3DCRT; p = 0.005), pelvic bones (V40Gy: 47.0% IMRT versus 56.9% 3DCRT; p = 0.005), and femoral heads (V40Gy: 3.4% IMRT versus 9.1% 3DCRT; p = 0.005), with an improvement in absolute volumes of small bowel receiving dose levels known to induce clinically-relevant acute toxicity (small bowel V15Gy: 138-cc IMRT versus 157-cc 3DCRT; p = 0.005). We found that the IMRT treatment volumes were typically larger than that covered by classic bony landmark-derived fields, without incurring penalty with respect to adjacent organs-at-risk. Conclusions: For rectal carcinoma, IMRT, compared to 3DCRT, yielded plans superior with respect to target coverage, homogeneity, and conformality, while lowering dose to adjacent organs-at-risk. This is achieved despite treating larger volumes, raising the possibility of a clinically-relevant improvement in the therapeutic ratio through the use of IMRT with a belly-board apparatus.
Although surgery is necessary to achieve long-term cure
for locally-advanced rectal cancer, randomized data has
demonstrated the role for adjuvant therapy in this
disease. The use of adjuvant radiation has been shown to
significantly reduce the rate of local failure , with
further improvement achieved with its concurrent
administration with chemotherapy [2,3]. Moreover,
Sauer and colleagues, demonstrated that preoperative
chemoradiation was superior with respect to the rates of
local recurrence and sphincter preservation compared to
postoperative therapy . The recently published
NSABP R-03 trial demonstrated a significant
improvement in 5-year disease-free survival with preoperative
therapy, and a trend toward improved overall survival at
The safe, effective, and tolerable administration of
preoperative chemoradiation in rectal cancer is not without
challenge, owing in part to the irradiation of a large
volume at risk for microscopic disease spread, with
potential toxicity to nearby bowel, bladder, and bones.
Indeed, acute grade 3 or higher gastrointestinal toxicity
in the form of severe diarrhea was reported to be 12%
by Sauer and colleagues , with modern series
reporting rates as high as 29%. Additionally, a strong
dosevolume relationship between the amount small bowel
receiving intermediate- and low-doses of radiation and
the rates of severe diarrhea has been demonstrated,
particularly at the 15-Gy dose level [7-10]. Higher rates of
acute severe toxicity may potentially lead to breaks in
treatment or mitigate compliance, which may confer
untoward consequences with respect to local control or
Techniques have been utilized with the aim to reduce
the volume of small bowel irradiated, such as the use of
prone positioning with a belly-board apparatus to
achieve bowel displacement away from the field .
Additionally, there has been interest in the application
of highly conformal treatment approaches, such as
intensity-modulated radiation therapy (IMRT).
Wholepelvis IMRT has been applied to gynecologic
malignancy, with less toxicity than traditional 3D conformal
radiation therapy (3DCRT). In anal cancer, IMRT
has been compared to 3DCRT, showing similar target
coverage with reduced dose to the genitals, femoral
heads, small bowel, and iliac crest [14,15]. In
comparison, the data for IMRT in rectal cancer are relatively
sparse. Guerrero Urbano and colleagues compared
IMRT with 3DCRT in five patients, and found small
bowel sparing with IMRT only at the 40-Gy level and
higher . Tho and colleagues selected eight patients
with the greatest volumes of small bowel irradiated from
their cohort of patients, and observed an overall
reduction in small bowel mean dose using IMRT, with
evidence of sparing at high- and low- dose levels on a
case-by-case basis . In one of the largest series to
date, Arbea and colleagues evaluated plans generated
from 15 patients, and found using IMRT a significant
reduction of dose to small bowel in the range of 40-Gy
and higher; relationships at the intermediate- and
lowdose levels were not explicitly reported .
Furthermore, the use of preoperative IMRT with concurrent
capecitabine and oxaliplatin is currently under
investigation in the recently completed phase II protocol, RTOG
Therefore, the aim of our study is to further elucidate
the potential role for IMRT in the management of
locally-advanced carcinoma of the rectum with respect
to minimizing dose to relevant normal tissue structures
including the bladder, bones, and bowel, through direct
dosimetric comparisons with 3DCRT techniques.
Ten patients recently treated preoperatively for
adenocarcinoma of the rectum at the University of Texas M.D.
Anderson Cancer Center were identified. These patients
were representative of the breadth of disease typically
encountered at this institution for preoperative
chemoradiotherapy. Six patients were male, and four were female.
All ten patients had clinical T3 disease. One patient was
clinically node-negative, while nine were clinically
nodepositive. No patient had evidence of distant metastasis.
All patients received concurrent fluoropyrimidine-based
chemotherapy, typically with capecitabine.
3-field belly board plans
All patients were simulated and received treatment in
the prone position using a carbon-fiber belly board
apparatus (CIVCO Medical Systems, #125012) to
achieve displacement of abdominal contents, which is
the current standard practice at our institution.
Computed tomography (CT) simulation was used in all
patients. No specific bladder filling instructions were
given to patients. No bowel contrast agent was used at
the time of simulation. The plans used clinically
[henceforth: 3-field belly board (3FBB)] consisted of a primary
treatment to a prescribed dose of 45-Gy using a 3-field
approach (PA and opposed laterals with wedges),
typically without the use of any field-in-field optimization,
followed by a localized boost for an additional 5.4-Gy
using opposed lateral fields, using exclusively 18-MV
photons and 1.8-Gy daily fractions. The intended
targeted tissues included the gross tumor and nodal
disease, which were contoured based on the CT simulation
scan, mesorectum, and the internal iliac and presacral
lymph nodes. Classic anatomical field borders were
employed, with the superior field border at L5/S1, and
inferior border at the level of the ischial tuberosities or
3-cm below the caudal-most extent of the tumor. For
the PA field, the lateral field borders were placed 2-cm
beyond the pelvic inlet. For the lateral fields, the
anterior border was 3-cm anterior to the sacral promontory,
and the posterior border was placed sufficient to expose
a 1-cm margin on the posterior sacral bony contour.
Multileaf collimator (MLC) blocking was utilized to
block normal tissues outside of the intended targeted
tissues. For the purposes of this study, given a lack of
consensus with regard to delineation of boost volumes
for rectal cancer , only the 45-Gy primary fields
Target volumes and dose prescription for 3DCRT and
An IMRT plan as well as a 3DCRT plan designed to
cover the PTV (henceforth: 3DCRT) were generated for
each patient from the initial CT simulation scan data.
All cases were contoured by a single physician, and
subsequently reviewed by an attending physician.
Delineation of the clinical target volume (CTV) included the
gross tumor and involved lymph nodes, mesorectum,
presacral and internal iliac lymph node regions, with
appropriate margin, as described in the RTOG
consensus contouring atlas for anorectal cancer . CTV to
planning target volume (PTV) expansions of 7-mm were
As noted above, the total prescription dose used in
this study was limited to 45-Gy in 1.8-Gy daily fractions,
without further boost.
Organs at risk (OAR)
The relevant OAR volumes for this study were the
bladder, femoral heads/necks, pelvic bones, small bowel,
sigmoid/colon, and normal tissues. The bladder was
contoured according to the CT simulation scan. The
femoral heads/necks contours consisted of the bilateral
femoral heads and necks to the level of the lesser
trochanter. The pelvic bones contours were defined as the
exterior of the bony table from top of the iliac crests to
the ischial tuberosities. Differentiation of small bowel
from sigmoid and colon was aided through correlation
with the diagnostic, contrast-enhanced CT study closest
in time to the date of simulation. The small bowel and
sigmoid/colon volumes consisted of individual loops of
bowel, contoured up to 2-cm above the superior-most
PTV slice. The normal tissues contours were defined by
the external contour, extending to 2-cm above and
below the superior- and inferior-most PTV slices,
All plans were generated using the Pinnacle version
8.0 m treatment planning system (Philips Healthcare),
using MLC-equipped megavoltage linear accelerator
delivery. For the 3DCRT and IMRT plans, the original
CT simulation datasets from each patient were restored,
and contoured as delineated above. For the 3DCRT
plans, the field borders were modified from the 3FBB
plans with the goal of covering greater than 95% of the
PTV volume with the prescription dose, which was
prescribed to the isocenter or a calculation point, and
renormalized based on PTV coverage. Additional
fieldin-fields were utilized in all cases for homogeneity
control, to limit hotspots to 107% of the prescription dose,
particularly to anterior, bowel-containing regions.
18-MV photons were used for all 3DCRT plans.
IMRT treatment plans were generated with respect to
delivery using only 6-MV photons via linear accelerators
equipped with Millennium 120 MLC (Varian Medical
Systems). Several beam arrangements were tested, with
optimal results achieved using a 7-beam arrangement
with the following gantry angles: 0, 40, 70, 95, 265,
290, and 320. The collimator was set to 90, with a
total of 70 control points allocated to all beams. Direct
machine parameter optimization (DMPO) was used, and
at the discretion of the optimization algorithm, fields
were split for all beam angles. In terms of general
planning strategy, highest priority was given to PTV
coverage, then to minimizing dose to small bowel. Of
intermediate priority were reducing dose to the pelvic
bones, bladder, and normal tissues outside the
contoured regions; no specific optimization for sigmoid/
colon volume was performed, but instead a general
anterior abdominal contents avoidance structure was
used. Lowest effort was applied to minimizing dose to
the femoral head/neck. Collapsed-cone (CC)
convolution methods were employed for final dose calculations.
The final IMRT plans were independently reviewed and
deemed clinically acceptable by both a gastrointestinal
clinical physicist and radiation oncologist.
Plan evaluation and statistical tools
Evaluated volumes included the PTV and relevant
normal tissue volumes. The PTV, bladder, pelvic bones,
femoral heads/necks, and small bowel were reported as
whole volumes. The sigmoid/colon and normal tissue
were reported exclusive of any overlapping/encompassed
Dosimetric parameters were calculated using tabular
cumulative dose volume histogram (DVH) data, set to a
bin size of 1-cGy, with median values reported. By
convention, DX% = dose received by X% of the volume of
interest, and VX Gy = percent volume of interest
receiving at least a dose of X Gy. Maximum dose was
expressed as D1%, minimum dose as D99%, mean dose as
Dmean, and maximum point dose as Dmax. The
homogeneity index (HI) and conformality index (CI) were
calculated for the 3DCRT and IMRT plans. HI was expressed
as (D5% - D95%) / prescription dose. CI was expressed as
the ratio of the absolute volume receiving the
prescription dose to the volume of the target, V45Gy / VPTV.
Plan average cumulative DVH values were calculated
by exporting tabular DVH data set to a bin size of
10-cGy, and were plotted. For the small bowel, a curve
based on the absolute volume irradiated was also
generated. Integral dose to all tissues (including PTV) was
calculated from the differential DVH data set to 10-cGy
For statistical analysis, each patients IMRT plan was
compared in a pairwise manner with both the 3FBB and
3DCRT plans, respectively. Non-parametric statistical
analyses were performed using the paired, two-tailed
Wilcoxon signed-rank test, with p-value < 0.05 taken to
Dose to target volumes
When comparing the 3FBB treatment volumes to the
contoured volumes based on RTOG consensus
guidelines, it was evident that the contoured PTV
encompassed a typically larger volume than that treated in the
3FBB plans. This was most pronounced superiorly, but
was also seen in the extent of the PTV anterior to the
sacral promontory, and occasionally in the inferior
extent of the field. Indeed, dosimetric comparisons
between 3FBB and IMRT plans, as shown in Table 1,
revealed that the percentage of the PTV receiving the
prescription dose was significantly lower for the 3FBB
plans than with IMRT (V45Gy: median 3FBB 87.2%
versus IMRT 99.5%; p = 0.005). Therefore, a 3DCRT plan
was generated in each case using techniques described
in the methods to adequately cover the PTV. This was
quite effective, as the 3DCRT V45Gy was increased to a
median of 98.4%, though still statistically inferior
compared with IMRT (p = 0.02). Mean doses were similar
between the 3DCRT and IMRT plans (p = 0.46).
With respect to target coverage, the minimum dose to
the PTV, D99%, was higher with IMRT compared to the
3FBB (p = 0.005) and 3DCRT (p = 0.01) plans. Maximum
dose to the PTV, D1%, was significantly lower with IMRT
in comparison to 3FBB (p = 0.007); results were similar
between IMRT and 3DCRT (p = 0.35). Both the
homogeneity and conformality indices were significantly better
with IMRT compared to 3DCRT (p = 0.007 and p = 0.005,
respectively). Graphically, these findings are reflected in
the averaged cumulative DVH plot (Figure 1A).
Dose to organs at risk and normal tissues
With respect to mean dose, IMRT compared to 3FBB
showed little difference for the bladder, femoral heads,
sigmoid, and small bowel. However, compared to
3DCRT, IMRT resulted in significantly lower mean dose
to the bladder (p = 0.007), sigmoid (p = 0.005), small
bowel (p = 0.005), and to the femoral heads (p = 0.03).
Mean dose to the pelvic bones was significantly lower
with IMRT compared with either 3FBB (p = 0.04) or
3DCRT (p = 0.005).
With respect to high dose, IMRT significantly
improved the V40Gy to the femoral heads (p = 0.01) and
pelvic bones (p = 0.005) compared to 3FBB, and to the
bladder (p = 0.005), femoral heads (p = 0.005), and
pelvic bones (p = 0.005) in comparison to 3DCRT. For the
dose to sigmoid/colon, IMRT was comparable to 3FBB
at all dose levels evaluated, but was significantly lower
compared to 3DCRT (p = 0.005).
Volumetric evaluation of total small bowel was
performed at dose levels ranging from 5- to 45-Gy. When
IMRT was compared to 3FBB, the V15Gy was
significantly reduced with IMRT (p = 0.03), but similar at
other doses. IMRT compared to 3DCRT showed
significant reductions in the volumes of small bowel irradiated
at levels ranging from 15- to 45-Gy (p < 0.01). With
respect to V15Gy, the magnitude of the difference in
median volumes was modest (138-cc IMRT versus
157cc 3DCRT; p = 0.005) when evaluating the ten patients
as a whole. However, the most profound bowel sparing
was evident in the subset of patients with the largest
volume of small bowel in proximity to the treatment
field. For example, in the 6 patients with the highest
volume of small bowel (range: 209 - 537-cc), the volume
of bowel receiving 15-Gy was reduced from a median of
231-cc in the 3DCRT plans to 185-cc with IMRT.
Conversely, in the remaining four patients, only a slight
absolute reduction was evident (median V15Gy: 13-cc
IMRT versus 22-cc 3DCRT).
Normal tissues outside the target were evaluated, and
IMRT plans had a significantly higher mean dose (p =
0.02) and V10Gy (p = 0.01) to V30Gy (p < 0.02) in
comparison to the 3FBB plans. However, at the highest
doses, IMRT was significantly lower (V40Gy, p = 0.02;
V45Gy, p < 0.01). IMRT, compared to 3DCRT, had a
significantly lower mean dose (p = 0.007), V40Gy (p =
0.005) and V45Gy (p = 0.005), with more modest, but
significant, differences at V10Gy (p = 0.005) and V20Gy
(p = 0.01).
Averaged cumulative DVH plots for organs-at-risk
and normal tissues are depicted in Figure 1.
Representative axial slices showing isodose distributions for an
IMRT and a 3DCRT plan for one patient are shown in
Plan summary characteristics
Monitor units were significantly higher with IMRT
compared to either 3FBB (p = 0.005) or 3DCRT (p = 0.005)
(Table 2). The overall plan maximum doses were similar
between IMRT and 3FBB, but higher with IMRT
compared to 3DCRT (p = 0.005). Integral dose, calculated
for all tissues including the target volume, was
significantly higher for IMRT compared to 3FBB (p = 0.007),
but lower compared to 3DCRT (p = 0.007).
In this study, we found that the application of IMRT for
rectal cancer gave excellent results in comparison to
non-IMRT based approaches. With respect to the PTV,
we found that IMRT plans achieved superior coverage,
homogeneity, and conformality in treating the gross
disease and at-risk pelvic nodal volume, in comparison to
3DCRT plans targeting the PTV. This was not at the
expense of adjacent organs-at-risk, as some measure of
sparing was evident for all organs-at-risk evaluated: small
bowel, sigmoid, pelvic bones, bladder, and femoral heads
(IMRT versus 3DCRT). In this comparison, IMRT
actually decreased the overall integral dose to all tissues, and
achieved lower mean doses to normal tissues outside the
PTV, which was evident especially in the high dose
range. As expected, IMRT required significantly more
monitor units per fraction, compared to 3DCRT.
We found quite interesting the discrepancy between the
size of the volumes encompassed by the PTV, which were
generated according to the RTOG consensus contouring
atlas , and the volumes treated according to classic
anatomic landmarks (3FBB), even considering the
anticipated patient-to-patient anatomical variation. This was
reflected in the significantly lower proportion of the PTV
Figure 1 Averaged cumulative dose-volume histograms. Averaged cumulative dose-volume histograms for (A) PTV, (B) bladder, (C) femoral
heads and necks, (D) pelvic bones, (E) sigmoid outside of PTV, (F) small bowel (relative), (G) small bowel (volumetric), and (H) normal tissues
outside PTV, for IMRT, 3FBB, and 3DCRT.
volume receiving the prescription dose in the 3FBB plans,
and to a certain extent the significantly lower overall
integral dose, compared to IMRT. We found that despite the
significantly larger volume targeted in the IMRT plans,
IMRT achieved either similar or improved dose levels to
all organs-at-risk evaluated. For example, the small bowel
irradiated had similar mean doses, and the absolute
volumes irradiated were similar from the 5- to 45-Gy
levels, except at 15-Gy, where IMRT was statistically
improved, compared to the 3FBB plans.
In terms of acute, severe treatment-related toxicity,
diarrhea is the most common, and studies have
Figure 2 Representative axial slices. Representative axial slices showing isodose distributions for two planes for an (A), (C) IMRT and (B), (D)
demonstrated a strong dose-volume relationship with
small bowel irradiated [7-10]. Baglan and colleagues
demonstrated a strong association between the rate of
small bowel toxicity and the V15Gy level; when the V15Gy
was below 150-cc, low rates of grade 2 or higher toxicity
were observed, while the majority of patients with V15Gy
over 300-cc had grade 3 or higher toxicity .
Subsequent studies by Robertson and colleagues have
confirmed the significance of the V15Gy dose level, as well
Table 2 Plan summary comparison of IMRT and 3DCRT
plans: median value (range)
MU/fraction 786 (730 - 950) 238 (224 - 272) 242 (232 - 276)
Dmax (Gy) 48.8 (48.4 - 49.4) 48.8 (48.1 - 51.0) 48.2 (47.8 - 49.2)
Integral dose 2.74 (2.39 - 4.03) 2.56 (2.15 - 3.60) 2.86 (2.49 - 4.12)
Abbreviations: MU = monitor units; IMRT = intensity modulated radiation
therapy; 3FBB = 3 field belly board; 3DCRT = 3 dimensional conformal
radiation therapy;denotes statistically significant difference with IMRT as
comparator, p < 0.05 (*) or p < 0.01 (); otherwise, not statistically significant.
as other intermediate dose levels, including the V20Gy
and V25Gy, with respect to severe diarrhea [9,10]. In our
study, we found IMRT achieved significant sparing in
terms of the mean dose to small bowel and absolute
volumes from V15Gy to V45Gy, whereas no difference was
seen at the lowest dose level evaluated, V5Gy, compared
to the 3DCRT plans. This sparing at the V15Gy level was
most pronounced in the cases with the highest volumes
of small bowel within or nearby the PTV. Therefore, we
would predict a lower rate of severe, acute
gastrointestinal toxicity in these patients treated with IMRT.
Furthermore, reduction in the small bowel V45Gy using
IMRT may lead to lower rates of late gastrointestinal
toxicity . Again, in the comparison between IMRT
and classic bony landmark-derived 3FBB fields, despite a
more extensive volume treated with IMRT, we would
predict similar, or based on the V15Gy, possibly
improved rates of severe, acute gastrointestinal toxicity
with IMRT compared to 3FBB.
In the context of other planning studies comparing
IMRT with 3DCRT, we feel overall our results are
superior and additive. Prior studies have demonstrated a
reduction in small bowel mean dose , or
improvement at the high-dose extreme [16,17], with the use of
IMRT. With respect to positioning, while all three
studies employed prone positioning, one achieved
immobilization using a foam cushion , whereas two made
no specific reference to the use of a bowel displacement
device [8,16]. In contrast, using a rigid, carbon-fiber
belly board apparatus, we observed a significant
improvement in small bowel dose from 15-Gy through
the 45-Gy level, as well as the mean dose, with IMRT
compared to 3DCRT plans. Therefore, our study
demonstrates a further significant interval improvement
in small bowel dose is realized with the use of IMRT in
conjunction with the carbon-fiber belly board. An
additional strength of our study is that our contoured
volumes conformed to the RTOG consensus guidelines.
We chose as a class-solution approach to use an
asymmetric, seven-beam arrangement, biased against
anterior-directed beams, thus minimizing beam entry
through anterior-lying bowel contents or through the
belly-board apparatus. This appeared to take advantage
of strengths of the 3-field beam arrangement, namely
sharp dose falloff in the intermediate- and low-dose
range anteriorly. Indeed, recently-published studies of
IMRT, using 5- to 9-equispaced beams, have principally
demonstrated reduced small bowel mean dose and
V40Gy, compared to 3DCRT [8,16,17]. In our study, in
addition to these findings, we found IMRT capable of
reducing small bowel volumes receiving potentially
toxicity-inducing intermediate- and low-dose irradiation, at
a statistically-significant level. Concomitantly, IMRT
achieved superior PTV target coverage, homogeneity,
and conformality, as well as evidence of sparing of all
other organs-at-risk evaluated in this study. Again, our
results support a clear dosimetric advantage for IMRT,
even with the use of prone-positioning on a belly-board
With respect to the volume of the irradiated target,
there are at least two different ways to consider this
issue. In our study, the PTVs, generated with a 7-mm
expansion, were typically larger than the volume treated
using classic 3FBB fields. Given the excellent historical
results obtained with the classic 3FBB fields, one
interpretation is that the target volumes, as delineated by the
RTOG consensus IMRT contouring atlas for anorectal
disease, may be more generous than necessary.
Alternatively, as we found that the more comprehensive PTV
target coverage was achieved without increasing dose to
the organs-at-risk including the small bowel, it is
conceivable that improved efficacy is attainable without
increasing acute- and long-term toxicities through the
use of IMRT. Long-term clinical data would be
necessary to provide evidence for this. As an additional point,
the use of IMRT does not automatically confer normal
tissue sparing, as an excessively voluminous target
volume may in fact lead to higher absolute volumes of
normal tissues treated. This reinforces the importance
of consensus target delineation to achieve
standardization from practice-to-practice.
Due to daily setup uncertainties using the rigid
carbon-fiber belly-board apparatus, for IMRT treatment of
a CTV-to-PTV expansion of 7-mm used in this study, it
may be worthwhile to consider daily kilovoltage imaging,
or perhaps modifications such as the incorporation of a
vacuum-cradle device to improve setup reproducibility.
One potential criticism for intensity modulated
treatment approaches is with respect to integral dose,
whereby larger volumes of normal tissues are exposed
to lower radiation doses, which may lead to increased
incidence of second malignancies . In our study, we
found a lower integral dose with IMRT compared to
3DCRT plans targeting the PTV. However, integral dose
was slightly higher with IMRT than in the classic 3FBB
Another potential downside of a static-field intensity
modulated therapy approach is a longer beam-delivery
time that is required as compared to 3DCRT, with
respect to intrafractional motion. This may be overcome
using volumetric-modulated arc therapy (VMAT) based
For the adjuvant treatment of rectal carcinoma, IMRT,
compared to 3DCRT, yielded plans superior with
respect to target coverage, homogeneity, and
conformality, while lowering dose to adjacent organs-at-risk. This
benefit was seen additive to the use of prone-positioning
on a belly-board apparatus, and with respect to small
bowel toxicity, could potentially be clinically significant.
HM carried out the study conception and design, drafted the manuscript,
and performed treatment planning. PD carried out the study conception
and design and drafted the manuscript. MBP performed treatment planning.
TMB and SB performed physics checks/plan evaluation. Patient accrual and
radiation field design were performed by CHC, MED, SK, and PD. CHC
provided mentorship for this work. All authors read and approved the final
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