Prone versus supine positioning for whole and partial breast radiotherapy

1. Partial breast radiotherapy
Prone versus supine positioning for whole and partial-breast radiotherapy:
A comparison of non-target tissue dosimetry
Anna M. Kirby a,*, Philip M. Evans b
, Ellen M. Donovan a
, Helen M. Convery a
, Joanna S. Haviland b
,
John R. Yarnold b
a
Royal Marsden NHS Foundation Trust, Sutton, UK; b
Institute of Cancer Research, Sutton, UK
a r t i c l e i n f o
Article history:
Received 14 January 2010
Received in revised form 17 May 2010
Accepted 23 May 2010
Available online 17 June 2010
Keywords:
Breast cancer
Prone breast radiotherapy
Partial-breast irradiation
Cardiac dosimetry
a b s t r a c t
Purpose: To compare non-target tissue (including left-anterior-descending coronary-artery (LAD))
dosimetry of prone versus supine whole (WBI) and partial-breast irradiation (PBI).
Methods and materials: Sixty-five post-lumpectomy breast cancer patients underwent CT-imaging supine
and prone. On each dataset, the whole-breast clinical-target-volume (WB-CTV), partial-breast CTV
(tumour-bed + 15 mm), ipsilateral-lung and chest-wall were outlined. Heart and LAD were outlined in
left-sided cases (n = 30). Tangential-field WBI and PBI plans were generated for each position. Mean
LAD, heart, and ipsilateral-lung doses (xmean), maximum LAD (LADmax) doses, and the volume of chest-
wall receiving 50 Gy (V50Gy) were compared.
Results: Two-hundred and sixty plans were generated. Prone positioning reduced heart and LAD doses
in 19/30 WBI cases (median reduction in LADmean = 6.2 Gy) and 7/30 PBI cases (median reduction
in LADmax = 29.3 Gy) (no difference in 4/30 cases). However, prone positioning increased cardiac doses in
8/30 WBI (median increase in LADmean = 9.5 Gy) and 19/30 PBI cases (median increase in LADmax
= 22.9 Gy) (no difference in 3/30 cases). WB-CTV > 1000cm3
was associated with improved cardiac
dosimetry in the prone position for WBI (p = 0.04) and PBI (p = 0.04). Prone positioning reduced ipsilat-
eral-lungmean in 65/65 WBI and 61/65 PBI cases, and chest-wall V50Gy in all WBI cases. PBI reduced normal-
tissue doses compared to WBI in all cases, regardless of the treatment position.
Conclusions: In the context of tangential-field WBI and PBI, prone positioning is likely to benefit left-breast-
affected women of larger breast volume, but to be detrimental in left-breast-affected women of smaller
breast volume. Right-breast-affected women are likely to benefit from prone positioning regardless of
breast volume.
Ó 2010 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 96 (2010) 178–184
Whole-breast irradiation (WBI) following breast-conserving
surgery (BCS) improves local control and survival from breast can-
cer but increases non-breast-cancer-related deaths by 1% at
15 years [1]. The majority of these deaths are cardiovascular in
origin [2] and irradiation of the left-anterior-descending coronary
artery (LAD) is implicated in pathogenesis [3,4]. Irradiation of lung,
chest-wall and other tissues also contributes to late mortality and
morbidity [1,2]. Improvements in radiotherapy techniques have
resulted in reduced normal-tissue doses [5], correlating with
reduced non-breast-cancer-related mortality [2]. Nonetheless
doses to heart, LAD and lung from current standard supine WBI
remain significant [6].
Methods by which normal-tissue doses could be decreased in-
clude optimization of patient positioning and use of partial-breast
irradiation (PBI). Prone positioning for WBI improves dose homo-
geneity within breast-tissue [7], reduces lung doses [8–10], and re-
duces wedge requirements with consequent reduction of scattered
dose, particularly in women of larger cup-size [8]. However, re-
ports comparing cardiac dosimetry from supine versus prone
WBI are inconsistent. One study using IMRT [11] reported that
prone positioning reduced within-field heart volume in 85% of wo-
men. However, studies using conventional tangential-field
arrangements have failed to show an overall benefit of prone posi-
tioning on cardiac dosimetry [9,12]. Furthermore, a study compar-
ing distances between anterior pericardium and chest-wall on
supine CT-images and prone MRI [13] found prone positioning to
systematically displace supero-lateral aspects of heart-tissue
closer to chest-wall, suggesting that prone positioning might be
detrimental where the target volume includes deeply-lying
breast-tissue. Thus far, LAD doses from prone positioning have
not been documented.
PBI could also reduce normal-tissue doses by restricting higher
radiation doses to the volume of breast-tissue at highest risk of
0167-8140/$ - see front matter Ó 2010 Elsevier Ireland Ltd. All rights reserved.
doi:10.1016/j.radonc.2010.05.014
* Corresponding author. Address: Department of Academic Radiotherapy, Royal
Marsden NHS Foundation Trust, Downs Road, Sutton, Surrey SM2 5PT, United
Kingdom.
E-mail address: anna.kirby@rmh.nhs.uk (A.M. Kirby).
Radiotherapy and Oncology 96 (2010) 178–184
Contents lists available at ScienceDirect
Radiotherapy and Oncology
journal homepage: www.thegreenjournal.com

2. tumour relapse [14,15]. The combination of prone positioning and
PBI is particularly attractive in terms of reducing normal-tissue
toxicity. Comparison of PBI techniques [12] found that prone tomo-
therapy (compared with supine 3-D-conformal-RT and tomothera-
py) decreased lung doses. However, heart doses again varied
between patients, prone positioning benefiting only women with
larger breasts and/or lesions further from the chest-wall.
Heart and LAD dosimetry from both prone WBI and PBI require
further evaluation before prone positioning can be adopted into
routine practice. As the majority of centres worldwide continue
to use conventional tangential-field arrangements, this study pro-
spectively evaluates non-target tissue exposure from WBI and PBI
planned conventionally in supine and prone positions using with-
in-patient comparison. This comparison aims to clarify optimal ap-
proaches to reducing normal-tissue exposure applicable to the
majority of current practices.
Methods and materials
This study was approved by the Royal Marsden Committee for
Clinical Research and the Regional Ethics Committee. All women
had undergone BCS for unifocal T1–2 G1–3 invasive-ductal
carcinoma or high-grade ductal-carcinoma-in situ, at which time
titanium-clips were placed in the tumour-bed (TB) according to a
UK protocol [16].
Patient positioning and image acquisition
Patients underwent non-contrast CT-imaging (slice-thickness
1.5 mm, C6 to below diaphragm) in the standard supine position
at which time radio-opaque wire was placed at the palpable edge
of breast-tissue circumferentially. Patients were repositioned and
imaged prone using an in-house-designed platform, with an aper-
ture through which index-breast could fall away from chest-wall
(Fig. 1). Radio-opaque wire remained in situ for prone CT-imaging.
Target and non-target tissue delineation
Whole-breast (WB) clinical-target-volume (CTV) was defined
using wire plus any additional breast-tissue visualized on CT,
limited by 5 mm from skin and chest-wall/lung interfaces. Par-
tial-breast CTV was defined as TB (clips plus seroma and/or
architectural distortion) expanded by 15 mm in 3-D (limited cir-
cumferentially by WB-CTV). Planning-target-volumes (PTV) were
generated by the addition of 3-D 10 mm margins to CTV, limited
by 5 mm from skin (according to UK National-Cancer-Research-
Institute Intensity-Modulated Partial Organ Radiotherapy
(IMPORT) study criteria [17]).
Heart and LAD were defined according to published criteria
[5]. Where LAD was difficult to visualize, its location was in-
ferred from the course of the anterior-interventricular groove
[5]. Consistent with previous practice, a 10 mm axial margin
was added to the LAD to allow for delineation uncertainty and
respiratory/cardiac motion [6]. Ipsilateral-lung was outlined
using an autocontour tool (edited to exclude major airways).
Chest-wall was defined as ipsilateral ribcage and intercostal
musculature.
Radiotherapy-planning
For each position, standard opposing tangential-fields were em-
ployed such that for WBI, P90% of WB-CTV was encompassed by
the 95%-isodose and, for PBI, P95% of partial-breast CTV was
encompassed by the 95%-isodose (according to UK IMPORT study
criteria [17]). (NB For PBI, fields encompassed the partial-breast
PTV with a 5 mm penumbra, also according to IMPORT criteria).
Plans fulfilled ICRU dose-homogeneity criteria, using 1–2 addi-
tional segment fields when required [18]. Dose-distributions were
reviewed in 3-D and using dose-volume-histogram (DVH) data.
50 Gy in 25 fractions over 5 weeks (6–10 MV photons) was pre-
scribed to the 100% isodose. MLC leaves were used as required to
reduce cardiac doses whilst maintaining satisfactory coverage of
WB and PB-CTV as defined above.
Analysis
This study was a planned prospective comparison of normal-tis-
sue doses. The difference in mean ipsilateral-lung (lungmean)dose
(biologically-weighted and normalised to 2 Gy fractions [19]) be-
tween supine and prone PBI was the primary endpoint. The study
had 95% power to detect an absolute difference of 2% between
lungmean doses for prone versus supine PBI. Secondary endpoints
included differences in mean heart (heartmean), mean LAD
(LADmean) and max LAD (LADmax) doses, and in the volumes of
chest-wall receiving 50 Gy (V50Gy). All values were determined
from DVH data.
Differences between heartmean, LADmean, lungmean, LADmax, and
chest-wall V50Gy for supine versus prone WBI, supine versus prone
PBI, and supine WBI versus PBI plans were calculated for each
patient and compared using the Wilcoxon signed-rank test. Lung
and chest-wall doses were compared in all patients, and heart/
LAD doses in left-breast-affected patients only (as a population
and by individual patient). Left-breast data were then pooled by
WB-CTV into tertiles (6500, 501–1000, and >1000 cm3
). The differ-
ences between supine and prone heartmean, LADmean and LADmax
were calculated for each patient, and compared by tertile using
the Kruskal–Wallis test.
Results
Sixty-five women were recruited (30 left-sided: 35 right-sided
BC). Mean age was 57 years (range 34–79 years). Median self-
reported UK cup-size was C (range A–G). 14 women (5 left-sided)
had WB-CTV 6 500cm3
, 29 (12 left-sided) had WB-CTV
501–1000cm3
, and 22 (13 left-sided) had WB-CTV > 1000cm3
.
There was no difference between supine and prone WB-CTVs
(p = 0.15). Satisfactory coverage of target-volumes was achieved
in 100% of plans.
Cardiac dosimetry
Heart and LAD doses for the patients with left BC (30 cases, 120
plans) are summarized in Table 1 and Fig. 2. Overall, for the
Fig. 1. Prone platform including polycarbonate centrepiece with aperture, polysty-
rene head and body supports, and polystyrene wedge to support contralateral
breast.
A.M. Kirby et al. / Radiotherapy and Oncology 96 (2010) 178–184 179

3. Table 1
Median normal-tissue doses (inter-quartile range).
Supine WBI Prone WBI p*
Supine PBI Prone PBI p*
Heartmean (Gy)a
0.9 (0.7–1.3) 0.8 (0.7–1.0) 0.10 0.3 (0.3–0.5) 0.4 (0.3–0.6) 0.09
LADmean (Gy) a
11.7 (6.1–15.5) 8.6 (4.7–13.7) 0.25 1.6 (0.9–2.4) 2.4 (1.1–5.4) 0.01
LADmax (Gy) a
49.6 (48.2–50.3) 47.6 (44.0–49.5) 0.01 29.8 (6.3–45.1) 41.4 (24.3–46.7) 0.06
Ipsilateral-lungmean (Gy)b
4.4 (3.5–5.2) 0.8 (0.3–1.4) <0.001 1.2 (0.8–2.1) 0.4 (0.2–0.8) <0.001
Chest-wall V50Gy (cm3
) b
26 (11–40) 1 (0–7) <0.001 14 (1–33) 0 (0–1) 0.03
a
n = 30.
b
n = 65.
*
Significance assessed using Wilcoxon signed-rank test.
Supine
W
B
I
Prone
W
B
I
Supine
PB
I
Prone
PB
I
0.0
0.5
1.0
1.5
2.0
HeartNTDmean(Gy)
Supine
W
B
I
Prone
W
B
I
Supine
PB
I
Prone
PB
I
0
10
20
30
LADNTDmean(Gy)
Supine
W
B
I
Prone
W
B
I
Supine
PB
I
Prone
PB
I
0
10
20
30
40
50
LADmax(Gy)
Supine
W
B
I
Prone
W
B
I
Supine
PB
I
Prone
PB
I
0
1
2
3
4
5
IpsilaterallungNTDmean(Gy)
Supine
W
B
I
Prone
W
B
I
Supine
PB
I
Prone
PB
I
0
20
40
60
80
100
Chest-wallV50Gy(cm3
)
a
c
e
d
b
Fig. 2. (a–e) Box and whisker plots displaying median normal-tissue doses (and inter-quartile ranges) by position and RT technique. (a) Heart-NTDmean (n = 30). (b) LAD-
NTDmean (n = 30). (c) LADmax (n = 30). (d) Ipsilateral-lung NTDmean (n = 65). (e) Chest-wall V50Gy (n = 65).
180 Normal-tissue dosimetry of prone versus supine breast radiotherapy

4. population of left-breast-affected women, there was no significant
difference in heart dose between prone and supine positions for
either WBI or PBI. Although differences in LADmean (for PBI) and
LADmax (for WBI) reached significance, the absolute LAD doses
were similar between the two groups as a whole.
Comparing doses by individual patient for WBI, prone position-
ing improved heart and LAD doses in 19/30 patients but increased
them in 8/30 patients. The magnitude of the difference between
LADmean doses was similar whether prone positioning improved
or worsened dosimetry (Table 2). In 3/30 patients, prone position-
ing increased LADmean but decreased LADmax. If heartmean had been
the only comparator, 20/30 would have been judged to benefit
from prone and 10/30 from supine positioning.
For PBI, prone positioning improved heart/LAD doses in 7/30
patients, but increased them in 19/30 patients. The magnitude of
the difference between LADmax doses was similar whether prone
positioning improved or worsened dosimetry. In 4/30 cases,
parameters failed to agree on optimal position. If heart-NTDmean
had been the only comparator, 13/30 patients would have been
judged to benefit from prone and 17/30 from supine positioning.
Heartmean and LADmean correlated (R = 0.80, p = 0.01), as did
heartmean and LADmax (R = 0.59, p = 0.01). Analysing results by vol-
umetric tertile, for WBI, prone positioning significantly reduced
cardiac doses for women with breast volumes >1000cm3
(p = 0.04) (Table 3). All women of D-cup or above benefited from
prone positioning with the exception of one patient (G cup) whose
TB was in the axillary-tail. In the context of PBI, prone positioning
significantly reduced heartmean and LADmean but not LADmax doses
for women with breast-volumes >1000 cm3
. For women with WB-
CTV 6 1000 cm3
, prone positioning significantly increased cardiac
doses.
Cardiac doses were significantly lower for PBI as compared to
WBI (p < 0.001). Median reductions (inter-quartile ranges) in
heartmean were 0.6 (0.4–0.9) Gy (supine) and 0.4 (0.3–0.6) Gy
(prone); in LADmean were 9.8 (4.6–12.8) Gy (supine) and 5.0 (2.7–
9.7) Gy (prone); and in LADmax were 18.6 (5.6–41.4) Gy (supine)
and 5.5 (1.2–20.7) Gy (prone).
Lung and chest-wall dosimetry
Ipsilateral-lungmean doses (65 cases, 260 plans) are summarized
in Table 1 and Fig. 2d. Prone, as compared to supine, positioning
significantly reduced lungmean in 65/65 WBI cases, and 61/65 PBI
cases. In 4/65 PBI cases, lungmean was similar in both positions be-
cause TBs were located at the lateral edge of breast-tissue such that
PB-CTV coverage would have been compromised by shallower
prone tangents. Lungmean was significantly lower for PBI as com-
pared to WBI (median reduction = 2.8 (2.0–3.8) Gy (supine) and
0.3 (0.1–0.6) Gy (prone) (p < 0.001)).
Prone positioning significantly reduced chest-wall V50Gy in the
context of both WBI and PBI (Table 1, Fig. 2e). Use of supine PBI
(compared to WBI) significantly reduced chest-wall V50Gy
(p < 0.001).
Discussion
This study aimed to compare normal-tissue (including left-
anterior-descending coronary-artery (LAD)) dosimetry from con-
ventional tangential-field whole- and partial-breast radiotherapy
planned in prone versus supine positions.
Cardiac dosimetry
We found the effects of prone positioning upon both heart and
LAD doses to be variable between patients, consistent with previ-
ous studies [8,12]. We also found a significant benefit of prone
positioning upon heart and LAD doses for women of WB-CTV >
1000 cm3
(equivalent to UK cup-size PD) for both WBI and PBI,
Table 2
Median cardiac doses (inter-quartile ranges) according to optimal position for WBI and PBI.
Supine position better (n = 8) Prone position better (n = 19)
Supine Prone Difference p*
Supine Prone Difference p*
WBI
Heartmean (Gy) 0.7 (0.6–0.8) 1.0 (0.9–1.4) 0.4 (0.2–0.7) 0.01 1.3 (0.9–1.4) 0.8 (0.6–0.9) 0.4 (0.2–0.6) <0.001
LADmean (Gy) 4.2 (2.6–6.3) 13.5 (11.5–17.0) 9.5 (5.5–10.8) 0.01 14.2 (9.5–18.6) 6.6 (3.4–9.0) 6.2 (3.0–7.8) <0.001
LADmax (Gy) 47.8 (42.7–49.2) 49.7 (48.1–50.7) 2.5 (0.5–7.1) 0.03 49.8 (48.9–50.6) 47.1 (41.9–47.7) 3.7 (2.5–7.2) <0.001
Supine position better (n = 19) Prone position better (n = 7)
PBI
Heartmean (Gy) 0.3 (0.2–0.4) 0.5 (0.4–0.6) 0.2 (0.0–0.3) <0.001 0.4 (0.3–0.5) 0.3 (0.2–0.3) 0.1 (0.0–0.2) 0.09
LADmean (Gy) 1.2 (0.8–1.6) 2.9 (2.2–7.0) 1.8 (1.2–5.3) <0.001 2.8 (1.8–4.0) 1.0 (0.8–1.9) 1.2 (0.3–3.1) 0.02
LADmax (Gy) 11.3 (4.3–29.0) 44.4 (40.8–47.8) 22.9 (15.0–37.8) <0.001 45.8 (34.4–47.9) 6.7 (3.2–39.0) 29.3 (7.8–36.6) <0.001
*
Significance tested using Wilcoxon signed-rank test.
Table 3
Median differences in cardiac doses (supine minus prone) by whole-breast clinical target volume (inter-quartile ranges in brackets).
Whole-breast clinical target volumetric tertile
<500 cm3
501–1000 cm3
>1000 cm3
p*
WBI
Heartmean (Gy) 0.2 (0.1–0.7) À0.1 (À0.4 to 0.3) À0.4 (À0.6 to À0.3) 0.01
LADmean (Gy) 5.0 (0.6–7.1) À1.3 (À6.8 to 7.8) À6.1 (À7.5 to À4.5) 0.04
LADmax (Gy) 1.6 (À1.3 to 2.0) À1.1 (À2.7 to 0.8) À4.4 (À8.1 to À2.5) 0.002
PBI
Heartmean (Gy) 0.1 (0.1–0.2) 0.2 (0.1–0.3) À0.1 (À0.1 to 0) 0.005
LADmean (Gy) 4.7 (1.4–5.5) 1.4 (0.6–2.1) À0.2 (À1 to 0.8) 0.04
LADmax (Gy) 21.3 (4.7–39.7) 19.3 (0.6–34.7) À0.6 (À30 to 21) 0.04
*
Significance of differences between tertiles assessed using Kruskal–Wallis test.
A.M. Kirby et al. / Radiotherapy and Oncology 96 (2010) 178–184 181

5. consistent with previous work reporting a trend towards a signifi-
cant reduction in heart dose in women of breast cup-size PE [9].
Our findings are also consistent with work reporting that heart-tis-
sue moves towards chest-wall in patients positioned prone [13].
Only patients in whom breast-tissue is pulled, under gravity, ante-
riorly in relation to chest-wall (such that shallower tangents can be
placed) are likely to gain from prone treatment. Otherwise, where
smaller breasts are not pulled anteriorly, prone tangents are likely
to encompass more cardiac tissue (Fig. 3). Other factors predictive
of a benefit of prone positioning may include TB location, heart-
size, and chest-wall breadth/curvature. Our study is underpowered
to detect these.
The proportion of women benefitting from prone positioning in
our study differed from interim reports of a large ongoing compar-
ative study [11]) of prone versus supine WBI, which suggest that
prone positioning reduces in-field heart volume in the majority
(85%) of left-sided BC patients. This could be firstly due to use of
volumetric rather than dosimetric comparators and/or to use of
IMRT rather than conventional tangential-fields. Using IMRT, an
in-field heart volume of 0 cm3
is achievable in many patients
regardless of position and prone positioning might then be judged
‘‘optimal” based on reduced in-field lung volume. Such an ap-
proach does not however detect differences in lower-dose irradia-
tion of cardiac tissues. Secondly, our use of LAD-dosimetry to
discern optimal position might have led to differing results.
Although heartmean and LADmean correlated, there was disagree-
ment between heart and LAD doses over optimal position in 7/60
plans. Had heartmean been the only comparator, prone positioning
would have been optimal in 20/30 WBI and 13/30 PBI cases.
Thirdly, the method of WB-CTV definition and clinicians’ decisions
on placement of the posterior field-edge in order to achieve target-
volume coverage will significantly alter normal-tissue doses close
to chest-wall. We used a method of WB-CTV definition agreed to
be more representative of the true volume than standard anatom-
ical landmarks [20]. Our cases also had clip-defined TB volumes,
without which, coverage of PB-CTV at depth cannot be ensured
[21].
The clinical impact of differences in cardiac doses is difficult to
quantify as radiation parameters determining excess cardiovascu-
lar disease (CVD) risk are poorly understood. Gagliardi [22] used
the relative-seriality model to quantitatively describe the dose–re-
sponse relationship for excess cardiac mortality and found a low
dependence of this endpoint upon irradiated-heart-volume, con-
cluding that cardiac mortality is more likely to be reduced by
decreasing dose than by restricting irradiated volume. Borger et
al. [23] also found no relationship between maximum heart dis-
Fig. 3. Relationship of breast-tissue (cyan-outline), heart (pink-outline), and LAD (red-outline with cyan-bullseye) to chest-wall for women of (i) cup-size B and (ii) cup-size F,
imaged (a) supine and (b) prone. Green colour-fill = partial-breast CTV.
182 Normal-tissue dosimetry of prone versus supine breast radiotherapy

6. tance (MHD) (a correlate of irradiated-heart-volume) and risk of
CVD but reported that, even where MHD = 0 mm, more cardiotoxic
effects occurred following left-sided as compared to right-sided-RT
suggesting that differences in doses <25 Gy may be important.
Other data supporting the hypothesis that low-dose radiation in-
creases CVD risk come from atomic-bomb survivors (4 Gy single
exposure) [24], patients irradiated for peptic-ulcer disease
(heartmean 1.6–3.9 Gy) [25], patients treated with para-aortic
irradiation for testicular cancer ($1 Gy scattered heart dose) [26]
and radiation workers [27]. Optimal positioning in our study de-
creased median heartmean from $1.3 to 0.7 Gy for WBI, and from
0.5 to 0.3 Gy for PBI. Based on the evidence above, the risks of
low-dose cardiac irradiation are not negligible. However, the
dose–effect relationship at these dose-levels is difficult to define.
Other studies suggest that LAD dose is the most relevant expo-
sure variable [3,4]. Retrospective review of patients irradiated
between 1977 and 1995 found a significantly higher prevalence of
cardiac stress-test abnormalities amongst left- versus right-side-
irradiated patients, 70% of which were in LAD territory [4]. Others
correlate a fall in mean LAD doses from WBI over the last 30 years
[5,6] with a decrease in CVD over the same period [2]. Furthermore,
it may be that Gagliardi’s finding of a low dependence of cardiac
mortality upon irradiated-heart-volume [22] relates to the fact that
the LAD is likely to remain within the high-dose volume from a tan-
gential-field arrangement even at low irradiated-heart-volumes.
Optimal positioning in our study decreased median LADmean from
$14 to 4 Gy for WBI, and from 3 to 1 Gy for PBI. Such reductions in
dose could be associated with a significant reduction in CVD [6].
As atherosclerosis anywhere along the LAD could cause CVD, LADmax
is a relevant additional variable. Optimal positioning in our study
decreased median LADmax from 50 to 47 Gy for WBI, and from 46
to 7 Gy for PBI. The latter could be particularly significant depending
partly upon the threshold dose for atherosclerosis. Gagliardi’s work
suggests that the risk of cardiac mortality rises steeply above doses
of around 20 Gy regardless of the volume irradiated [22].
Lung and chest-wall dosimetry
Our study confirms previous reports [8–10] that prone position-
ing reduces mean lung doses for both WBI and PBI, furthermore
demonstrating that benefits are applicable to women of all cup-
sizes. The main threat of death in relation to lung-tissue irradiation
is from low-dose stochastic effects, the relative-risk of death from
second-primary lung cancer (SPLC) ranging from 1.5 to 2.8 at
15 years [28,29], with odds ratios of up to 37.6 in smokers [30]. Data
on SPLC deaths in $9000 women irradiated in 1935–1971 [28]
suggest a dose–response relationship with an incremental RR of
0.2 per Gy to ipsilateral-lung (equating to 9 cases of SPLC/year/
10,000 women receiving 10 Gy to lung and living to 10 years). The
SEER registry cohort demonstrates a similar relationship between
lungmean and risk of SPLC [2] in women irradiated in 1973–2001.
In our study, prone positioning for WBI reduced median lungmean
from 4.4 to 0.8 Gy. Based on evidence above, this reduction in dose
might prevent around 3 SPLC/year/10,000 women at 10 years
post-RT. The effect is likely to be larger in women who smoke.
The START trial suggests that 40% of women experience chest-
wall discomfort at 10 years post-RT [31], whilst the incidence of
rib-fracture following WBI is reported to be 0.3–2.2% [32,33]. A
recent study of external-beam-accelerated-PBI found the incidence
of chest-wall pain and rib fracture to relate to the volume of chest-
wall receiving 35 Gy or more (based on 38.5 Gy/10 fractions 5 days
[34]). This is equivalent to 48 Gy in 2 Gy fractions, in keeping with
previously-reported tolerance doses (TD5/5 ribcage $50 Gy) [35]. In
our study, prone positioning significantly reduced chest-wall V50Gy
for WBI, thereby warranting consideration as a technique by which
chest-wall morbidity might be reduced.
PBI versus WBI
The normal-tissue dosimetric advantages of PBI have been as-
sumed but not proven. Indeed a recent study reported that 3-D-
conformal PBI increased the volume of lung exposed to low-dose
radiation whilst decreasing the volume exposed to higher-dose
radiation [36]. We found that supine PBI reduced mean heartmean
(by 0.6 Gy), LADmean (10 Gy), LADmax (19 Gy), ipsilateral-lungmean
(3 Gy) and chest-wall V50Gy (17 cm3
) compared to supine WBI.
With dose-sparing of these magnitudes, it seems likely that PBI
will reduce long-term cardiovascular side-effects of breast RT,
reduce second-primary lung malignancies by around 2 lung can-
cers/year/10,000 women at 10 years post-RT, and reduce the inci-
dence of late chest-wall discomfort.
Conclusions
In the context of tangential-field WBI and PBI, prone positioning
is likely to benefit left-breast-affected women of larger breast vol-
ume, but to be detrimental in left-breast-affected women of smal-
ler breast volume. Right-breast-affected women are likely to
benefit from prone positioning regardless of breast volume.
Conflict of interest
None declared.
Acknowledgements
Dr A Kirby was funded by a Royal College of Radiologists’ Clin-
ical Oncology Research Fellowship and by the Breast Cancer Re-
search Trust. The work was also supported by Cancer Research
UK (Section of Radiotherapy: Grant number C46/A3970). The
authors are grateful to Mr. Craig Cummings in the Institute of
Cancer Research Workshop for his assistance in the design and
manufacture of the prone breast platform. The work was under-
taken in The Royal Marsden NHS Foundation Trust which receives
a proportion of its funding from the NHS Executive; the views ex-
pressed in this publication are those of the authors and not neces-
sarily those of the NHS Executive.
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184 Normal-tissue dosimetry of prone versus supine breast radiotherapy