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Boost : What margin?

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E. Maggiulli et al.

In any case, our findings suggest that after few

fractions it is likely that the irradiation of the whole

rectum contributes to a reduction of the ability of

rectum to hold air and stool with the consequent

“ normalisation” of the rectal ampulla.

In the rectal cancer scenar io, most repor ts deal

with mesorectum variations and are mostly based on

the analysis of few fractions of a conventionally frac-

tionated treatment or in shor t-course approaches

and consequently based on relatively limited statis-

tics [12–14] ; these studies have repor ted systematic

large ( 10 mm) variations of mesorectum during

treatment together with a quite large random vari-

ability especially at the anterior border, mainly due

to variable bladder filling.

To our knowledge, only two papers have recently

% envelope included

100

margin (mm)

% volume

Figure 4. T he percentage of rectal envelope (with range) included

in the expanded median rectum vs. isotropic margin referring to

ions (0.67 0.09 vs. 0.75 0.03; p 0.00001, t-test).

imilarly, the average value referring to the first four

ractions was worse than the remaining fractions

0.70 0.04 vs. 0.75 0.03; p 0.00001, t- est).

of the rectal envelope was included by an expansion

of 12 mm and 5 mm of the median contours when

considering the whole or the second half of the treat-

ment respectively.

When considering the first MVCT or the kVCT

rectum as the reference contours, smaller margins

were adequate compared to the expansions from the

median contour of the whole treatment (Table II).

T his result is not surprising and depends on the

Time-trend

100

120

140

160

180

200

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

Fraction

igure 1. T ime-trend analysis of rectal volume variation during

reatment. T hin dotted lines (& black squares): patients with

ignificant trend (p 0.05); grey squares: patients without trend;

ontinuous thick black line: average values; thick black dotted line:

olynomial fit of the average trend.

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18

fraction

Average DSC

0.60

0.65

0.70

0.75

0.80

0.55

Figure 3. T he mean similarity coefficients (DSC) referring to the

agreement between the rectum at each k

fraction and at the

others N-1 fractions are shown: the values ignificantly i crease

after the first few fractions.

Maggiulli E et al. Acta Oncol 2012

Rectal motion during neo-adjuvant radiochemotherapy for rectal cancer

321

T he time-trend analysis revealed a significant

correlation (p 0.05) between volume and time in

6/10 patients, as shown in Figure 1.

On average, the Spearman’s rank correlation coef-

ficient R was 0.84 (p 0.0005, 95% confidence inter-

val (CI) 0.62; 0.94); of interest, R was correlated

with the rectal volume at the first MVCT (p 0.03).

T he time-trend was significant in the first par t of

the treatment (R 0.98, 95% CI 0.92; 0.997,

p 0.005) but not when consider ing the second par t

(R 0.42, 95% CI 0.85; 0.34 p 0.24), confirm-

ing that the rectal volume reduction occurred in the

first fractions.

By fitting the data with a polynomial curve, as

shown in Figure 1, the average reduction was found

to be around 35 cm

in the first 9 fractions (4 cm

fraction). When consider ing t nvelope v lume ,

the values referring to the first par t of the treatment

were always larger than thos referring to the second

half (Figure 2) with average values equal to 129

76 cm

and 87 23 cm

(p 0.002), respectively.

Contour agreement analysis using DSC

M ean DSC values were found to increase after the

first few fractions (Figure 3). The mean DSC refer-

ring to the first fraction was significantly worse than

the mean DSC value referring to all the other frac-

tions (0.67 0.09 vs. 0.75 0.03; p 0.00001, t-test).

Similarly, the average value referring to the first four

fractions was worse than the remaining fractions

(0.70 0.04 vs. 0.75 0.03; p 0.00001, t-test).

Margins for rectal motion

A summary of the results is shown in Figure 4 and

Table I I .

When consider ing the median rectum of the

whole treatment as the reference contour, margins of

10 mm and 15 mm included about 95% of the rectal

envelope for seven and 10 patients, respectively.

Instead, when consider ing the second half of treat-

ment, margins of 5 mm and 7mm were adequate for

eight and 10 patients, respectively; on average, 95%

of the rectal envelope was included by an expansion

of 12 mm and 5 mm of the median contours when

consider ing the whole or the second half of the treat-

ment respective y.

When consider ing the first MVCT or the kVCT

100

150

200

250

300

350

1 2 3 4 5 6 7 8 9 10

patient

rectal envelope (cm

)

Figure 2. T he volumes of the rectal envelope of the first half

(dark), second half (grey) and total treatment (white) are plotted

for each patient.

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For personal use only.

12mm = 95% of the rectum covered

5mm sufficie t in the sec nd half