Shaping of the fields can be achieved using cerrobond
customized conformal blocks, a multileaf collimator (MLC)
or a mini/micro MLC [
13,
14]. MLC’s are commercially
available offers for conformal fields, and rapid treatment
procedures but beam shaping is less precise due to 10 mm
stepping of the leaves at isocentre [
14].
This study is planned to analyze and compare the dose
volume relationship of the prostate and the adjacent normal
structures with each of the two shaping techniques. The
DVH’s revealed that a complete coverage of the prostate
and seminal vesicles is achieved and the PTV was suitably
delineated with two techniques as reported by Lo Sasso et
al. and Adams et al. [10,15].
The rectum seems to be the most critical dose limiting
structure in prostate radiotherapy. The risk of developing
rectal complications increased with larger target volumes
[16,17]. In the previous studies, the late rectal toxicity was
seen particularly at doses greater than 60 Gy with 3D-CRT
[2,18,19]. Recently, Huang et al. [4] identified the optimal
cut points that most significantly discriminate those patients
at high risk of late toxicity from those patients at low risk.
To reduce the risk of late toxicity; < 40% of the defined
rectal volume should receive 60 Gy and < 25% should
receive 70 Gy and < 5% should receive 78 Gy. The rectal
toxicity related data shows that, between the low to moderate
doses range, even a larger percentage volume of rectum is
irradiated and toxicity does not occur. This may indicate
that a large surrounding region of intermediate dose may
interfere with the ability to repair the effects of central high
dose region.
In our data, the mean DVH of rectum was higher for
the plans generated with MLC than cerrobond blocks for
the irradiated volumes of rectum at the dose points of
V40Gy, V50Gy, statistically (p=0.04). On the other hand
the dose points for V60 and V70 that are adjusted to be the
critical volumes of interest did not differ significantly.
Whilst the percentage of the volume irradiated at all dose
levels with MLC was worse than the other plan; none of
the shaping configurations did show a volume of dose point
greater than the accepted critical cut points for the DVH
of the rectum.
Our data revealed that the mean volumes irradiated at
dose points for 30 Gy and 40Gy showed a statistical
difference for bladder. Whilst irradiating larger volumes
with MLC in the moderate (30-50Gy) dose region, at the
higher dose points, the irradiated volumes with MLC did
not differ from conformally individualized configurations
which indicated that with each of the blocking techniques,
the critical dose for the bladder was not achieved. As it’s
stated by some of the investigators, none of them was able
to find out any correlation between the irradiated volume
and bladder toxicity, and it was suggested that the bladder
toxicity is to be assessed as in the case of rectum [7,20].
The complication probability for the femoral heads is
in fact gathered from the calculations of NTCP and this
issue is not as clinically relevant as for rectal morbidity
[21]. Khoo and associates [3] adopted a non-pragmatic
indicator of femoral head tolerance. This was that, no more
than 10% of the femoral head volume should receive a
dose greater than 52 Gy and if a prescription dose of 74
Gy is administered, than a percentage volume of greater
than 70% was required to be below the threshold of 100%.
None of the configurations of the plans irradiated a volume
greater than 10% at 52 Gy for the right femur. Left femur
had been irradiated with a higher mean dose distribution
than 10% for the doses of greater than 52 Gy, but since it
did not reach a statistical difference, the conformal shaping
method was slightly better.
Several groups investigated normal tissue and organ
dose volume effects for conformally irradiated volumes.
Adams et al. [15] stated the normal tissue increases were
due to larger penumbral region required for MLC as in the
case of stereotacticly irradiated brain tumors. Similarly
Fernandez et al. [22], reported that the 5.5% of the treatment
plans generated with MLC shaping could not achieve an
adequate coverage of the target volume especially for
lymphoma and brain tumors because of the difficulties in
the shielding of critical structures such as back of the eye
where the larger penumbra caused inadequate irradiation
of the target volume. One of the most problematic dosimetric
differences between the two shaping techniques is the
penumbra width. The irregularity of the field edge introduced
by MLC leaf widths of 1-1.25 centimeter relative to the
smoothly verifying C blocks, is a potential disadvantage. The effective penumbra which is defined as the difference
between a line connecting the crests of the 80% isodose
level and a line connecting through of the 20% level has
been reported to be 3-5 mm larger than the corresponding
values for cerrobond shaped fields [10]. Secondly the zigzag
arrangement of the leaf edges imprints it’s own on the
penumbra which appears as the ‘scalloping effect’ of the
isodose lines, for a MLC field. In fact this effect is even
smaller than expected due to some other factors. Photon
and electron scatter in the patient is one of these factors.
The 90% isodose line have minimal ‘scalloping’ as compared
to the 50% isodose line since more of the secondary
electrons in the 50% region are due to interactions of
primary photons closer to the edge of the aperture. As
reported by Lo Sasso et al. [10] and Powlish et al. [14],
when the individual radiation beams are composed in a
multifield treatment, the effect of scalloping dose distributions
are moderated by the dose distributions from the other
fields. When daily set up variations are included in the analysis, the penumbra for the block edge is only minimally
different than the penumbra for an MLC positioned with
maximum stepping (leaf displacement relative to its neighbor
is equal to the leaf width) and setup variations over a period
of time tend to smooth the MLC beam edges [23-25].
Our analysis suggest that the MLC dosimetry is less
favorable than that of the cerrobond blocks in the treatment
of six-field irradiation of prostate carcinoma on the computerized
planning system without daily setup variations
taken into account. However, none of the normal tissues
e.g. rectum, bladder or femoral head were irradiated to the
critical doses and thus with each of the beam shaping
configurations, higher volumes did not receive critical
doses of irradiation. Even though the scalloping effect and
larger penumbra, MLC shaped fields seems to be problematic
features; it’s reported that when the set up uncertainties
and the multifield treatments are taken into account, these
features would also reduce the differences between using
conformal blocks and MLC.
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