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ABSTRACT
Radiotherapy
has an important role in rectal cancer treatment. Radiotherapy
can be applied as adjuvant therapy, therapy of the recurrent disease,
palliative therapy and as primary therapy in early stages. Meta-analyses
in numerous studies confirmed that radiotherapy reduced local
relapses, but unfortunately, overall survival could not be improved
only with radiotherapy (1,2). It can be administered as a transcutaneous
therapy on the mega voltage machines (LINAC) as well as brachytherapy
(endoluminal and/or interstitial), and as combination of transcutaneous
therapy and brachytherapy. In combination with surgery radiotherapy
can be preoperative, postoperative and intraoperative. Preoperative
radiotherapy is administered by short-term technique and tumor
dose that ranges between 10 and 20 Gy with obligatory planned
surgery in following 5 to 7 days or by protracted technique with
tumor dose that ranged between 40 and 50 Gy and planned operation
usually 4 to 6 weeks after radiotherapy with preliminary assessment
of tumor response to administered radiotherapy (3).The advantages
of preoperative radiotherapy are: tumor down staging and possibility
for radical surgery or sphincter preserving surgery, reduction
of tumor cell viability and dissemination and low incidence of
acute toxicity.The disadvantage of preoperative radiotherapy is
the potential of over treatment (early stage) and problem with
perineal scar after abdominoperineal resection. Pelvic radiation
is associated with acute and long-term toxicity. These complications
are in function of the irradiated volume, radiation energy, total
dose and technique (4). Because of that treatment should be designed
with the use of computerized radiation dosimetry, which by nature
of their depth dose characteristics deliver a higher dose to the
tumor volume, while sparing the surrounding normal structures.
The treatment of all fields each day results in a lower integral
dose and more homogeneous dose distribution.The clinical, prospective,
non-randomized study with 48 patients with locally advanced rectal
cancer (T3 and T4 stage) was conducted in the Institute of Oncology
and Radiology of Serbia under the project Quality control in radiotherapy
and radiology physics. The aims of this study were to compare
the dose distribution in two transcutaneous techniques (technique
with three fields and four fields technique in isocenter) and
to analyze the acute radiation complications. The patients were
divide into two groups according to the radiation technique: the
first group of 28 patients irradiated with three fields technique
(direct posterior and two lateral fields), and the second group
of 20 patients irradiated with four fields technique (anterior/posterior
and two lateral fields). Volumes of interest (GTV - gross tumor
volume, PTV - planning target volume and radiosensitive normal
tissues) and isodose distribution were calculated according the
ICRU 50 recommendations (5). Preoperative radiotherapy was administered
with tumor dose of 45 Gy applied in 25 fractions, 1.8 Gy per fraction,
with photons of 18 MeV energy, isocentric technique, on linear
electron accelerator. Calculation of median GTV values was performed
based on tumor dimensions obtained by computerized tomography
(based on longitudinal and two transversal diameters), while the
calculation of median PTV values was performed based on the chosen
margins from isodose distribution and was precisely defined by
methodology of the study. Acute sequelae during five-weeks radiotherapy
were analyzed and graded according to the WHO recommendations
(6). Comparison of dose distribution was conducted to define the
degree of dose homogeneity in relation to applied various irradiation
techniques (technique with three and four fields respectively)
in GTV and PTV as well as to the dose applied to the adjacent
normal structures of interest (bladder, small bowel) in the irradiated
volume. The following parameters were analyzed: median volume
of GTV and PTV, median minimal and maximal doses in PTV applied
median doses to the bladder and small bowel in irradiated volume
related to the applied irradiation techniques. The analysis of
median target volume values (50.4:90.6 cm3) and median values
of PTV (1168:1467.4 cm3) showed statistically significant difference
(p=0.007) or (p=0.014) respectively. Comparison of median minimal
doses (cold spots) in PTV (43.6 Gy for the first group, 43.4 Gy
for the second group) revealed no statistical significance (p=0.311)
while the analysis of median maximal doses (hot spots) in PTV
(46.8 Gy: 49.9 Gy) showed statistically very significant difference
(p=0.000). The analysis of this important geometric and dosimetry
radiotherapy parameters showed that radiotherapy plan in the first
group of patients was less homogenous in relation to the second
group of patients irradiated with four field technique (7). Analysis
of median applied doses to the bladder (30.3 Gy for the first
group: 39.8 Gy for the second group) and small bowel (11.7 Gy:
20 Gy) we concluded that patients in the second group received
higher dose to this sensitive structure. In order to define the
degree of tolerability of radiotherapy the incidence and most
frequent acute complications in relation to applied irradiation
techniques were compared. In the first group 21 patients (75%)
had complications during radiotherapy and in the second group
12 patients (60%). There was no statistically significant difference
in acute complications between these two groups. Analysis of most
frequent complications as diarrhea (21.43%: 20%), cystitis (3.57%:
25%) and perineal dermatitis (57.14%: 60%) showed that only cystitis
occurred more frequently in group II of patients (7). Although
acute complications were registered both groups (I and II) of
investigated patients, they were of low grade. Concerning that
the homogeneity of dose in three-fields technique is satisfactory
and in concordance with ICRU 50 recommendations, with less dose
applied to the critical structures of interest (urinary bladder
and small bowel), we could conclude that this technique was simpler
and more acceptable for routine practice.
References
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7. Stojanović S.: Preoperative radiotherapy in locally advanced
rectal cancer - analysis of radiation technique and acute complications.
Master thesis Medical School University of Belgrade, 1999.
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