, vol. 81: ELSEVIER IRELAND LTD ELSEVIER HOUSE, BROOKVALE PLAZA, EAST PARK SHANNON, CO, CLARE, 00000, IRELAND, pp. S364–S365, 2006.
Purpose/Objective: Although 3D-treatment-planning reduced cardiac
dose in postoperative breast radiotherapy, overall cardiac toxicity
increased due to the more widespread use of anthracyclines in the
adjuvant treatment. Modelling of toxicity based on data from patient
series treated for breast cancer and for Hodgkin's disease suggests
that reduction of the maximal dose to the left heart might be
particularly beneficial.We compared treatment plans created with
an aperture based IMRT-approach (an approach that offers some advantages
over fully inverse treatment planning) to three-dimensionally
planned tangent fields with regard to physical and biological
optimisation of the dose distribution, its robustness to positioning
errors and its treatment time requirements.
Materials/Methods: CT- and structure data of 14 patients that had
been treated postoperatively with external beam irradiation for
early breast cancer at our department formed the basis of our plan
comparison study. For each patient data set, a conventional 3D
treatment plan and an IMRT plan were created on ELEKTA Precise-
Plan. Dose Volume Histograms (DVH) were evaluated with regard
to target coverage, dose homogeneity, and dose to organs at risk
(heart, lung, contralateral breast). Normal tissue complication probability
(NTCP) for cardiac mortality as the endpoint was calculated
for both approaches based on a relative seriality model. For a representative
patient, the influence of positioning errors on the DVH was
simulated by moving the isocenter in various directions.
Results: IMRT reduced the maximum dose to the left ventricle from
a mean of 49 Gy to a mean of 34 Gy. Heart volume that was exposed
to <30 Gy was reduced from a mean of 45 ccm3 to a mean of 5.8
ccm3. Mean dose to the heart was increased by an average of 15
%. The change in DVH characteristics translated into a projected reduction
of the probability for therapy associated long term cardiac
death from 6 % (conventional 3D) to 2.5 % (IMRT) with the relative
seriality model. On average, mean dose to the contralateral (right)
breast was increased from 1.15 Gy to 5.4 Gy.Coverage of the ventral
portion of the breast was robust to positioning errors, coverage of
the posterior portion was not as a consequence of steep dose gradients
towards the lung/heart.Monitor units, segment numbers and
treatment time (<15 min) were within acceptable limits
Conclusions: Aperture based IMRT for left sided breast cancer significantly
reduces maximum dose to the left ventricle. This reduction
may translate into reduced cardiac mortality based on an appropriate
NTCP model. The clinical significance of increased exposure to
the right breast is as of now unclear but may be less relevant than
cardiac toxicity. As for any conformal technique, patient positioning
has to be improved in the direction of the steep gradients (e.g. at
the thoracic wall)
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