Significance of breast boost volume changes during radiotherapy in relation to current clinical interobserver variations.
OBJECTIVE
Nowadays, many departments introduce CT images for breast irradiation techniques, aiming to obtain a better accuracy in the definition of the relevant target volumes. However, the definition of the breast boost volume based on CT images requires further investigation, because it may not only vary between observers, but it may also change during the course of treatment. This study aims to quantify the variability of the CT based visible boost volume (VBV) during the course of treatment in relation to the variability between observers.


METHODS
Ten patients with stage T1-2 invasive breast cancer treated with breast conservative surgery and post surgical radiotherapy were included in this study. In addition to the regular planning CT which is obtained several days prior to radiotherapy, three additional CT scans were acquired 3, 5 and 7 weeks after the planning CT scan. Four radiation oncologists delineated the VBV in all scans. Conformity of the delineations was analysed both between observers, and between scans taken at different periods of the radiotherapy treatment.


RESULTS
The VBV averaged over all patients decreased during the course of the treatment from an initial 40 cm(3) to 28 cm(3), 27 cm(3) and 25 cm(3) after 3, 5 and 7 weeks, respectively. Assuming the VBV to be spherical, this corresponds to a reduction in diameter of 5-6mm. More detailed analysis revealed that this reduction was more pronounced when radiotherapy started within 30 days after surgery. These boost volume changes over time were found to be significant (p=0.02) even in the presence of interobserver variations. Moreover, the conformity index (CI) for the volume changes was of the same magnitude as the conformity index for the interobserver variation (0.25 and 0.31, respectively).


CONCLUSIONS
Breast boost volume variations during a course of radiotherapy are significant in relation to current clinical interobserver variations. This is an important finding to take into account when introducing CT based planning, especially when applying an integrated boost technique.
Hurkmans
Coen
C
Catharina Hospital, Department of Radiation Therapy, Eindhoven, The Netherlands. coen.hurkmans@cze.nl
Admiraal
Marjan
M
van der Sangen
Maurice
M
Dijkmans
Ingrid
I
D001943:Breast Neoplasms / Q000000981:diagnostic imaging / Q000473:pathology / Q000532:radiotherapy*; D019583:Dose Fractionation, Radiation; D005260:Female; D006801:Humans; D009361:Neoplasm Invasiveness; D009367:Neoplasm Staging; D015588:Observer Variation; D015641:Radiography, Interventional; D011879:Radiotherapy Dosage; D011880:Radiotherapy Planning, Computer-Assisted / Q000379:methods*; D050397:Radiotherapy, Intensity-Modulated; D014057:Tomography, X-Ray Computed; D016896:Treatment Outcome