105 Intensity modulated tangential beam irradiation of the intact breast

Proceedings

of the 39th Annual

ASTRO

Meeting

187

104 LUNG Indra

AND

HEART

J. Das, Ph.D.,

Department

DOSE Elizabeth

of Radiation

VOLUME

ANALYSES

WITH

C. Cheng,

MS., Barbara

Fowble,

Oncology,

Fox Chase

Cancer

CT SIMULATOR

IN TANGENTIAL

FIELD

IRRADIATION

OF BREAST

CANCER

M.D.

Center,

Philadelphia,

PA 19111.

Objective: Radiation pneumonitis and cardiac effects are directly related to the irradiated lung and heart volumes in the treatment fields. The central lung distance (CLD) from a tangential breast radiograph is shown to be a significant indicator of ipsilateral irradiated lung volume based on empirically derived functions which accuracy depends on the actual measured volume in treatment position. A simple and accurate linear relationship with CLD and retrospective analysis of the pattern of dose volume of lung and heart is presented with actual volume data from a CT simulator in the treatment of breast cancer. Materials & Methods: The heart and lung volumes in the tangential treatment fields were analyzed in 45 consecutive (22 left and 23 right breast) patients referred for CT simulation of the cone down treatment. All patients in this study were immobilized and placed on an inclined breast board in actual treatment setup. Both arms were stretched over head uniformly to avoid collision with the scanner aperture. Radiopaque marks were placed on the medial and lateral borders of the tangential fields. All patients were scanned in spiral mode with slice width and thickness of 3 mm each, respectively. The lung and heart structures as well as irradiated areas were delineated on each slice and respective volumes were accurately measured. The treatment beam parameters were recorded and the digitally reconstructed radiographs (DRRs) were generated for the CLD and analysis. Results: Table 1 shows the volume statistics of patients in this study. There is a large variation in the lung and heart volumes among patients. Due to differences in the shape of right and left lungs the percent irradiated volume (PIV) are different. The PIV data have shown to correlate with CLD with 2nd and 3rd degree polynomials: however, in this study a simple straight line regression is used to provide better confidence than the higher order polynomial, The regression lines for the left and right breast are very different based on actual CT data. The slopes of regression lines for the left and right lung are 0.64%/mm and OS4%/mm, respectively with a combined slope of 0.6%/mm. With the selection of proper beam parameters, the heart volume can be minimized. As expected, there is no correlation between heart PIV and the CLD. A maximum heart PIV of 5.6% is observed with one fourth of patients having a PIV of 0%. The heart PIV is inversely correlated with gantry angle as shown in Figure 2. Due to the radiation scatter in the body, the geometrical volume may be of limited importance and, hence, dose volume histogram (DVH) analyses were performed. A representative DVH of a patient whose lung and heart PIV were 14.4% and 4.0%, respectively is shown in Figure 3. Table 1. Volume Statistics of Lung and Heart Fig. 1. % volume vs. CLD Fig. 2. % Heart vs. gantry angle Fig. 3. DVH of a patient

Volume

(cm3j

Rieht

Lun

Total ~~~~~~~:~

Irradiated % Irradiated (PIV)

C,.,i”L.“,b,,..,~~,,,

DNIW

Conclusions: The CT-simulator provides an accurate volumetric information of the heart and lungs in the treatment fields. The lung PIV is directly correlated to the CLD. Left and right lungs have different volumes and, hence, different regression lines are recommended. Heart volume is not correlated with the CLD. The heart PIV is associated to the beam angle. Heart volume may not be accurately visualized in a tangential radiograph; however, this can be easily seen in a DRR with contour delineation and can be minimized with proper beam parameters. Lung and heart PIV along with DVH are essential in reducing oulmonarv and cardiac comolications.

105 INTENSITY

MODULATED

TANGENlTAL

L. Hong, Ph.D., M. Hunt, MS.. J. Yahalom, M.D.*,

BEAM

IRRADIATION

OF THE INTACT BREAST

C. Chui, Ph.D., K. Forster. Ph.D., H. Lee, M.D., Ph.D. *,W. Lutz, Ph.D.,

G.J. Kutcher, Ph.D., B. McCormick,

M.D.*

Departments of Medical Physics and *Radiation Oncology. Memorial Sloan Kettering Cancer Center, New York, NY Fbpose/Obja3ive: The purpose of this study was to evahmte the potential The primary goal was to develop an intensity modulated contralateral Ixeast while still using a standard tangential opposed fields, was also desired. Since a major goal of population of patients, the design of “standard” Intensity

benefits of intensity modulated tangential beams in the irradiation of the imact breast. trearment which would substantially decrease the dose to coronary arteries, lung and beam arrangement. Improved target dose homogeneity, withii the limits imposed by the study was the development of a technique which was practical for use on a large profiles analogous in function to conventional wedges was also investigated.

Materids & Methods Three dimensional treatment planning was performed using both conventional and intensity modulated tangential beams. Plans were. developed for both the right and left breast for a mnge of patient sizes and shapes. For each patient, FTV, lung, heart, origin and peripheral branches of the coronaty artery, and contmlate.ral breast were contoured. Mum tangential beam direction and shape were designed using Beams-EyeView display and then used for both the conventionaJ and intensity modulated plans. For the conventional plan, the optimum wedge combInatlon and beam weighting were chosen based on tie dose distribution in a single transverse plane through the field center. Intensity modulated plans were designed using an algorithm which allows the user to specify the prescribed, maximum and minimum acceptable doses sod dose volume consuaints for each organ of interest. Plans were compared using multiple dose disaibutions and DVHs. Results: Significant improvements in the doses to critical structures were achiex-d using the intensity modulated plan. Coronary artery dose decreased substantially for padents !xeated to the leii breast. Ipsilateral lung and conaalatexal breast doses decreased for all patients. For one patient atatcd to the left bmst, the mm dose to the peripheral branches of the coronary ane-ry decreased 50% with intensity modulation compared with the conventional plan (from 36 Gy to 18 Gy based on a 46 Gy prescription). Dose to the origin of the coronary artery decreased by 50% aIs0 (from 26 Gy to 13 Gy). ‘TXe lung volume irradiated to doses of 20 Gy or greater decreased by 25%. In general, dose homogeneity within the targex vohune improved appmximately 10% with intensity modulation. The improvement was observed primarily in the superior and inferior regions of the. breast although some decrease in the medial and lateral hot spots were observed. Improved homogeneity compared to conventional tangential wedged fields can also be obtained using “standard” intensity profiles chosen from a library. Conclusion: The USC.of intensity modulation with standard tangential breast fields can significantly reduce the dose received by normal lung, anmalateral breast and coronary arteries during treamnt of the intact breast. Impmvements in dose homogeneity throughout the target volume can also be achievai, particularly in the superior and inferior regions of the breast. By continuing to use a standard tangential beam arrangement, the improvements in nmmal tissue doses and target uniformity gained through intensity modulation can be achieved without significantly increasing mammt complexity.