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BPI02
Breast Treatment Planning: A retrospective comparison of present and past techniques

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Breast Treatment Planning: A retrospective comparison of present and past techniques

Vivian Mang Chou (student) 1, Janos Juhasz (Ph.D.) 2, Jeremy Siemens (MRT (T)) 2
1Mohawk-McMaster University, Medical Radiation Sciences Department, Hamilton, Ontario

2Juravinski Cancer Centre, Medical Physics Department, Hamilton, Ontario

Introduction:

In the past 20 years, how we deliver the tangential parallel opposed pair technique has changed considerably. Traditionally, patients was planned using two-dimensional (2D) techniques without tissue density corrections.[1][2] Therefore with 2D planning, there was limited knowledge of precise dose distributions.Currently the field in field technique with three-dimensional (3D) planning, gives anatomical information of the entire breast, corrects for tissue density and has shown to achieve better dose homogeneity. [3]This study compares between the field in field technique, wedged technique and the use of cobalt-60 to review how much treatment planning has advanced over the years in terms of target volume coverage, reduction in dose inhomogeneities, lung and heart doses.


Methods:

Three treatment plans were generated for each of the 10 patients chosen for this study using the Pinnacle treatment planning software.

Plan A:

Field in field breast plans using 3D treatment planning and density corrections were created. Dose distributions were optimized using static multileaf collimator segments (MLC) by minimizing hotspots and shielding organs at risk as seen in Figure 1 and Figure 2

Plan B:

This was a traditional wedged technique using either a cobalt-60 (patient separation < 15cm), 6MV or 6/18 MV mixed beam (patient separation ≥ 15 cm). Density corrections was turned off to better simulate the dose distributions generated in the past. Asymmetric field sizes were used in 6MV or mixed beam plan as seen in Figure 3. However, the collimators of Cobalt-60 machines does not have asymmetric, therefore symmetric collimator jaws were set and a half beam block was drawn as seen in Figure 4. 

Plan C:

All parameters for this plan were kept constant with Plan B by prescribing the same monitor units for each beam. However, density correction was turned on.

Image J

Dose grids were exported from Pinnacle and were run through Image J to have CT slices 6 cm superior to central axis, central axis, and 6 cm inferior to central axis subtracted from Plan A to Plan C.

Dose Volume Histogram Evaluation

The volume of lung receiving 1700 cGy (V17) was compared between Plan A to Plan C. The cardiac volume receiving 213 cGy for patients with right sided disease and 426 cGy for patients with left sided disease were compared between Plan A to Plan C.

Plan A (Figure 5a) and Plan B (Figure 5b) would be considered to be a good plan under ICRU-50 criteria.[4]However, once tissue density correction was on (Plan C), an inhomogeneous dose distribution representative of what the patient would have actually received can be fully appreciated in central axis (Figure 5c).

Figure 6 shows inadequate coverage of the target volume since 2D planning does not give dosimetric information in positions other than central axis.

Dose grids were generated using Image J. Comparisons of Plan A, Plan B and Plan C can be made (Figure 7a, 7b and 7c).


The differences in dose between Plan B and Plan C at a CT slice inferior to central axis (Figure 8a), central axis (Figure 8b) and a CT slice superior to central axis (Figure 8c) is displayed. The areas of white shows an overdose, while black shows areas of underdose.

Dose Volume Histograms were generated and an overall decrease in dose to the lung and heart with the field in field technique can be seen in Figure 9.

Table 1 was generated using the DVH and was assessed under local quality assurance guidelines. It was recommended that the V17 of lung should be approximately under 13% - 15% of lung irradiated. It also recommended that no more than 5% of the heart should receive doses of 213 cGy for right sided disease and 426 cGy for left sided disease.


Conclusion:
The results of this single institution retrospective study follow the same findings to what is published in literature.[3][5] The 10 patients planned in this study using the 3D field in field technique produced a more homogenous plan, improved target volume coverage and decrease dose to normal organs compared to conventional 2D radiotherapy techniques.

References:

1. Fraass, B. A., Lichter, A. S., McShan, D. L., Yanke, B. R., Diaz, R. F., Yeakel, K. S., & Van de Geinjn, J. (1988). The influence of lung density corrections on treatment planning for primary breast cancer. International Journal of Radiation Oncology* Biology* Physics, 14(1), 179-190.

2. Kestin, L. L., Sharpe, M. B., Frazier, R. C., Vicini, F. A., Yan, D., Matter, R. C., ... & Wong, J. W. (2000). Intensity modulation to improve dose uniformity with tangential breast radiotherapy: initial clinical experience. International Journal of Radiation Oncology* Biology* Physics, 48(5), 1559-1568.

3. Aref, A., Thornton, D., Youssef, E., He, T., Tekyi-Mensah, S., Denton, L., & Ezzell, G. (2000). Dosimetric improvements following 3D planning of tangential breast irradiation. International Journal of Radiation Oncology* Biology* Physics, 48(5), 1569-1574.

4. ICRU: Prescribing, recording, and reporting photon beam therapy.ICRU report. Volume 50. Bethesda: International Commission on Radiation Units and Measurements; 1993.

5. Hurkmans, C. W., Cho, B. C., Damen, E., Zijp, L., & Mijnheer, B. J. (2002). Reduction of cardiac and lung complication probabilities after breast irradiation using conformal radiotherapy with or without intensity modulation. Radiotherapy and oncology, 62(2), 163-171.



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