Jaw position uncertainty and adjacent fields in breast cancer radiotherapy

Abstract

Locoregional treatment of breast cancer involves adjacent, half blocked fields matched at isocenter. The objective of this work is to study the dosimetric effects of the uncertainties in jaw positioning for such a case, and how a treatment planning protocol including adjacent field overlap of 1 mm affects the dose distribution. A representative treatment plan, involving 6 and 15 photon beams, for a patient treated at our hospital is chosen. Monte Carlo method (EGSnrc/BEAMnrc) is used to simulate the treatment. Uncertainties in jaw positioning of ± 1 mm are addressed, which implies extremes in reality of 2 mm field gap/overlap when planning adjacent fields without overlap and 1 mm gap or 3 mm overlap for a planning protocol with 1 mm overlap. Dosimetric parameters for PTV, lung and body are analyzed. Treatment planning protocol with 1 mm overlap of the adjacent fields does not considerably counteract possible underdosage of the target in the case studied. PTV-V95% is for example reduced from 95% for perfectly aligned fields to 90% and 91% for 2 mm and 1 mm gap, respectively. However, the risk of overdosage in PTV and in healthy soft tissue is increased when following the protocol with 1 mm overlap. A 3 mm overlap compared to 2 mm overlap results in an increase in maximum dose to PTV, PTV-D2%, from 113% to 121%. V120% for 'Body-PTV' is also increased from 5 cm(3) to 14 cm(3). A treatment planning protocol with 1 mm overlap does not considerably improve the coverage of PTV in the case of erroneous jaw positions causing gap between fields, but increases the overdosage in PTV and doses to healthy tissue, in the case of overlapping fields, for the case investigated.

Figure 1

Illustration of the patient specific plan. Field numbers indicated in the figure. Fields 1 to 3 cover the cranial part of the target (lymph nodes); fields 4 to 6 are tangential fields covering the residual breast tissue.

Figure 2

Illustration of the dose distributions in a plane 3 cm dorsal from isocenter for the situation of a) fields overlapping 3 mm and b) with a 2 mm gap. Color scale in Gy per fraction ranging from 0.2 to 3.16. DVH (c) for the case of 3 mm overlapping fields (solid line) and 2 mm gap (dashed line) between the fields covering the lymph nodes and the tangential fields irradiating the breast tissue. Perfectly aligned jaws shown for reference (red line).

Figure A.1

Dose profiles in water at 3 cm depth for the main fields of the validation plan: (a) tangential anterior field; (b) lymph nodes anterior field; (c) tangential posterior field; (d) lymph nodes posterior field. $\text{Black lines}=\text{MC}$ calculations, $\text{gray dots}=\text{IC}$ Profiler measurements, $\text{large crosses}=\text{ionization}$ chamber measurements.

Figure A.2

The STT for the wedge tangential posterior field (a). Backscatter corrected (dashed) shown together with the original uncorrected STT (solid) given by the manufacturer for wedge angle 15°. Validation profile (b) at 3 cm depth in water for the tangential wedge field (6 MV). MC calculations (solid black line) compared to IC Profiler measurement (grey dots) — a relative measurement normalized to the absolute ion chamber measurement (cross).

Figure A.3

To the left the dose distribution for 3 mm overlapping fields is shown where the black line indicates the position of the dose profile shown in the graph to the right. MC dose profiles for 2 mm gap and 3 mm overlap (green and yellow) compared to AAA dose profiles (blue and red) for the same cases.