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 cm3 to 14 cm3. 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.PACS numbers: 87.55.D‐, 87.55.dk, 87.55.Gh, 87.55.K‐, 87.56.J‐
Highlights
Adjuvant radiotherapy after breast conserving surgery is used to reduce the risk for local recurrences
Test calculations are performed with the dose calculation algorithm currently used at our hospital for this type of treatment, namely the analytical anisotropic algorithm (AAA) version 10.0.28 implemented in Eclipse (Varian Medical Systems)
Validation of MC beam calculations The difference between the ionization chamber measurements centrally in the field and MC data is 0.2%–0.9 % for the four separate main fields (MLC and wedges excluded) where the uncertainty of the MC data is negligible and the measurement error (95% significance level) is estimated to below 0.2% for all fields
Summary
Adjuvant radiotherapy after breast conserving surgery is used to reduce the risk for local recurrences. In the locoregional case the target volume is treated in two parts, a cranial and a caudal one. The cranial part covers the lymph nodes and consists of anterior–posterior fields, whereas tangential fields are most commonly used for the caudal part (i.e., the breast tissue). The isocenter is positioned at the junction between the cranial and the caudal fields to eliminate beam divergence, which makes the treatment fields strongly asymmetrical (half-blocked fields). This is a well-established technique introduced many years ago,(1) and is still used, in combination with advanced respiratory gating systems.[2]
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