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In the United Kingdom (UK), the inclusion of internal mammary chain (IMC) radiotherapy (RT) has been adopted within national guidelines since the publication of clinical trials which show benefit. Currently in our institution a 3arcs volumetric modulated arc therapy (VMAT) technique is offered to patients where a wide tangents (WT) technique does not meet defined dosimetric constraints. However, the beam on time is considerably longer, increasing the number of breath holds a patient has to undergo. This study aims to investigate dosimetric differences in terms of target volume coverage and organs at risk (OAR) sparing between 2arcs and 3arcs VMAT versus WT.

Planning computed tomography (CT) scans of 20 patients previously treated with whole breast/chestwall RT at our institution were included.  For all datasets, a single observer (a breast consultant clinical oncologist) delineated breast/chestwall clinical target volumes (CTV) and axillary levels 1-4 and IMC nodal CTV on the planning CT with a planning target volume (PTV) margin of 5mm; ipsilateral/contralateral lungs, heart, contralateral breast/chestwall, spinal cord, thyroid and oesophagus were contoured using ESTRO guidelines. For each patient, 3 RT planning techniques: WT, 2arcs and 3arcs VMAT were planned by a single radiation therapist with the prescription dose of 40Gy in 15 fractions following the departmental protocol. PTV coverage and OAR doses were measured for each technique. Kruskai-Wallis ANOVA was performed to detect statistically significant differences in dosimetric parameters between techniques followed by Bonferroni-type multiple comparisons with adjusted P-values ≤ 0.05 indicating statistical significance

Within the cohort, there were 10 left sided (6 breast and 4 chestwall) and 10 right sided (8 breast and 2 chestwall) patients resulting 60 plans in total. The median (range) of total monitor units (MU) were 567 (536-592), 781 (667-863) and 773 (647-1080) for WT, 2arcs and 3arcs VMAT respectively. No statistically significant difference was found between 2arcs and 3arcs VMAT but the MU of VMAT groups were significantly higher than WT.

As illustrated in table 1, statistically significant differences were found in all PTV and OAR dosimetric parameters (p<0.01) apart from heart V17Gy (p=0.83) between all three techniques. For multiple pairwise comparisons, WT group achieved statistically significant poorer target volumes and better OARs sparing than the VMAT techniques (p<0.01); there were no dosimetric benefits of 3arcs VMAT over the 2arcs technique. 

This study suggested that VMAT techniques had dosimetric advantages in target volume coverage over WT; 2arcs VMAT technique has been suggested to be the optimum technique compared to 3arcs due to its shorter treatment beam on time. Caution should be made about post RT toxicities to thyroid and oesophagus with VMAT techniques for breast IMC RT due to the expense of larger low dose coverage to surrounding OARs.