Effects of different bolus strategies on doses in postmastectomy radiotherapy / 中华放射医学与防护杂志

ABSTRACT

Objective:To comprehensively evaluate the effects of different bolus usages in postmastectomy intensity-modulated radiotherapy (PM-IMRT) on doses.Methods:Fifty patients receiving PM-IMRT at Fudan University Shanghai Cancer Center from April to October 2021 were retrospectively studied. The planning target volume (PTV) was divided into four parts, namely chest wall (CW), internal mammary node, retained axillary lymph node, and supraclavicular node. The prescription dose was 50 Gy/25 fractions. Three PM-IMRT plans applying boluses with different thicknesses (3, 5 and 10 mm) were designed for each patient. The effects of different thicknesses and usage frequencies of boluses on PTV coverage, high dose volume of the CW skin, and dose to surrounding normal tissues were comprehensively evaluated.Results:When boluses were applied throughout the PM-IMRT, the PTV V95% of plans applying 10 mm-thick boluses was lower than that of plans applying 3 and 5 mm-thick boluses ( F=3.340, P < 0.05), the CI of plans applying 3 mm-thick boluses was higher than that of plans applying 5 and 10 mm-thick boluses ( F = 50.05, P < 0.05), and there was no statistically significant differences in the skin V105% and V110% of three plans( P > 0.05). Both PTV V95% and skin V105% were reduced with a decrease in the usage frequency of boluses. At a frequency of 20, PTV V95% decreased slightly (< 1%), while skin V105% decreased sharply to nearly half of the original values. At a frequency of 15, the PTV V95%, CI, and HI in the three plans showed no statistically significant dosimetric differences ( P > 0.05). The PTV Dmax of plans applying 3 mm-thick boluses was lower than that of plans applying 5 and 10 mm-thick boluses ( F = 9.21, P < 0.05). As for the dose to surrounding normal tissues, different bolus thicknesses and frequencies had negligible effects on doses to heart and lung, causing little different biological effects. Conclusions:For PM-IMRT, different bolus thicknesses have similar effects on doses to the PTV, skin, heart, and lung. Bolus usage frequency, rather than thickness, was the major factor determining the PTV coverage and the dose to CW skin.