A spreadsheet to determine the volume ratio for target and breast in partial breast irradiation*

Abstract

The technical feasibility of Partial Breast Irradiation (PBI) using external beam radiotherapy depends on the ratio between the evaluation planning target volume (PTV_eval) and the whole breast volume ("PBI volume ratio" = PVR). We aimed to develop a simple method to determine PVR using measurements performed at the time of the planning CT scan. A PVR calculation tool was developed using a Microsoft Excel spreadsheet to determine the PTV from three orthogonal dimensions of the seroma cavity and a given margin on the CT scans. The breast volume is estimated from the separation and breast height in five equally spaced CT slices. The PTV_eval and whole breast volume were determined for 29 patients from two centres using the spreadsheet calculation tool and compared to volumes delineated on computerised treatment planning systems. Both the PTV_eval and whole breast volumes were underestimated by approximately 25% using the spreadsheet. The resulting PVRs were 1.05 +/- 0.35 (mean +/- 1SD) times larger than the ones determined from planning. Estimations of the PVR using the calculation tool were achievable in around 5 minutes at the time of CT scanning and allow a prompt decision on the suitability of the patients for PBI.

The effect of imaging modality (magnetic resonance imaging vs. computed tomography) and patient position (supine vs. prone) on target and organ at risk doses in partial breast irradiation

“…TROG 06.02 credentialing publications have demonstrated feasibility of selecting patients for PBI based on the PBI breast volume ratio. 32 This ratio was set at 0.25; however, other publications have not considered for PBI breast volume ratio in patient selection. 8,11,12,14,19 Ultimately, there was no significance between ratio below 0.25 and compliance or conversely ratio larger than 0.25 and non-compliance in this cohort.…”

“…This indicates that patients should not be excluded for PBI on the basis of PBI breast volume ratio. Patients may also still be eligible for PBI if the seroma is larger than 2 cm, which has been used as selection criteria in recent clinical trials 14,30,32 and < 3 cm as per international consensus statements and guidelines. 5,6,13 Our study reported a 21% incidence of UIQ targets for the imaging study and 26% for the positioning component, which is higher than the 16% reported elsewhere.…”

“…Within this group of patients, the PBI volume ratio data were collected for analysis, but not used as the basis of exclusion. TROG 06.02 credentialing publications have demonstrated feasibility of selecting patients for PBI based on the PBI breast volume ratio 32 . This ratio was set at 0.25; however, other publications have not considered for PBI breast volume ratio in patient selection 8,11,12,14,19 .…”

“…This indicates that patients should not be excluded for PBI on the basis of PBI breast volume ratio. Patients may also still be eligible for PBI if the seroma is larger than 2 cm, which has been used as selection criteria in recent clinical trials 14,30,32 and < 3 cm as per international consensus statements and guidelines 5,6,13 …”