The Severity Weighted Assessment Tool (SWAT) was introduced by Stevens et al. (2002) to quantify the skin burden of mycosis fungoides by weighting the extent of patches, plaques and tumours, and it correlated closely with the percentage of body surface area involved and with the time to remission under phototherapy. The modified SWAT (mSWAT) replaced the original grid counting with twelve body regions carrying fixed shares of body surface area and raised the tumour weight from three to four; it was first used in the phase IIb vorinostat trial (Olsen et al. 2007) and was then adopted as the standard skin measure in the consensus statement of the International Society for Cutaneous Lymphomas, the United States Cutaneous Lymphoma Consortium and the EORTC Cutaneous Lymphoma Task Force (Olsen et al. 2011).

For each of the twelve regions (head 7%, neck 2%, anterior trunk 13%, arms 8%, forearms 6%, hands 5%, posterior trunk 13%, buttocks 5%, thighs 19%, legs 14%, feet 7% and groin 1% of body surface area), the percentage of total body surface area occupied by patches, by plaques and by tumours is recorded, with the patient's palm including the fingers taken as approximately 1% of body surface area. A patch is any size lesion without induration or significant elevation; a plaque is elevated or indurated; a tumour is a solid or nodular lesion of at least 1 cm with deep infiltration or vertical growth. The percentages for each lesion type are summed across the regions and multiplied by the weighting factors 1, 2 and 4, and the three weighted totals are added to give the mSWAT, from 0 to 400. The lesion areas within a region cannot exceed that region's share of body surface area.

Skin response in the consensus criteria is defined from the change in the mSWAT: a complete response is 100% clearance of skin lesions, a partial response is a 50% to 99% clearance from baseline without new tumours in patients whose skin disease was patches, plaques or erythroderma only, stable disease lies between a 25% increase and a 50% clearance, and progressive disease is an increase of 25% or more from baseline, the appearance of new tumours, or loss of a previous response. Trials such as MAVORIC (Kim et al. 2018) and ALCANZA (Prince et al. 2017) operationalise the partial response as a reduction of at least 50% in the mSWAT, and later reports describe this threshold as mSWAT 50, with mSWAT 90 for a 90% reduction. The calculator evaluates the 50%, 90% and 100% reductions and the 25% progression threshold from an optional baseline score; new tumours and biopsy findings must be judged clinically.

The mSWAT measures skin disease only and is combined with blood, lymph node and visceral assessments in the global response of the consensus. Estimating lesion areas by palm counts introduces observer variability, particularly for tumours and erythrodermic disease, and no dedicated psychometric validation of the modified tool has been published; its use rests on the consensus and on trial practice. The 2022 update of the recommendations (Olsen et al. 2022) retained the twelve regions and the weighting factors, defined erythroderma as involvement of at least 80% of body surface area, and aligned the response categories with the Lugano classification.