Objective

To evaluate margins of excision and long term survival in dogs following excision of cutaneous and subcutaneous mast cell tumours (MCTs) using a modified proportional margins (MPM) approach.

Design

Retrospective study.

Animals

25 dogs with 27 MCTs.

Procedure

MCTs were removed by taking lateral margins equal to the diameter of the tumour up to a maximum lateral margin of 4 cm with a deep margin of at least one fascial plane. Samples were submitted for histological grading and assessment of margins. Tumours were graded according to the Patnaik system and the two-tier system proposed by Kiupel et al. Survival time for subcutaneous MCTs was predicted based on the guidelines proposed by Thompson et al. Followup information was obtained through repeat examination by referring vets or communication with owners.

Results

Twenty-seven tumours were evaluated from 25 dogs. Histologically, 18 masses were cutaneous and nine subcutaneous. Using the MPM approach, margins were 2 cm or less on 14 occasions. Twelve cutaneous tumours were Patnaik grade I and six grade II. Under the Kiupel system 16 were low grade and 2 were high grade. Seven of the cutaneous tumours had dirty margins and 11 were clean. Lateral margins were dirty on five occasions and the deep on five occasions. Six subcutaneous MCTs were considered likely to have decreased survival time based on histology. Eight subcutaneous masses had clean margins and one was dirty.

Using this MPM approach margins were considered dirty in 38.88% of cutaneous masses and 11.11% of subcutaneous masses (29.63% overall). However, long term follow-up was available for all but one case (mean follow up period 512 days; range, 30 to 1322 days). At the time of writing none of the dogs had recurrence at the site of surgery despite only 3 cases receiving adjunctive treatment following tumour removal.

Conclusion

Excision with a lateral margin equal to tumour diameter and a deep margin of at least one fascial plane may result in satisfactory excision of Patnaik grade I and II (or Kiupel low and high grade) MCTs in dogs despite the description of histologically 'dirty' margins found in 29.63% of the cases. This raises interesting questions regarding how accurate current methods used for assessment of histological margins are in canine MCTs and whether routine adjunctive therapy is indicated following a report of histologically 'dirty' margins.