Collectively, it may be concluded from clinical studies evaluating the effectiveness of systemic chemotherapy for treating mast cell tumors that adjuvant systemic chemotherapy for treating residual microscopic disease is an alternative to curative-intent radiation therapy. However, additional studies are required to better define the true effect of different systemic chemotherapy protocols for treating residual neoplastic mast cell disease. Systemic chemotherapy administration is not restricted to referral centers, so it is a more widely available therapeutic option for both pet owners and veterinary practitioners. If you choose to treat an animal with adjuvant systemic chemotherapy, you must know the basic precautions required for the safe handling and administration of chemotherapy.

Intraregional deionized water

Mast cells are sensitive to changes in osmolality and when exposed to hypotonic solutions, will undergo cellular swelling and ultimately membrane lysis. Based on this cellular response, several studies have evaluated the safety and efficacy of intraregional deionized water injections as an adjuvant treatment for incompletely resected cutaneous mast cell tumors. Although all studies suggest that intraregional deionized water injections are well-tolerated, a marked disparity exists among studies regarding the efficacy of the injections in preventing local tumor recurrence.^28-30

In direct contrast, two other reports discuss intraregional deionized water injections as being effective when used in an adjuvant setting. In one study, 74 dogs with 118 mast cell tumors were treated with surgery alone or surgery combined with intraregional deionized water injections.²⁸ The local tumor recurrence rate in dogs receiving only surgery was 52.6% (10/19), while dogs treated with surgery and the injections had a recurrence rate of 26.2% (26/99). Furthermore, in a recent pilot investigation, only one dog out of 17 with incompletely resected cutaneous mast cell tumors treated with adjuvant intraregional deionized water injections developed local tumor regrowth.³⁰ Although intraregional deionized water injections may be reasonable for the adjuvant treatment of small, low- to moderate-grade mast cell tumors, their use is unlikely to be beneficial in treating diffuse, infiltrative tumors or aggressive grade III mast cell tumors.

The cost-effectiveness and ease of administering intraregional deionized water injections makes this investigational adjuvant therapy attractive. However, the direct contradictory results from clinical trials assessing their efficacy for treating resected cutaneous mast cell tumors remain problematic and a reason for concern. Until further studies are conducted to better clarify the role of deionized water injections, it remains difficult to wholeheartedly support this adjuvant treatment option. Additionally, despite being reported as well-tolerated, it is the experience of many clinicians that deionized water injections can cause moderate pain and discomfort.

Interstitial brachytherapy

Brachytherapy is a radiation treatment modality in which radioactive material sealed in needles, seeds, wires, or catheters is placed directly into or near a tumor. Treatment with brachytherapy may be intracavitary, intraluminal, or interstitial. For interstitial brachytherapy, radioactive materials are placed directly through the body tissue encompassing the tumor. Iridium-192 is a radioisotope capable of releasing both gamma and beta particles and can be used as a radioactive source for brachytherapy.

Recently, interstitial brachytherapy with iridium-192 has been evaluated as an adjuvant treatment for cutaneous mast cell tumors in dogs.³¹ In this study, nine dogs with microscopic residual disease and two dogs with macroscopic disease were treated with interstitial brachytherapy. Five of the 11 treated dogs ultimately had local tumor regrowth at a median of 1,391 days. Although a high percentage of dogs experienced tumor recurrence, the long median latency period before tumor regrowth was impressive. Given the extended period between initial brachytherapy and local regrowth, it remains a possibility that some of the dogs identified as treatment failures may actually have been affected with de novo tumor formation.

Interstitial brachytherapy offers some advantages over conventional external beam megavoltage radiation therapy, including more localized energy deposition, greater normal tissue sparing effect, and decreased total treatment duration. Unfortunately, the number of facilities offering brachytherapy is limited, and in conjunction with the observed high incidence for local tumor regrowth (5/11), it is difficult to recommend interstitial brachytherapy as a realistic or practical adjuvant treatment option for incompletely resected mast cell tumors.