Tumour Margins: A Medical and Radiation Perspective

In radiation oncology, margins are determined by a number of factors of which some are related to tumor type, others to previous treatments, diagnostic modalities used, and technical aspects. The veterinary literature on radiotherapy margins is scarce and mostly related to technical (so-called set-up) margins. Human guidelines are far better established and based on recommendations made in the ICRU (International Commission on Radiation Units & Measurements) reports 29, 50 and 62. Since most veterinary patients are relatively small, and an increase of the lateral field margins with 1mm, would increase a treated volume of 6 cm in diameter with as much as 10%, there is a continuous struggle to reduce margins as much as possible, without jeopardizing treatment effect.

In general, the radiation oncologist will identify the tumor or, following resection, the tumor bed/scar based on palpation or surgery and pathology reports. The tissue volume identified in this first step is referred to as the Gross Tumor Volume (GTV). In the case of a post-operative radiotherapy, determining the GTV based on only the scar can result in serious underestimation of the total area that requires treatment or, especially following major reconstructive surgery, even misplacement of the exact tumor bed location. In case of pre-planned post-operative radiotherapy, it is therefore advisable to place radiopaque markers, for instance, surgical staples, along the edges of the surgical field. When this is not possible pre- and operative (digital) pictures may prove useful. Another factor in determining GTV is related to the imaging technique used for establishing tumor size, the accuracy for outlining the size of a brain tumor is higher with a MRI scan compared to that with a CT-scan.

Following the establishment of the GTV, a first margin is added to compensate for delineation errors when drawing the GTV and the microscopic tumor spread/subclinical disease. For this purpose it is common use to apply the (veterinary) surgical lateral and deep margins, as well as the concept of tissue barriers. This second volume now created is referred to as the Clinical Target Volume (CTV).  [...]