Session Item

Subcutaneous tissue fibrosis (STF) is common in patients receiving RT for soft tissue sarcoma of the extremities (STSE). Grade 2 or above (grade2+) STF  has been previously been reported as  31% and 48%  respectively, for neoadjuvant and adjuvant RT. Traditionally, normal tissue sparing for STSE is done by adding a normal tissue corridor as an avoidance structure for RT plan optimisation. However, the corridor does not represent a discrete normal tissue structure. This preliminary work explores the dose-volume relationships of anatomically-defined normal tissues of the lower limb using novel outlining guidance.

This exploratory data analysis was conducted in 38 patients with lower limb STSE treated in either VorteX (NCT00423618) or IMRiS (NCT02520128) UK clinical trials. The VorteX phase III trial studied whether a reduction in radiotherapy treatment volumes reduced normal tissue toxicity for STSE treated with adjuvant radiotherapy to 66Gy. The IMRiS phase II trial studied the feasibility of Intensity-Modulated Radiotherapy (IMRT) in the neo-adjuvant (50Gy) and adjuvant (60 to 66Gy) settings. Novel outlining guidance was developed and used to delineate the anterior, posterior, and adductor muscle compartments of the thigh (MCTA, MCTP and MCTAD, respectively). An atlas of complication incidence (ACI) was generated to relate the dose-volume histograms for each normal tissue structure and  incidences of SFT grade 2+ (Jackson A. Semin Radiat Oncol 2016). The relative incidence of toxicity is presented as a colour map indicating the relative incidence. This analysis was conducted for all patients and separately for patients receiving neo-adjuvant RT.

Thirty-one patients received neo-adjuvant RT, all recruited in the IMRiS trial. Adjuvant RT was given to 7 patients (4 in Vortex and 3 in IMRiS trials). IMRT was delivered to 34 patients (31 neo- and 3 adjuvant RT). 3DCRT was given to 4 patients. STF grade 2+ incidence was 42% amongst all patients and 29% for the neo-adjuvant RT cohort. Grade2+ STF was higher when larger volumes of MCTA received doses above 45Gy (figure 1) for the entire cohort of 38 patients. The same trend was observed for the MCTP. Figure 2 shows a trend in higher incidences of STF when delivering relatively low doses of radiation (V5Gy – V25Gy) to the MCTA for neo-adjuvant RT. This could be attributable to the low-dose bath in IMRT, however, a larger cohort of patients is needed. There were no identifiable trends in the MCTAD ACIs at this stage.

This work identifies trends towards the development of grade 2+ STF when high doses are delivered to MCTA and MCTP and lower doses to the MCTA for neo-adjuvant RT only.   Further data is required to confirm. Novel outlining guidelines were developed and are consistently being used to outline. Future work will also include deriving and validating dose-volume constraints to avoid the  grade 2+ STF.

The first author is a clinical doctoral research fellow CDR-2018-04-ST2-004, funded by HEE/ NIHR