Session Item

Technological advances in radiotherapy technique have allowed dose escalation in many tumour types. There is better local tumour control with increased radiation dose.

In this centre, rectal radiotherapy treatment using volumetric modulated arc radiotherapy (VMAT) with an additional simultaneous integrated boost (SIB) up to 50, 52 or 54Gy to either primary tumour, pelvic lymph nodes (LN) or both has been developed. The purpose of this retrospective audit was to investigate the toxicity of treatment and patient outcomes of this new technique.

All patients treated with rectal VMAT and SIB between 2015-18 were identified. Data was collected on patient characteristics, acute toxicity (within 3 months of completion of CRT), long-term toxicity (greater than 3 months after completion of CRT), radiotherapy parameters (such as the boost planning target volume (PTV_boost ) and dose to organs at risk (OARs)), and response parameters (clinical, pathological and radiological).

A total of 66 patients, 49 men and 17 women, were identified (median age of 66 years). Fifty two percent of tumours were located in the lower third of the rectum, 24% mid and 21% upper. The SIB was given to pelvic LN regions in 50% of patients, to primary site in 44%, and to both regions in 6%.

The median PTV_boost treated with SIB was 154cc (IQR 41 to 265). The primary tumour PTV_boost was larger (266cc IQR(178-367), than the pelvic LN PTV_boost (44cc (IQR 22-103). 

Patients who received SIB to pelvic LN regions were more likely to receive 54Gy (73%, n=24) compared to SIB to the primary with 62% (n=18) receiving 52Gy (figure 1).

Figure 1. Number of patients assigned a 50Gy, 52Gy or 54Gy SIB, boost location and dose constraint of two OARs.

All patients completed CRT except one patient who discontinued at fraction 22 due to grade 3 diarrhoea. The most commonly reported acute toxicities were skin reactions (29%) and proctitis (24%). The most commonly reported long-term toxicities were fatigue (8%) and small/large intestine problems (6%). 

There was an association between size of PTV_boost and acute toxicity; 36% of patients with PTV _boost <100cc experienced acute toxicity compared to 71% with volume of >200cc (OR 4.4 95%CI (1.3 to 15.5), p=0.019). After adjusting for sex and level of cancer the odds ratio reduced to 3.1 (0.8 to 11.9), p=0.1 (table 1).

Table 1. Comparison of % acute toxicity with PTV boost volume^*

Rectal VMAT with SIB is a relatively new technique. Our results demonstrate an association with PTV_boost volume and acute toxicity but no association with PTV_boost dose was found in this study.