Studying the use of Aperture Shape Controller (ASC) and Convergence Mode (CM) in Eclipse (Varian Medical System) in terms of plan quality and complexity of volumetric modulated arc therapy (VMAT).

Forty VMAT plans were re-optimised for the prostate, prostate + lymph nodes, breast and head & neck patients retrospectively, changing the ASC settings (off, moderate, very high) and CM settings (off, on and extended).

Using ‘on’ or ‘extended’ CM increased plan quality in terms of planning target volume homogeneity and low-dose spread to the organs at risk (OAR). ‘Extended’ CM increased the optimisation time 4·3-fold compared to ‘on’, and deteriorated the plan quality in several simple planning cases. ‘Moderate’ ASC decreased plan complexity with minor effect on plan quality compared to ‘off’, but ‘very high’ ASC had larger adverse dosimetric effects. However, the ASC decreased the plan complexity only if the CM was turned ‘on’.

Using ‘on’ CM increases the plan quality but using ‘extended’ CM is not recommended. The ‘moderate’ ASC decreased complexity without significant adverse effects on plan quality, and even ‘very high’ ASC may be used when plan simplicity is prioritised. However, if CM is not used, the ASC should also be turned off.

Prostate cancer is one of the most common solid malignancies and has a high morbidity rate. The uncertainty of the prostate location compromises the overall treatment plan optimisation. To account for the location uncertainty, the radiation oncologist needs to expand the margin of the planning target volume (PTV), which may increase the radiation toxicity to organs in proximity.

In this study, we investigated the quality of treatment plans for a patient with different ring sizes (2 and 3 cm). A small ring-shaped structure circumferentially around the PTV helps in defining the location of PTV. Prostate and pelvic node plans were analysed with dose prescription to 99% of PTV.

Additional ring-shaped structures led to more conformal dose coverage for target with reduced radiation side effects to nearby organ at risk (OAR). Expected treatment time was slightly higher for 2 cm ring compared to 3 cm ring. In case of prostate, expected duration was 4% higher, while for node plan, expected duration for 2 cm ring was 16% higher compared to 3 cm ring plan.

It was observed that using a smaller size ring can lead to improved dose sparing to OAR with same target coverage as with larger dimension ring. The composite plans do not show any clinically significant difference in dose to OARs.