Optimization of the dosimetric leaf gap for use in planning VMAT treatments of spine SABR cases

Abstract

The dosimetric leaf gap (DLG) is a beam configuration parameter used in the Varian Eclipse treatment planning system, to model the effects of rounded MLC leaf ends. Measuring the DLG using the conventional sliding‐slit technique has been shown to be produce questionable results for some volumetric modulated arc therapy (VMAT) treatments. This study therefore investigated the use of radiochromic film measurements to optimize the DLG specifically for the purpose of producing accurate VMAT plans using a flattening‐filter‐free (FFF) beam, for use in treating vertebral targets using a stereotactic (SABR, also known as SBRT) fractionation schedule. Four test treatments were planned using a VMAT technique, to deliver a prescription of 24 Gy in 3 fractions to four different spine SABR treatment sites. Measurements of the doses delivered by these treatments were acquired using an ionization chamber and radiographic film. These measurements were compared with the doses calculated by the treatment planning system using a range of DLG values, including a DLG identified using the conventional sliding‐slit method (1.1 mm). An optimal DLG value was identified, as the value that produced the closest agreement between the planned and measured doses (1.9 mm). The accuracy of the dose calculations produced using the optimized DLG value was verified using additional radiochromic film measurements in a heterogeneous phantom. This study provided a specific initial DLG (1.9 mm) as well as a film‐based optimization method, which may be used by radiotherapy centers when attempting to commission or improve an FFF VMAT‐based SABR treatment programme.

On the need for tuning the dosimetric leaf gap for stereotactic treatment plans in the Eclipse treatment planning system

“…Some studies have found good agreement between calculated and delivered doses by using the DLG measured with sweeping gap tests or the dynamic chair test . However, other authors found substantial discrepancies and reported on the need for tuning the DLG value configured in the Eclipse TPS. Kielar et al observed discrepancies between calculated and measured doses around 5% for the Varian's high‐definition multileaf collimator (HDMLC), which were greatly reduced by increasing the DLG entered into the TPS by more than 1 mm.…”

A study of minimum segment width parameter on VMAT plan quality, delivery accuracy, and efficiency for cervical cancer using Monaco TPS

“…Over the last decade, volumetric modulated arc therapy (VMAT) has been explored and implemented to treat a variety of cancers including in the prostate, head and neck, lung, and spine 1, 2, 3, 4. VMAT is a dynamic treatment technique in which the radiation dose rates, gantry speeds, and movements of the multi‐leaf collimator and jaws are simultaneously varied while the beam is on 5.…”

Technical note: A modified gamma evaluation method for dose distribution comparisons

“…The 3% ΔD criterion was selected in this study because it is widely used, in combination with a range of Δd values, but the use of 3% as a ΔD criterion is not universal. Similarly, GAI values of 100% and 95% were selected as examples of extreme and commonly‐used PSQA action levels, respectively.…”

“…Recent surveys suggested that 3%/3 mm are currently the most commonly used gamma evaluation criteria, and the AAPM's task group report on IMRT commissioning (TG‐119) used 3%/3 mm and the AAPM's more recent task group report on modulated radiotherapy quality assurance (TG‐218) recommended 3%/2 mm. Investigations of cranial and extra‐cranial stereotactic radiotherapy have reported the use of 3%/2 mm, 3%/1.5 mm, 3%/1 mm and 3%/0.3 mm criteria.…”