Stereotactic radiotherapy for small and very small tumours (≤1 to ≤3 cc): evaluation of the influence of volumetric-modulated arc therapy in comparison to dynamic conformal arc therapy and 3D conformal radiotherapy as a function of flattened and unflattened beam models

Abstract

Purpose: The objective of this article is to evaluate the dosimetric efficacy of volumetric modulated arc therapy (VMAT) in comparison to dynamic conformal arc therapy (DCAT) and 3D conformal radiotherapy (3DCRT) for very small volume (≤1 cc) and small volume (≤3 cc) tumours for flattened (FF) and unflattened (FFF) 6 MV beams. Materials and methods: A total of 21 patients who were treated with single-fraction stereotactic radiosurgery, using either VMAT, DCAT or 3DCRT, were included in this study. The volume categorisation was seven patients each in <1, 1–2 and 2–3 cc volume. The treatment was planned with 6 MV FF and FFF beams using three different techniques: VMAT/Rapid Arc (RA) (RA_FF and RA_FFF), dynamic conformal arc therapy (DCA_FF and DCA_FFF) and 3DCRT (Static_FF and Static_FFF). Plans were evaluated for target coverage (V100%), conformity index, homogeneity index, dose gradient for 50% dose fall-off, total MU and MU/dose ratio [intensity-modulated radiotherapy (IMRT) factor], normal brain receiving >12 Gy dose, dose to the organ at risk (OAR), beam ON time and dose received by 12 cc of the brain. Result: The average target coverage for all plans, all tumour volumes (TVs) and delivery techniques is 96·4 ± 4·5 (range 95·7 ± 6·1–97·5 ± 2·9%). The conformity index averaged over all volume ranges <1, 2, 3 cc> varies between 0·55 ± 0·08 and 0·68 ± 0·04 with minimum and maximum being exhibited by DCA_FFF for 1 cc and Static_FFF/RA_FFF for 3 cc tumours, respectively. Mean IMRT factor averaged over all volume ranges for RA_FF, DCA_FF and Static_FF are 3·5 ± 0·8, 2·0 ± 0·2 and 2·0 ± 0·2, respectively; 50% dose fall-off gradient varies in the range of 0·33–0·42, 0·35–0·40 and 0·38–0·45 for 1, 2 and 3 cc tumours, respectively. Conclusion: This study establishes the equivalence between the FF and FFF beam models and different delivery techniques for stereotactic radiosurgery in small TVs in the range of ≤1 to ≤3 cc. Dose conformity, heterogeneity, dose fall-off characteristics and OAR doses show no or very little variation. FFF could offer only limited time advantage due to excess dose rate over an FF beam.

Effect of inaccurate small field output factors on brain SRS plans

“…Monasor Denia et al compared the detector performance in a Versa HD accelerator [40]. Mamballikalam et al evaluated the dosimetric efficacy of VMAT compared to dynamic conformal arc therapy and 3D conformal radiotherapy for very small (<1 cm 3 ) and small (<3 cm 3 ) volume tumours [41]. Lam et al reviewed small-field output factor measurements using various recent detectors [42].…”

Point dose verification of Cranial Stereotactic Radiosurgery using micro Ionization Chamber and EBT3 film for 6MV FF and FFF beams in Varian TrueBeam® LINAC

“…• Low spectral sensitivity. 1 The sensitive layer of the Gafchromic films has very close mass energy absorption coefficient (µ en /ρ) and Stopping power (S/ρ) to that of water and skeletal muscle which makes this detector water equivalent. 22 The ratios of (µ en /ρ) and (S/ρ) from gafchromic to water is within 3%.…”

“…There are mainly three techniques to deliver radiation dose; (a) multiple static conformal beams, (b) Direct Conformal Arc (DCA) and (c) Volumetric Arc Therapy. 1 VMAT technique is having in addition complex nature due to composite fields and inverse planning. It is important to verify radio-surgical absolute dose while using intensity modulation for SRS, because of its complex fields.…”

“…Modern Linacs with HD MLC, 6D Robotic Couch, and Exactrac system is capable of delivering frameless cranial stereotactic radiosurgery. 1 Patient-specific dose verification is an integral part of SRS treatment, where a dosimeter or radiochromic film chamber is used to verify that the plan created in the Treatment Planning System (TPS) is executable within the permissible deviation of + or -3%.…”

Time motion study to evaluate the impact of flattening filter free beam on overall treatment time for frameless intracranial radiosurgery using Varian TrueBeam® linear accelerator

“…In MLC based Intensity Modulated Radiotherapy (IMRT) required fluence distribution can be achieved by modulating the un-flattened non-uniform fluence distribution directly [11]. The fluence of photons that comes out of an unflattened beam is dosimetrically different from a flattened beam [12,13]. Beam hardening does not occur since there is no flattening filter, and the fluence of photons contains a larger number of low energy photons (< 1 MeV).…”