The dosimetric effect of electron density overrides in 3DCRT Lung SBRT for a range of lung tumor dimensions

Abstract

The combined effects of lung tumor motion and limitations of treatment planning system dose calculations in lung regions increases uncertainty in dose delivered to the tumor and surrounding normal tissues in lung stereotactic body radiotherapy (SBRT). This study investigated the effect on plan quality and accuracy when overriding treatment volume electron density values. The QUASAR phantom with modified cork cylindrical inserts, each containing a simulated spherical tumor of 15‐mm, 22‐mm, or 30‐mm diameter, was used to simulate lung tumor motion. Using Monaco 5.1 treatment planning software, two standard plans (50% central phase (50%) and average intensity projection (AIP)) were compared to eight electron density overridden plans that focused on different target volumes (internal target volume (ITV), planning target volume (PTV), and a hybrid plan (HPTV)). The target volumes were set to a variety of electron densities between lung and water equivalence. Minimal differences were seen in the 30‐mm tumor in terms of target coverage, plan conformity, and improved dosimetric accuracy. For the smaller tumors, a PTV override showed improved target coverage as well as better plan conformity compared to the baseline plans. The ITV plans showed the highest gamma pass rate agreement between treatment planning system (TPS) and measured dose (P < 0.040). However, the low electron density PTV and HPTV plans also showed improved gamma pass rates (P < 0.035, P < 0.011). Low‐density PTV overrides improved the plan quality and accuracy for tumor diameters less than 22 mm only. Although an ITV override generated the most significant increase in accuracy, the low‐density PTV plans had the additional benefit of plan quality improvement. Although this study and others agreed that density overrides improve the treatment of SBRT, the optimal density override and the conditions under which it should be applied were found to be department specific, due to variations in commissioning and calculation methods.

Acuros® dose verification of ultrasmall lung lesions with EBT‐XD film in a homogeneous and heterogeneous anthropomorphic phantom setup

“…[20][21][22] Although an anthropomorphic phantom is nowadays the best representation of the actual patient anatomy, the sizes of the water-equivalent targets are in general still large compared to the small tumor volumes encountered in patients that eligible for SBRT. 23 For example, the phantom used by Sepp€ al€ a et al 24 contains a spherical target of 1.5 and 4.0 cm in diameter, which would clinically be treated with SBRT (maximum diameter ≤5.0 cm, RTOG 0236) but such a large volume does not pose a serious challenge to type "b" or "c" algorithms as their results showed. [25][26][27] It is therefore interesting to investigate when type "c" dose calculation algorithms start to deviate beyond a relative uncertainty of 5%, 12 especially for very small tumors with tumor diameters below 1 cm, that are nowadays treated with SBRT.…”

“…A dosimetric plan verification setup is chosen using either an inhomogeneous slab phantom or more sophisticated anthropomorphic phantoms with a spherical water‐equivalent target inside lung‐equivalent material to verify the TPS dose prediction with actual measurements 20–22 . Although an anthropomorphic phantom is nowadays the best representation of the actual patient anatomy, the sizes of the water‐equivalent targets are in general still large compared to the small tumor volumes encountered in patients that eligible for SBRT 23 . For example, the phantom used by Seppälä et al 24 contains a spherical target of 1.5 and 4.0 cm in diameter, which would clinically be treated with SBRT (maximum diameter ≤5.0 cm, RTOG 0236) but such a large volume does not pose a serious challenge to type “b” or “c” algorithms as their results showed 25–27 …”

Guía para el control de calidad y seguridad de los sistemas de planificación y planes de tratamiento de radioterapia externa

“…Esto puede ser conveniente en algunos casos en los que el CTV esté rodeado de grandes heterogeneidades, por ejemplo, pulmón. 203,204 • Disminuyendo el grado de modulación de intensidad cuando haya movimientos respiratorios/cardíacos o, en general, riesgo de movimientos intrafracción (ver siguiente apartado).…”

Technical Note: A 3D‐printed phantom for radiochromic film evaluation of moving lung tumor SBRT without dose convolution

“…9 This effect can be offset by mathematical transformation, where the blurring is considered as a convolution of the static dose distributions that would result when there is no motion. 9 However, while this strategy has been applied in several studies using radiochromic films and has been proven to function well in most clinical situations, [10][11][12] it is not used clinically because it is not available in most treatment planning systems (TPSs) or QA tools. Moreover, it can introduce uncertainties that are difficult to estimate, such as when using deformable image registration maps to accumulate the dose matrix.…”