Influence of tumor location on the intensity-modulated radiation therapy plan of helical tomotherapy

The impact of the offset distance between the planning target volume and isocenter on irradiation time in TomoTherapy: A phantom study

PURPOSE

The influence of the offset distance from treatment target to gantry isocenter (GIC) on the dosimetric parameter and irradiation time was investigated using TomoTherapy METHODS: The reference position was defined as the centers of both the I'mRT phantom and planning target volume (PTV) with a spherical of 4 cm diameter aligned with the GIC. The dose calculations were performed in two offset methods with 2 and 12 Gy/fr, Method 1. The PTV was moved from 0.0 to 12.5 cm along the RL direction and -5.0 to 5.0 cm along the AP direction (PTV offset), Method 2. The phantom was moved from 0.0 to -7.5 cm along the RL direction and -5.0 to 5.0 cm along the AP direction (Phantom offset). The maximum, minimum and mean doses, homogeneity index, conformity index, irradiation time, and monitor unit were compared.

RESULTS

The irradiation times increased with increasing PTV offset. The increases in the irradiation time were 54.4% and 40.8% at PTV offsets of 12.5 cm along the RL direction for 2 and 12 Gy/fr, while the increases were 20.1% and 15.0% at a PTV offset of 5.0 cm along the AP direction. An increased irradiation time was not observed for the phantom offset. The offset didn't affect the other parameters.

CONCLUSIONS

The PTV location offset of ≥5 cm from the GIC along the RL and AP axes increased the irradiation time; therefore, the PTV should be aligned with the GIC as much as possible to reduce the irradiation time on TomoTherapy.

Emphysema quantification on computed tomography and its value in predicting radiation pneumonitis in lung cancer treated by stereotactic body radiotherapy

A large portion of patients with early-stage non-small-cell lung cancer (NSCLC) who are receiving stereotactic body radiation therapy (SBRT) are medically inoperable due to compromised pulmonary function, and among these patients pulmonary emphysema (PE) is common. However, the relationship between PE and radiation-induced lung injuries remains unclear. In this study, we aimed to describe the full spectrum of computed tomography (CT) features after SBRT for NSCLC, and to explore their relationship with variables, including PE and dosimetric factors. In all, 71 patients were enrolled. PE was quantified as the percentage of low attenuation area [attenuation values of <-860 Hounsfield units (HU)] within the radiation field (%LAA-860). Spearman's correlation and logistic regression were used to explore factors related to radiological features and radiation pneumonitis (RP). At the 1-year follow-up, acute radiological changes included: (i) diffuse consolidation, 11.3%; (ii) patchy consolidation and ground-glass opacities, 42.3%; and (iii) patchy ground-glass opacity, 14.1%. Late morphological changes occurred in 61.9% of patients (50.7% with a modified conventional pattern, 5.6% with a mass-like pattern and 5.6% with a scar-like pattern). Lower %LAA-860 was the only factor that was significantly associated with consolidation changes at 6 months after SBRT [odds ratio (OR), 0.008; P = 0.009], and it was also a significant predictor for Grade ≥ 2 RP (OR, 0.003; P = 0.04). Our study showed that patients with PE can benefit from SBRT on the condition that good control of dose-volume constraints is achieved.