2326 - Evaluation of Lung Dose Reduction in VMAT Treatment Plans for Esophageal Cancer: A Comparative Study of Avoidance Strategies

D. K. Kim, B. Park, T. Y. Park, J. Y. Park, C. Y. Kim, and D. Y. Kwon; Department of Radiation Oncology, Samsung Medical Center, Seoul, Korea, Republic of (South)

Purpose/Objective(s): This study aims to compare 3 Volumetric Modulated Arc Therapy (VMAT) treatment plans, namely Full Arc-Avoidance Sector (FA-AS), Full Arc-Avoidance Structure (FA-ST), and Partial Arc (PA), designed to reduce lung dose in esophageal cancer patients. Materials/

Methods: The Planning Target Volume (PTV) of 10 esophageal cancer patients treated at our institution were transferred to the ATOM Phantom CT image. All treatment plans were executed using an advanced dose calculation algorithm. The plan aimed for 66Gy in 33 fractions. The treatment plans were categorized into three methods by restricting the incident beams in the Gantry Angle (GA) range of 240° to 300° and 60° to 120°. In the FA-AS, we generated 4 arcs with GA of 181° to 179° and set Avoidance Sectors in the range of 240° to 300° and 60° to 120°, ensuring Beam Off during these intervals. The FA-ST involved creating a ring-shaped structure with a diameter of 15 cm and a thickness of 5 mm at the center of the ATOM Phantom CT image, matching the SI range of each PTV. Avoidance Structures were generated using only the portions corresponding to GA of 240° to 300° and 60° to 120°. 4 arcs with GA of 181° to 179° were created, applying Entry Mode to the structure. The PA generated a total of 12 arcs (4 arcs each for GA 181° to 240°, 300° to 60°, and 120° to 179°) to apply the same control points as other methods based on a Full Arc approach. For PTV evaluation, we utilized Homogeneity Index (H.I) and Conformity Index (C.I). Evaluation of Organs at Risk included D_mean of heart and D_mean, V_5Gy of both lung. For low dose region assessment, the 50% Isodose volume of the prescribed dose was compared. Treatment plan efficiency was evaluated by comparing Total MU and Beam On Time. Differences were assessed for statistical significance (P<0.05) by use of One-way ANOVA.
Results: FA-ST demonstrated superior results in PTV H.I and mean lung dose but exhibited similar outcomes to the FA-AS (P>0.05). FA-AS exhibited advantages in terms of simplicity, reducing Total MU by 41.3% (P<0.001), and decreasing the 50% Isodose Volume by 1.7% (P<0.05). Both FA-AS and FA-ST outperformed PA in mean lung dose reduction by approximately 4% (P<0.001).
Conclusion: For esophageal cancer VMAT, adopting FA-AS is recommended due to its simplicity, significant reduction in total MU, and a lung dose reduction similar to that of FA-ST. These findings emphasize the importance of selecting an optimal avoidance technique to enhance treatment efficiency and minimize radiation-induced side effects such as radiation pneumonitis. Abstract 2326 – Table 1

Parameters		FA-AS	FA-ST	PA		P-value
		Mean				FA-AS vs. FA-ST	FA-AS vs. PA	FA-ST vs. PA
PTV	H.I	1.06	1.05	1.07		<0.001	<0.001	<0.001
	C.I	0.97	0.96	0.98		0.065	0.04	<0.001
Heart	Dmean (cGy)	3558.1	3576.6	3398.1		0.729	0.003	0.003
Lung	V5Gy (%)	69.3	69.7	72		0.471	<0.001	<0.001
	Dmean (cGy)	1549.7	1542.3	1607.4		0.479	<0.001	<0.001
50% Isodose Volume (cc)		1834.8	1868	1736.3		0.023	<0.001	<0.001
Total MU		641	1093	603		<0.001	<0.001	<0.001
Beam On Time (sec)		151.3	146.6	167.8		<0.001	<0.001	<0.001