A Dosimetric Study of Inhomogeneous Dose Delivery: A Novel Approach to Dose Escalation

Abstract

Purpose/Objective(s)To evaluate the dosimetric feasibility of creating inhomogeneous plans as an approach to overall target dose escalation.Materials/MethodsThe study included volumetric modulated arc plans obtained with an Eclipse planning system from 11 localized prostate cancer cases. The clinical target volume (CTV) was the prostate gland. The planning target volume (PTV) was an expansion of CTV by 3 mm posteriorly and 4 mm in other directions. The prescribed dose (PD) to the PTV was 40 Gy (100%) to be delivered in 5 fractions with a RapidArc technique, similar to an ongoing SBRT protocol in our department. The intent in this exercise was to perform dose escalation within the PTV by increasing mean doses, rather than the minimum or peripheral doses. To create PTV dose inhomogeneity, a PTV sub-volume was defined to receive a higher dose. This high-dose volume (HDV) was created by shrinking the CTV by 5 mm and subtracting the urethral volume with a margin of 3 mm. The HDV was intended to receive at least 150% of the PD with no maximal dose limit. The mean PTV was 90 cc (range 54 to 127 cc). The HDV represented an average of 24% of the prostate PTV volume (range 15 to 38%). Rectal and bladder constraints were the same at: 50% < 40 Gy, 20% < 32 Gy, 12% < 36 Gy, and 8% < 40 Gy.ResultsThe profiles of the PTV dose volume histograms resemble those achieved with high dose rate brachytherapy of the prostate. With respect to the target areas, the average PTV dose was 132% of the PD (range 110 to 140%). The HDV received an average of 155% of the PD (range 120 to 183%). The average of the HDV maximal doses was 195% of the PD (range 128.3 to 236.3%). Critical structure constraints were met in all cases. Overall, the mean rectal volume receiving 36 Gy was <5% (range 3 to 11%); the mean bladder volume receiving 36 Gy was <3% (range 1 to 12%) The urethral mean dose was 106% of the PD (range 91 to 128%).ConclusionsIn this study, allowing dose inhomogeneity within the prostate gland resulted in large dose escalation without compromise of critical normal tissues. Brachytherapy target dose/volume profiles can be safely achieved with this novel external beam technique. Purpose/Objective(s)To evaluate the dosimetric feasibility of creating inhomogeneous plans as an approach to overall target dose escalation. To evaluate the dosimetric feasibility of creating inhomogeneous plans as an approach to overall target dose escalation. Materials/MethodsThe study included volumetric modulated arc plans obtained with an Eclipse planning system from 11 localized prostate cancer cases. The clinical target volume (CTV) was the prostate gland. The planning target volume (PTV) was an expansion of CTV by 3 mm posteriorly and 4 mm in other directions. The prescribed dose (PD) to the PTV was 40 Gy (100%) to be delivered in 5 fractions with a RapidArc technique, similar to an ongoing SBRT protocol in our department. The intent in this exercise was to perform dose escalation within the PTV by increasing mean doses, rather than the minimum or peripheral doses. To create PTV dose inhomogeneity, a PTV sub-volume was defined to receive a higher dose. This high-dose volume (HDV) was created by shrinking the CTV by 5 mm and subtracting the urethral volume with a margin of 3 mm. The HDV was intended to receive at least 150% of the PD with no maximal dose limit. The mean PTV was 90 cc (range 54 to 127 cc). The HDV represented an average of 24% of the prostate PTV volume (range 15 to 38%). Rectal and bladder constraints were the same at: 50% < 40 Gy, 20% < 32 Gy, 12% < 36 Gy, and 8% < 40 Gy. The study included volumetric modulated arc plans obtained with an Eclipse planning system from 11 localized prostate cancer cases. The clinical target volume (CTV) was the prostate gland. The planning target volume (PTV) was an expansion of CTV by 3 mm posteriorly and 4 mm in other directions. The prescribed dose (PD) to the PTV was 40 Gy (100%) to be delivered in 5 fractions with a RapidArc technique, similar to an ongoing SBRT protocol in our department. The intent in this exercise was to perform dose escalation within the PTV by increasing mean doses, rather than the minimum or peripheral doses. To create PTV dose inhomogeneity, a PTV sub-volume was defined to receive a higher dose. This high-dose volume (HDV) was created by shrinking the CTV by 5 mm and subtracting the urethral volume with a margin of 3 mm. The HDV was intended to receive at least 150% of the PD with no maximal dose limit. The mean PTV was 90 cc (range 54 to 127 cc). The HDV represented an average of 24% of the prostate PTV volume (range 15 to 38%). Rectal and bladder constraints were the same at: 50% < 40 Gy, 20% < 32 Gy, 12% < 36 Gy, and 8% < 40 Gy. ResultsThe profiles of the PTV dose volume histograms resemble those achieved with high dose rate brachytherapy of the prostate. With respect to the target areas, the average PTV dose was 132% of the PD (range 110 to 140%). The HDV received an average of 155% of the PD (range 120 to 183%). The average of the HDV maximal doses was 195% of the PD (range 128.3 to 236.3%). Critical structure constraints were met in all cases. Overall, the mean rectal volume receiving 36 Gy was <5% (range 3 to 11%); the mean bladder volume receiving 36 Gy was <3% (range 1 to 12%) The urethral mean dose was 106% of the PD (range 91 to 128%). The profiles of the PTV dose volume histograms resemble those achieved with high dose rate brachytherapy of the prostate. With respect to the target areas, the average PTV dose was 132% of the PD (range 110 to 140%). The HDV received an average of 155% of the PD (range 120 to 183%). The average of the HDV maximal doses was 195% of the PD (range 128.3 to 236.3%). Critical structure constraints were met in all cases. Overall, the mean rectal volume receiving 36 Gy was <5% (range 3 to 11%); the mean bladder volume receiving 36 Gy was <3% (range 1 to 12%) The urethral mean dose was 106% of the PD (range 91 to 128%). ConclusionsIn this study, allowing dose inhomogeneity within the prostate gland resulted in large dose escalation without compromise of critical normal tissues. Brachytherapy target dose/volume profiles can be safely achieved with this novel external beam technique. In this study, allowing dose inhomogeneity within the prostate gland resulted in large dose escalation without compromise of critical normal tissues. Brachytherapy target dose/volume profiles can be safely achieved with this novel external beam technique.