Attempted “seamless matchline” of intensity-modulated radiation therapy headsstatic neck fields for nasopharyngeal cancer: Treatment planningsdosimetric verification
2004; Elsevier BV; Volume: 60; Linguagem: Inglês
10.1016/s0360-3016(04)01964-9
ISSN1879-355X
AutoresXuxia Huang, K.M. Kuan, GuangJun Xiao, S.Y. Tsao, Xibin Qiu, Kelvin K. Ng,
Tópico(s)Lung Cancer Diagnosis and Treatment
ResumoPurpose/ObjectiveFor nasopharyngeal cancers (NPC), if the PTV covers also the lower neck, intensity-modulated radiation therapy (IMRT) with a monoisocentric technique is commonly employed. However, dedicated isocenters for the vastly different anatomies of the head and the neck are, in fact, more preferable, especially with the advent of thin-leaf multileaf collimators (MLC) and more versatile treatment planning systems (TPS), provided that there is satisfactory progress on the arduous task for field-matching. Working on the ultimate goal of a "seamless matchline" for IMRT fields for the head and static fields for the neck regions, we made an initial attempt.Materials/MethodsAt the Kiang Wu Hospital, indicated NPC patients in thermoplastic shells were simulated on a Varian simulator for static neck and IMRT head fields (by a 120-leaf Millennium MLC) on a Varian linac coupled with Eclipse TPS. PTV and OAR were delineated with appropriate cross-sectional imaging of the head and neck. Isocenter 1 for the lower neck static field (6 MV photons) was placed - the field had 3 segments with identical isocenters and equal weighting, forming an overlapping "dose-buffering zone". Resembling "stepsshoot", the upper border had 3 steps: "Neck 1": at the lower margin of the PTV for the head region as viewed with beam's eye view (BEV); "Neck 2s3": 0.5 cm and 1.0 cm respectively above "Neck 1" - creating a multi-matchline zone. Isocenter 2 was in the head region for IMRT with sliding window, 7 or 9 fields, 6 MV photons. Field size was PTV + 0.5 cm. Inverse IMRT optimization yielded a uniform composite dose distribution in the PTV and matchline which was an "intelligent" junctional zone, with due consideration of all dose contributions from the neck fields. Prior to treatment, dosimetric verification was done by ionization chamber and X-ray films.ResultsNo significant "hot spots" nor "cold spots" were detected. After fluence mapping with ionization chamber on 138 fields, less than 6% had a variation of the calculated versus measured value of over ±3%; no single field had any variation of over ±5%; average variation was 0.1%; all resultant absolute dose value as measured by ionization chamber had a variation of less than ±3%. The isodoses as measured by film matched well with that calculated by TPS. At the matchline, the variation was within 5% for combined fields. Our final composite dose distribution showed definite improvements on matchline dose inhomogeneity.ConclusionsWith multiisocentric techniques, field-matching methods, e.g., a gap between the head and the neck fields or, IMRT optimization for a simple overlapping zone, are prone to disproportionately large dosage variations even for minor matchline parameter errors on setup. Monoisocentric non-divergent "half-beam" techniques are less prone but still cannot tailor adequately for the very unique anatomy of the head - highly complex yet rigid enough for good reproducibility. Utilizing fully the new Base Plan optimization feature of Eclipse for enhanced matchline dosimetry, we have initial success to obviate such unpredictable setup errors - by our "dose-buffering zone". The thinnest (5 mm) leaves of the MLC were also fully utilized by placing them directly over the head region as far as feasible. Moreover, with matchlines away from critical structures, safety was also enhanced. Thus, dedicated isocenters for the head and the neck are now more practicable - a step in the right direction towards the ultimate goal of a "seamless matchline" for NPC treatment. As the measured value at the matching zone still varies by 5%, further fine-tuning is still required, e.g., with more segments for the neck field or by designing better "dose-buffering zones" with collimator and/or MLC adjustments. As the strictest QA is paramount for IMRT, actual documentation of dose variation is mandatory regardless of the method employed Purpose/ObjectiveFor nasopharyngeal cancers (NPC), if the PTV covers also the lower neck, intensity-modulated radiation therapy (IMRT) with a monoisocentric technique is commonly employed. However, dedicated isocenters for the vastly different anatomies of the head and the neck are, in fact, more preferable, especially with the advent of thin-leaf multileaf collimators (MLC) and more versatile treatment planning systems (TPS), provided that there is satisfactory progress on the arduous task for field-matching. Working on the ultimate goal of a "seamless matchline" for IMRT fields for the head and static fields for the neck regions, we made an initial attempt. For nasopharyngeal cancers (NPC), if the PTV covers also the lower neck, intensity-modulated radiation therapy (IMRT) with a monoisocentric technique is commonly employed. However, dedicated isocenters for the vastly different anatomies of the head and the neck are, in fact, more preferable, especially with the advent of thin-leaf multileaf collimators (MLC) and more versatile treatment planning systems (TPS), provided that there is satisfactory progress on the arduous task for field-matching. Working on the ultimate goal of a "seamless matchline" for IMRT fields for the head and static fields for the neck regions, we made an initial attempt. Materials/MethodsAt the Kiang Wu Hospital, indicated NPC patients in thermoplastic shells were simulated on a Varian simulator for static neck and IMRT head fields (by a 120-leaf Millennium MLC) on a Varian linac coupled with Eclipse TPS. PTV and OAR were delineated with appropriate cross-sectional imaging of the head and neck. Isocenter 1 for the lower neck static field (6 MV photons) was placed - the field had 3 segments with identical isocenters and equal weighting, forming an overlapping "dose-buffering zone". Resembling "stepsshoot", the upper border had 3 steps: "Neck 1": at the lower margin of the PTV for the head region as viewed with beam's eye view (BEV); "Neck 2s3": 0.5 cm and 1.0 cm respectively above "Neck 1" - creating a multi-matchline zone. Isocenter 2 was in the head region for IMRT with sliding window, 7 or 9 fields, 6 MV photons. Field size was PTV + 0.5 cm. Inverse IMRT optimization yielded a uniform composite dose distribution in the PTV and matchline which was an "intelligent" junctional zone, with due consideration of all dose contributions from the neck fields. Prior to treatment, dosimetric verification was done by ionization chamber and X-ray films. At the Kiang Wu Hospital, indicated NPC patients in thermoplastic shells were simulated on a Varian simulator for static neck and IMRT head fields (by a 120-leaf Millennium MLC) on a Varian linac coupled with Eclipse TPS. PTV and OAR were delineated with appropriate cross-sectional imaging of the head and neck. Isocenter 1 for the lower neck static field (6 MV photons) was placed - the field had 3 segments with identical isocenters and equal weighting, forming an overlapping "dose-buffering zone". Resembling "stepsshoot", the upper border had 3 steps: "Neck 1": at the lower margin of the PTV for the head region as viewed with beam's eye view (BEV); "Neck 2s3": 0.5 cm and 1.0 cm respectively above "Neck 1" - creating a multi-matchline zone. Isocenter 2 was in the head region for IMRT with sliding window, 7 or 9 fields, 6 MV photons. Field size was PTV + 0.5 cm. Inverse IMRT optimization yielded a uniform composite dose distribution in the PTV and matchline which was an "intelligent" junctional zone, with due consideration of all dose contributions from the neck fields. Prior to treatment, dosimetric verification was done by ionization chamber and X-ray films. ResultsNo significant "hot spots" nor "cold spots" were detected. After fluence mapping with ionization chamber on 138 fields, less than 6% had a variation of the calculated versus measured value of over ±3%; no single field had any variation of over ±5%; average variation was 0.1%; all resultant absolute dose value as measured by ionization chamber had a variation of less than ±3%. The isodoses as measured by film matched well with that calculated by TPS. At the matchline, the variation was within 5% for combined fields. Our final composite dose distribution showed definite improvements on matchline dose inhomogeneity. No significant "hot spots" nor "cold spots" were detected. After fluence mapping with ionization chamber on 138 fields, less than 6% had a variation of the calculated versus measured value of over ±3%; no single field had any variation of over ±5%; average variation was 0.1%; all resultant absolute dose value as measured by ionization chamber had a variation of less than ±3%. The isodoses as measured by film matched well with that calculated by TPS. At the matchline, the variation was within 5% for combined fields. Our final composite dose distribution showed definite improvements on matchline dose inhomogeneity. ConclusionsWith multiisocentric techniques, field-matching methods, e.g., a gap between the head and the neck fields or, IMRT optimization for a simple overlapping zone, are prone to disproportionately large dosage variations even for minor matchline parameter errors on setup. Monoisocentric non-divergent "half-beam" techniques are less prone but still cannot tailor adequately for the very unique anatomy of the head - highly complex yet rigid enough for good reproducibility. Utilizing fully the new Base Plan optimization feature of Eclipse for enhanced matchline dosimetry, we have initial success to obviate such unpredictable setup errors - by our "dose-buffering zone". The thinnest (5 mm) leaves of the MLC were also fully utilized by placing them directly over the head region as far as feasible. Moreover, with matchlines away from critical structures, safety was also enhanced. Thus, dedicated isocenters for the head and the neck are now more practicable - a step in the right direction towards the ultimate goal of a "seamless matchline" for NPC treatment. As the measured value at the matching zone still varies by 5%, further fine-tuning is still required, e.g., with more segments for the neck field or by designing better "dose-buffering zones" with collimator and/or MLC adjustments. As the strictest QA is paramount for IMRT, actual documentation of dose variation is mandatory regardless of the method employed With multiisocentric techniques, field-matching methods, e.g., a gap between the head and the neck fields or, IMRT optimization for a simple overlapping zone, are prone to disproportionately large dosage variations even for minor matchline parameter errors on setup. Monoisocentric non-divergent "half-beam" techniques are less prone but still cannot tailor adequately for the very unique anatomy of the head - highly complex yet rigid enough for good reproducibility. Utilizing fully the new Base Plan optimization feature of Eclipse for enhanced matchline dosimetry, we have initial success to obviate such unpredictable setup errors - by our "dose-buffering zone". The thinnest (5 mm) leaves of the MLC were also fully utilized by placing them directly over the head region as far as feasible. Moreover, with matchlines away from critical structures, safety was also enhanced. Thus, dedicated isocenters for the head and the neck are now more practicable - a step in the right direction towards the ultimate goal of a "seamless matchline" for NPC treatment. As the measured value at the matching zone still varies by 5%, further fine-tuning is still required, e.g., with more segments for the neck field or by designing better "dose-buffering zones" with collimator and/or MLC adjustments. As the strictest QA is paramount for IMRT, actual documentation of dose variation is mandatory regardless of the method employed
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