UME UNIVERSITY June 11, 2008 DEPARTMENT OF RADIATION SCIENCES, RADIATION PHYSICS SE-901 87 UME SWEDEN
Intensity modulated radiotherapy (IMRT) in head and neck cancer
A comparative treatment planning study using
physically and biologically based optimization
Elin Styf
Examiner
Heikki Tlli
Supervisors
Per Nilsson, Michael Blomquist and Mikael Karlsson
Thesis for Master of Science in Medical Radiation Physics
Abstract The ordinary way of optimizing intensity modulated radiotherapy (IMRT) treatment
plans is by defining physical dose-volume objectives and/or constraints to target volumes and organs at risk (OAR). This project aims to evaluate another approach of IMRT optimization. Instead of using physically based treatment goals, biological optimization parameters, derived from radiobiological models, were used in the optimization process. Both normal tissue complication probability (NTCP) and equivalent uniform dose (EUD) based optimization were investigated.
The main purpose of the project was to investigate advantages and/or disadvantages with this method compared to conventional IMRT optimization and, hopefully, a continuation of this study will result in improvements of the treatment planning in head and neck cancer in the future.
Five patients with head and neck cancer, already treated with IMRT, were chosen for the study. Physical dose constraints were defined for the target volumes in all plans, i.e. the optimizations using the biological models were limited to the organs at risk (OARs). The OARs included in the optimization process were spinal cord, brainstem and parotid glands.
The treatment planning system (TPS) used in this project was the research prototype ORBIT Workstation (RaySearch Laboratories AB, Stockholm, Sweden).
A decrease in mean dose to the parotid glands were obtained when using biologically compared to physically based optimization and still keeping the same target dose coverage. The maximum dose to the serially organized OARs, spinal cord and brainstem, obtained with biological optimization, were not significantly different from the maximum dose constraints set in the physical optimization process.
The physically and biologically optimized plans were further compared in terms of NTCP for the parotids. NTCP was significantly reduced with the biologically optimized plans. Spinal cord and brainstem NTCP were zero with both techniques and could therefore not be analyzed in this way.
It could be argued that both optimization techniques can in principle produce the same result. Several hours were, however, spent on the physically optimized plans to make them optimal and to meet the dose-volume criteria set for target volumes and OARs in the optimization process. In the biological optimization the adjustable parameter is only one (EUDmax or NTCPmax) for each OAR, which makes this method much simpler and easy to apply. An improved result was always obtained regarding the mean dose to the parotids when using biologically based optimization.
Sammanfattning Det vanliga sttet att optimera dosplaner med s.k. intensitetsmodulerad radioterapi
(IMRT) r genom att definiera fysikaliska dos-volymml och/eller dos-volymbivillkor fr tumrvolymer och riskorgan (eng. Organs At Risk, OAR). Det hr projektet syftar till att studera en annan metod fr IMRT optimering. Istllet fr att anvnda fysikaliska optimeringsml, anvnds biologiska kriterier i optimeringsprocessen, baserade p radiobilologiska modeller. Bde optimering baserad p risk fr biverkningar hos den normala vvnaden (eng. Normal Tissue Complication Probability, NTCP) och ekvivalent homogen dos (eng. Equivalent Uniform Dose, EUD) undersktes.
Huvudsyftet med projektet var att underska fr- och nackdelar med denna metod och jmfra den mot konventionell IMRT optimering. Frhoppningsvis kan detta s smningom leda till frbttringar av metoden fr dosplaneringen vid behandling av huvud och hals cancer.
Dosplaneringsunderlag (CT-bilder med utridade tumrvolymer och riskorgan) fr fem patienter med huvud-halscancer, redan behandlade med IMRT, valdes ut fr studien. Fysikaliska doskriterier definierades fr behandlingsvolymerna i alla planerna, d.v.s. optimering med biologiska modeller begrnsades till riskorganen. De riskorgan som inkluderades i studien var ryggmrgen, hjrnstammen och ronspottkrtlarna.
Dosplaneringssystemet som anvndes var forskningsprototypen ORBIT Workstation frn RaySearch Laboratories AB i Stockholm.
I samtliga planer optimerade med parametrar baserade p biologiska modeller uppnddes en lgre medeldos till ronspottkrtlarna, med bibehllen dostckning i behandlingsvolymerna, jmfrt med fysikaliskt baserad optimering.
Den maximala dosen till de seriellt organiserade OARs, d.v.s. ryggmrgen och hjrnstammen, frndrades inte nmnvrt frn de maximala doskriterierna satta i den fysikaliskt baserade optimeringen.
De fysikaliskt och biologiskt baserade planerna jmfrdes ven med NTCP fr ronspottkrteln. NTCP minskade vsentligt med biologiskt baserad optimering, medan NTCP fr ryggmrgen och hjrnstammen var noll med bda metoderna och kunde drfr inte evalueras p detta stt.
Det kan diskuteras huruvida bda metoderna i princip kan uppn samma resultat. Flera timmar spenderades dremot p att frska uppn optimala planer med fysikaliskt baserad optimering. Dosplaneringsprocessen baserad p biologiska parametrar tog betydligt kortare tid i ansprk fr att uppfylla stllda krav p planen.
I de biologiskt baserade optimeringarna var det endast en justerbar parameter (EUDmax eller NTCPmax) fr varje OAR, vilket medfr att denna metod r mycket enklare att tillmpa.
Ett frbttrat resultat uppnddes allts fr samtliga planer vad gller medeldos och NTCP till ronspottkrtlarna genom att anvnda biologiskt styrd optimering.
Contents Introduction............................................................................................................... 1
1.1 Intensity modulated radiotherapy (IMRT)................................................... 2
1.2 Treatment planning techniques Optimization ........................................... 3
1.3 Aim of the report ........................................................................................ 4
Background............................................................................................................... 5
2.1 Optimization process .................................................................................. 5
2.1.1 Optimization criteria....................................................................................................5
2.2 Biological models ....................................................................................... 5
2.2.1 Normal Tissue Complication Probability, NTCP ........................................................7
2.2.2 Tumour Control Probability, TCP ...............................................................................7
2.2.3 Equivalent Uniform Dose, EUD..................................................................................8
2.3 RaySearch ORBIT Workstation ............................................................... 8
2.4 Optimization process (continuation).......................................................... 11
2.4.1 Objective functions....................................................................................................11
2.4.1 Optimization algorithms............................................................................................11
2.5 Terminology in Radiotherapy ................................................................... 15
2.5.1 General volume definitions in photon beam therapy.................................................15
2.5.2 TNM classification ....................................................................................................16
2.6 Head and neck cancer ............................................................................... 17
2.6.1 OAR serial and parallel organs...............................................................................17
2.6.2 Dose volume restrictions and dose objectives ...........................................................18
Materials and methods ............................................................................................ 20
3.1 Patient material......................................................................................... 20
3.2 OARs included in the optimization ........................................................... 25
3.3 Treatment planning................................................................................... 25
3.3.1 Physically optimized treatment plans ........................................................................25
3.3.2 Biologically optimized treatment plans .....................................................................26
3.4 Evaluation of the treatment plans .............................................................. 27
3.5 Additional investigations .......................................................................... 28
3.5.1 Effect of changing number of segments ....................................................................28
Results .................................................................................................................... 29
4.1 Evaluation of the treatment plans .............................................................. 29
4.1.1 Target volumes ..........................................................................................................29
4.1.
