Intensity Modulated Radiotherapy Treatment Planning with OTP/TMS for Head&Neck Carcinomas in the ARTSCAN study

Cancer är en otrevlig sjukdom, som drabbar många människor. Man söker ständigt nya och bättre sätt att bota patienter eller åtminstone lindra sjukdomens framfart. Ett vanligt sätt att behandla cancer är med strålning. Strålningen kan bestå av sk fotoner. Fotoner är samma slags partiklar som t.ex. vanligt ljus. Skillnaden är att de fotoner som används vid strålbehandling av cancer har högre energi och kan ta sig in i kroppen och förstöra celler. Man vill att strålningen bara ska förstöra tumörceller och inte celler i friska organ som ligger nära tumören. Detta är dessvärre inte så lätt att åstadkomma, men man försöker minimera biverkningarna. Biverkningar som inte är livshotande får ofta accepteras eftersom man annars riskerar att låta tumören överleva. Exempelvis förlorar patienter med cancer i huvud-hals området ofta sin salivproduktion helt pga strålskador på spottkörtlarna. Detta är ett stort problem för patienterna, som därmed får svårighet att tugga och svälja. Smaken försvinner och risken för tandlossning ökar. Det har dock visat sig att patienten kan få tillbaka sin salivproduktion om man kan ge lite mindre strålning till den ena av spottkörtlarna. Det finns nu en ny metod inom strålbehandling, som kallas IMRT, och som innebär att man på ett bättre sätt kan koncentrera strålningen till tumören utan att skada känsliga organ i närheten. IMRT står för Intensitetsmodulerad Radioterapi. Radioterapi betyder strålbehandling och intensitetsmodulerad innebär att man delar upp sitt strålfält i många mindre fält, där man strålar olika mycket på varje litet fält. Resultatet blir ett strålfält med olika intensitet på olika ställen, därav ordet intensitetsmodulerad. Hur mycket man ska stråla på varje ställe låter man en dator räkna ut. Man specificerar hur mycket strålning man vill ge till tumören och anger samtidigt hur mycket friska organ tål. Dator-programmet räknar sedan ut på vilket sätt man bäst kan uppnå det resultat man vill. Målet är att Universitetssjukhuset i Lund ska kunna använda sig av IMRT inom en snar framtid, men innan detta är möjligt måste många tester utföras. Olika sjukhus har ofta olika utrustning och de metoder som fungerar bäst på ett sjukhus är inte nödvändigtvis de bästa på ett annat. Syftet med det här arbetet var att ta reda på om man, med hjälp av IMRT och den utrustning som finns här, kan ge patienter med cancer i huvud-hals området en bättre behandling än med de metoder som används idag. Framför allt vill vi undvika förlust av salivproduktionen. Arbetet har visat att det går att ge mindre strålning till en av spottkörtlarna, men tyvärr så leder det oftast till att tumören inte får riktigt så mycket strålning som man önskar. Detta beror dock på vilken utrustning man använder. Tester med annan utrustning visade att IMRT definitivt ger en bättre behandling för de här patienterna. Förhoppningen är ändå att efter fler tester och förbättringar kunna börja använda IMRT i Lund inom kort."Purpose: The aim of this study was to investigate whether inversely planned intensity modulated radiation therapy (IMRT), using the clinical radiotherapy equipment at hand in our department, renders any advantages regarding dose distribution over conventional radiotherapy for head and neck cancer patients in the ARTSCAN (Accelerated RadioTherapy of Squamous cell Carcinoma in the head And Neck) study, with special consideration to parotid sparing. Materials and Methods: The treatment plans were optimized for an Elekta SLi accelerator using the Oncentra Treatment Planning (OTP) system (version 1.2) from Nucletron for the inverse planning of the patient cases. For the final forward calculations, the Helax-TMS ver 6.1A, SP 1, (Nucletron) was used. The treatment planning systems (TPS), as well as the Elekta accelerator, only support step-and-shoot IMRT technique. A total of five patients have been used, all diagnosed with squamous cell carcinoma of the tonsil. The patients had all prior to this work received treatment according to the ARTSCAN protocol using conventional planning and treatment technique. The influence of different planning parameters (such as number of fields, number of intensity levels and segments per beam) on the dose distribution was investigated. The dose-volume distribution aimed for are according to the ARTSCAN protocol, i.e. 95 – 105 % of the prescribed dose (46 Gy in 23 fractions) to the planning target volume (PTV), including bilateral neck nodes, and a maximum spinal cord dose of 45 Gy (allowing for additional dose contribution from the boost treatment). Furthermore, the mean dose to the contralateral parotid gland was decided to be kept below 26 Gy to avoid permanent xerostomia. The treatment plans were evaluated in terms of physical quantities based on dose–volume histograms and isodose distributions. The IMRT plans were compared to the existing conventional plans (used for treatment of the patients) and also to plans optimized on the same patient at the Department of Radiation Physics in Umeå (using the same treatment planning equipment as in this study) and in Göteborg (using different planning and delivering techniques). Results: It is shown that the optimal IMRT settings for the given situation, is to use seven coplanar beams, separated by equal angles, ten intensity levels and a maximum of ten segments per beam. The dose-volume constraints for the PTV need to be stricter than the 95 – 105 % aim and the organ at risk (OAR) constraints must usually be set to a lower level than the actual tolerance doses. The IMRT plans show slightly worse PTV coverage and a larger standard deviation than the conventional plans, but the doses to the organs at risk are on the other hand lower. It is also shown that the quality of an IMRT plan is strongly dependent on the equipment used. The plan optimized with the other equipment showed better dose conformity and lower standard deviation as well as lower normal tissue doses. Conclusions: This work shows the possibilities of normal tissue sparing using IMRT. The contralateral parotid gland mean dose is easily kept below the threshold dose of 26 Gy in the IMRT plans. The spinal cord maximum dose is also usually lower as compared to the conventional plans. This normal tissue sparing is however always achieved at the cost of target coverage. The use of a different TPS may produce superior treatment plans, which is shown to be true in one particular case.

Cone beam CT for QA of synthetic CT in MRI only for prostate patients

Purpose: Magnetic resonance imaging (MRI)-only radiotherapy is performed without computed tomography (CT). A synthetic CT (sCT) is used for treatment planning. The aim of this study was to develop a clinically feasible quality assurance (QA) procedure for sCT using the kV-cone beam CT (CBCT), in an MRI-only workflow for prostate cancer patients. Material and method: Three criteria were addressed; stability in Hounsfield Units (HUs), deviations in HUs between the CT and CBCT, and validation of the QA procedure. For the two first criteria, weekly phantom measurements were performed. For the third criteria, sCT, CT, and CBCT for ten patients were used. Treatment plans were created based on the sCT (MriPlannerTM). CT and CBCT images were registered to the sCT. The treatment plan was copied to the CT and CBCT and recalculated. Dose–volume histogram (DVH) metrics were used to evaluate dosimetric differences between the sCT plan and the recalculated CT and CBCT plans. HU distributions in sCT, CT, and CBCT were compared. Well-defined errors were introduced in the sCT for one patient to evaluate efficacy of the QA procedure. Results: The kV-CBCT system was stable in HU over time (standard deviation <40 HU). Variation in HUs between CT and CBCT was <60 HU. The differences between sCT–CT and sCT–CBCT dose distributions were below or equal to 1.0%. The highest mean dose difference for the CT and CBCT dose distribution was 0.6%. No statistically significant difference was found between total mean dose deviations from recalculated CT and CBCT plans, except for femoral head. Comparing HU distributions, the CBCT appeared to be similar to the CT. All introduced errors were identified by the proposed QA procedure, except all tissue compartments assigned as water. Conclusion: The results in this study shows that CBCT can be used as a clinically feasible QA procedure for MRI-only radiotherapy of prostate cancer patients

MR-PROTECT : Clinical feasibility of a prostate MRI-only radiotherapy treatment workflow and investigation of acceptance criteria

Background: Retrospective studies on MRI-only radiotherapy have been presented. Widespread clinical implementations of MRI-only workflows are however limited by the absence of guidelines. The MR-PROTECT trial presents an MRI-only radiotherapy workflow for prostate cancer using a new single sequence strategy. The workflow incorporated the commercial synthetic CT (sCT) generation software MriPlanner™ (Spectronic Medical, Helsingborg, Sweden). Feasibility of the workflow and limits for acceptance criteria were investigated for the suggested workflow with the aim to facilitate future clinical implementations. Methods: An MRI-only workflow including imaging, post imaging tasks, treatment plan creation, quality assurance and treatment delivery was created with questionnaires. All tasks were performed in a single MR-sequence geometry, eliminating image registrations. Prospective CT-quality assurance (QA) was performed prior treatment comparing the PTV mean dose between sCT and CT dose-distributions. Retrospective analysis of the MRI-only gold fiducial marker (GFM) identification, DVH- analysis, gamma evaluation and patient set-up verification using GFMs and cone beam CT were performed. Results: An MRI-only treatment was delivered to 39 out of 40 patients. The excluded patient was too large for the predefined imaging field-of-view. All tasks could successfully be performed for the treated patients. There was a maximum deviation of 1.2% in PTV mean dose was seen in the prospective CT-QA. Retrospective analysis showed a maximum deviation below 2% in the DVH-analysis after correction for rectal gas and gamma pass-rates above 98%. MRI-only patient set-up deviation was below 2 mm for all but one investigated case and a maximum of 2.2 mm deviation in the GFM-identification compared to CT. Conclusions: The MR-PROTECT trial shows the feasibility of an MRI-only prostate radiotherapy workflow. A major advantage with the presented workflow is the incorporation of a sCT-generation method with multi-vendor capability. The presented single sequence approach are easily adapted by other clinics and the general implementation procedure can be replicated. The dose deviation and the gamma pass-rate acceptance criteria earlier suggested was achievable, and these limits can thereby be confirmed. GFM-identification acceptance criteria are depending on the choice of identification method and slice thickness. Patient positioning strategies needs further investigations to establish acceptance criteria

Target definition in radiotherapy of prostate cancer using magnetic resonance imaging only workflow

In magnetic resonance (MR) only radiotherapy, the target delineation needs to be performed without computed tomography (CT). We investigated in thirteenpatients with prostate cancer, how the clinical target volume (CTV) was affected, when the target delineation procedure was changed from using both CT and MRimages to using MR images only. The mean volume of the CTVCT/MR was 61.0 cm3 as compared to 49.9 cm3 from MR-only based target delineation, corresponding toan average decrease of 18%. Our results show that CTVMR-only was consistently smaller than CTVCT/MR, which has to be taken into consideration before clinicalcommissioning of MR-only radiotherapy